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The one where we reverse engineered Microsoft’s C++ Unit Test Framework (Part 1)

Have you ever really been interested in Microsoft’s C++ Unit Test Framework? I mean really interested? Like where you’d go to great lengths to figure out how it works. Well, I have… The story goes back a few years, maybe 3 or so.

At this point in my career I was deep into C++ development, and I had fallen in love with unit testing. I had fallen in love with Visual Studio’s Test Window, the ease it allowed me to get quick feedback about my development. My defacto standard was the built-in C++ Unit Test Framework. It was accessible. I didn’t need to install anything (past VS), and I didn’t need to download anything special. The project templates came built in, it was the easiest path to unit testing my work. I loved it. However, as with many things in life, there are always the ‘little things’ you have to learn to love.

My main gripe, was that if I wrote a function and a std::exception would escape, for whatever reason, the test would fail with the message “An unexpected C++ exception occurred”. Thanks Microsoft, for this useless information… I put up with it. I would use tactics like wrapping my calls in try/catch, I even wrote a new header that made an extension to the TEST_METHOD macro that would make a function level try/catch. It wasn’t enough for me, I could not believe that this wasn’t built in. For instance, if an Exception escapes in the C# test framework, you get the data about the Exception. This is a novel idea, so why doesn’t it work in C++? My second major stumbling block, was that if you didn’t have the right dependencies in the right directory. You would get an error, on all your tests, something along the lines of “Failed to setup execution context.” Also, a very very helpful error. This was the straw that broke the camels back. The amount of times I had run into this, the amount of times that junior developers scrambled to figure it out. It was too much. Something had to be done. Rather than divorce myself from Microsoft’s framework, and use something like boost::test, like so many had said I should do. I decided to do the sane thing, and just write my own. Not write my own, like re-invent the wheel. Just write my own test executor. I already had all the tests written, I didn’t want to redo that work in some new framework. I wanted to just build my own engine to run the tests I already had. My thought was that if someone at Microsoft could build it, so could I. They’re humans too — I think. Armed with only naive curiosity, my trusty Visual Studio, and the internet. I set out to do just that.

Where do I even start? Let’s just start at discovering the tests. How can we discover what tests are available in the binary? Well, if you’re familiar with the C# Unit Test framework, defining test classes and methods is done with attributes, similar to the C++ macros. My thought is that the C# Test Discoverer, must use reflection, look for the attributes, discovering the test classes and methods. I don’t know this for sure, but I would bet that is the case. Cool. Well, apart from some third party libraries, there’s no built in reflection in C++. So that can’t be the case for the C++ tests, can it? Maybe they load the assembly and have it tell the discoverer what tests are available? That’s what I would do if I engineered this.

Stop for a minute. Let’s reflect.

I said that my second problem with the framework, was that when you tried to run the tests and the dependencies couldn’t be loaded, you would get the error “Failed to load execution context”. Now — let’s think about this. If you’re able to see all the tests, yet the assembly can’t be loaded due to missing dependencies. How are we able to see what tests are in the binary? Magic! That’s how. Just kidding — I don’t believe in magic. It means that they’re not “loading” the library, which means that information about the tests, lives somewhere in metadata in the binary… Reflection… Could it be???

Well, the magic was right there in front of us the whole time, if you’re using the framework. The magic lies in the ‘ CppUnitTest.h’ header file. It took me a few beers, and a few hours to figure out just exactly WTF they were doing in there. It was essentially like trying to decipher Cuneiform .

If you’re unfamiliar, a typical TEST_CLASS and TEST_METHOD(s) looks like this.

#include "CppUnitTest.h"

TEST_CLASS(DummyClass)
{
    TEST_METHOD(DummyAssert)
    {
        /// My Code Under Test Here
    }
};

If you build and discover this, you’ll end up with a test class named DummyClass and a test in your Test Window, that says DummyAssert. So the magic lives in that TEST_METHOD macro. We will ignore the TEST_CLASS for now. Let’s look at TEST_METHOD. This is the macro, pulled directly from ‘CppUnitTest.h’

///////////////////////////////////////////////////////////////////////////////////////////
//Macro for creating test methods.
#define TEST_METHOD(methodName)\
    static const EXPORT_METHOD ::Microsoft::VisualStudio::CppUnitTestFramework::MemberMethodInfo* CALLING_CONVENTION CATNAME(__GetTestMethodInfo_, methodName)()\
    {\
        __GetTestClassInfo();\
        __GetTestVersion();\
        ALLOCATE_TESTDATA_SECTION_METHOD\
        static const ::Microsoft::VisualStudio::CppUnitTestFramework::MethodMetadata s_Metadata = {L"TestMethodInfo", L#methodName, reinterpret_cast<const unsigned char*>(__FUNCTION__), reinterpret_cast<const unsigned char*>(__FUNCDNAME__), __WFILE__, __LINE__};\
\
        static ::Microsoft::VisualStudio::CppUnitTestFramework::MemberMethodInfo s_Info = {::Microsoft::VisualStudio::CppUnitTestFramework::MemberMethodInfo::TestMethod, NULL, &s_Metadata};\
        s_Info.method.pVoidMethod = static_cast<::Microsoft::VisualStudio::CppUnitTestFramework::TestClassImpl::__voidFunc>(&methodName);\
        return &s_Info;\
    }\
    void methodName()

Okay — so humour me and let’s ignore the __GetTestClassInfo(); and __GetTestVersion(); calls and look to the line ALLOCATE_TESTDATA_SECTION_METHOD, which if we scan a little higher in the file is found here.

///////////////////////////////////////////////////////////////////////////////////////////
//Macros for creating sections in the binary file.
#pragma section("testvers$", read, shared)
#pragma section("testdata$_A_class", read, shared)
#pragma section("testdata$_B_method", read, shared)
#pragma section("testdata$_C_attribute", read, shared)

#define ALLOCATE_TESTDATA_SECTION_VERSION __declspec(allocate("testvers$"))
#define ALLOCATE_TESTDATA_SECTION_CLASS __declspec(allocate("testdata$_A_class"))
#define ALLOCATE_TESTDATA_SECTION_METHOD __declspec(allocate("testdata$_B_method"))
#define ALLOCATE_TESTDATA_SECTION_ATTRIBUTE __declspec(allocate("testdata$_C_attribute"))

But what does it all mean Basil? Well, without diving into too much history, we need to at least know about Windows’ binary formats. If you didn’t already know, every form of executable “binary” on the Windows platform is in the format of a Portable Executable (which was an extension of the COFF format). This is what allows the operating system to load and run executables, dynamic libraries, etc. It’s a well defined format, see the Wiki link above if you don’t believe me. A PE file looks like this.

Portable_Executable_32_bit_Structure_in_SVG_fixed

I’m not going to explain everything in this image, only the relevant information. If you look down just passed the DOS STUB, on the very right (your right) you’ll see a 2 byte number called #NumberOfSections, this tells us the count of sections in the binary. That’s something we care about. I know this, because I know they’ve made sections where the data lives. I know this, because of the

#pragma section("testdata$_B_method", read, shared)

and the

#define ALLOCATE_TESTDATA_SECTION_METHOD__declspec(allocate("testdata$_B_method"))

Then, if you look at the bottom, you’ll see the ‘Section Table’. It means, that from the COFF Header, the offset of the Optional Header, there lives N sections in the Sections Table. In there, we will find our “testdata$_B_method” section, and in there, we will find SOMETHING! Are you bored yet? Because when I got this far, you couldn’t pull me away. I was like a 13 year old watching my first R rated movie. What did they store in there? What was it used for? The only thing I could do, is dive a little deeper. My best bet, was that these MethodMetadata were stored in that section.

ALLOCATE_TESTDATA_SECTION_METHOD\
static const ::Microsoft::VisualStudio::CppUnitTestFramework::MethodMetadata s_Metadata = {L"TestMethodInfo", L#methodName, reinterpret_cast<const unsigned char*>(__FUNCTION__), reinterpret_cast<const unsigned char*>(__FUNCDNAME__), __WFILE__, __LINE__};

It would be a block of data, that would contain a bunch of strings. The first being a wide character string, “TestMethodInfo”, the next a wide character string of the ‘methodName’ defined in the macro, the next a character string of the __FUNCTION__, next the string of __FUNCDNAME__, a wide character string of the filename __WFILE__ , and lastly the __LINE__. (If you’re interested in a list of Predefined Macros there you go.)

This was my assumption, but I couldn’t know for sure unless I saw it with my own two eyes. But how do I do that? Well there are a few third-party tools that will dump the PE (I’ll let you figure out what to search…), but I needed to write my own tool anyways so I just jumped in feet first. A few quick Bing searches (just kidding I used Google), and I found out I needed to open the binary as a flat file, and then map that file into memory. From there, I could get a pointer to the start of the file and use some macros, structures and functions in Windows.h to move about this file.  The “pseudo” algorithm is as follows

1) Open the binary as a flat file
2) Map the binary into memory
3) Obtain a view of the map (a pointer to the map)
4) Navigate the PE to understand file type, and number of sections
5) Iterate through each section definition until we find the correct section
6) Using the mapping offset, along with the section definition, find our data

There we go, simple as that. Let’s try it. The memory when we do that, and we point to the section table, looks like this.

0x07440210  2e 74 65 78 74 62 73 73 00 00 01 00 00 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 a0 00 00 e0 2e 74 65 78 74 00 00 00 82 15 02 00 00 10 01 00 00 16 02 00 00 04 00 00 00 00 00 00 00 00 00 00 00 00 00 00 20 00 00 60 2e 72  .textbss............................ ..à.text............................... ..`.r
0x07440262  64 61 74 61 00 00 c3 bd 00 00 00 30 03 00 00 be 00 00 00 1a 02 00 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 40 2e 64 61 74 61 00 00 00 cc 0e 00 00 00 f0 03 00 00 0c 00 00 00 d8 02 00 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 c0 2e 69 64 61  data..Ã....0......................@..@.data...Ì....ð.......Ø..............@..À.ida
0x074402B4  74 61 00 00 86 29 00 00 00 00 04 00 00 2a 00 00 00 e4 02 00 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 40 2e 6d 73 76 63 6a 6d 63 79 01 00 00 00 30 04 00 00 02 00 00 00 0e 03 00 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 c0 74 65 73 74 64 61  ta...).......*...ä..............@..@.msvcjmcy....0......................@..Àtestda
0x07440306  74 61 49 06 00 00 00 40 04 00 00 08 00 00 00 10 03 00 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 50 74 65 73 74 76 65 72 73 09 01 00 00 00 50 04 00 00 02 00 00 00 18 03 00 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 50 2e 30 30 63 66 67 00 00  taI....@......................@..Ptestvers.....P......................@..P.00cfg..
0x07440358  04 01 00 00 00 60 04 00 00 02 00 00 00 1a 03 00 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 40 2e 72 73 72 63 00 00 00 3c 04 00 00 00 70 04 00 00 06 00 00 00 1c 03 00 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 40 2e 72 65 6c 6f 63 00 00 69 1a  .....`......................@..@.rsrc...<....p......................@..@.reloc..i.
0x074403AA  00 00 00 80 04 00 00 1c 00 00 00 22 03 00 00 00 00 00 00 00 00 00 00 00 00 00 40 00 00 42 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ...€......."..............@..B....................................................
0x074403FC  00 00 00 00 cc cc cc cc cc e9 86 36 01 00 e9 41 78 01 00 e9 9c 33 00 00 e9 b7 93 01 00 e9 b3 73 01 00 e9 7d b4 00 00 e9 68 a8 00 00 e9 13 62 00 00 e9 7e 76 00 00 e9 09 6f 01 00 e9 a4 59 00 00 e9 8f e4 00 00 e9 aa 9d 01 00 e9 98 73 01 00 e9 ce ab

So what gives??? I don’t see any section called “testdata$_B_method”.  I can however see a ‘testdata’ section. At this point no amount of research other than this anecdotal evidence, leads me to believe the ‘$’ is some kind of delimiter on the section name. I guess we have to assume. We assume that the “testdata” section will contain our test method metadata. The problem is now, there are other things that sit in this section. There’s class, method, and attribute metadata. So, if it’s all lined up in a single section, how do we decipher what is what? Meaning, if we’re just trying to use pointers to walk around, how will we ever know what type we’re pointing to?

Did anything strike you as odd about the MethodMetadata structure? Maybe, if I show you the structure definitions of all the metadata objects, you might see something.

struct ClassMetadata
{
    const wchar_t *tag;
    const unsigned char *helpMethodName;
    const unsigned char *helpMethodDecoratedName;
};

struct MethodMetadata
{
    const wchar_t *tag;
    const wchar_t *methodName;
    const unsigned char *helpMethodName;
    const unsigned char *helpMethodDecoratedName;
    const wchar_t *sourceFile;
    int lineNo;
};

struct ModuleAttributeMetadata
{
    enum AttributeType { MODULE_ATTRIBUTE };
    const wchar_t *tag;
    const wchar_t *attributeName;
    const wchar_t *attributeValue;
    AttributeType type;
};

struct ClassAttributeMetadata
{
    enum AttributeType { CLASS_ATTRIBUTE };
    const wchar_t *tag;
    const wchar_t *attributeName;
    const void *attributeValue;
    AttributeType type;
};

struct MethodAttributeMetadata
{
    enum AttributeType { METHOD_ATTRIBUTE };
    const wchar_t *tag;
    const wchar_t *attributeName;
    const void *attributeValue;
    AttributeType type;
};

Huh. If you look carefully, the first actual member of these structures is a wchar_t* called tag. Then if we go to our use of it.

static const ::Microsoft::VisualStudio::CppUnitTestFramework::MethodMetadata s_Metadata = {L"TestMethodInfo", L#methodName, reinterpret_cast<const unsigned char*>(__FUNCTION__), reinterpret_cast<const unsigned char*>(__FUNCDNAME__), __WFILE__, __LINE__};

You might notice, that there’s a L”TestMethodInfo” set as the tag, so one could deduce, dear Watson that that is how we can decipher our different metadata components. By their tag! Let’s readjust our rudders, and fly! If we get our ‘testdata’ section, then move to the area with the data, we should see a bunch of nicely spaced wide strings in memory, right? Wrong!

0x07471120  94 43 03 10 b8 43 03 10 d8 43 03 10 40 44 03 10 f8 44 03 10 67 00 00 00 00 00 00 00 94 43 03 10 6c 46 03 10 98 46  ”C..¸C..ØC..@D..øD..g.......”C..lF..˜F
0x07471146  03 10 08 47 03 10 f8 44 03 10 78 00 00 00 00 00 00 00 94 43 03 10 cc 47 03 10 08 48 03 10 80 48 03 10 f8 44 03 10  ...G..øD..x.......”C..ÌG...H..€H..øD..
0x0747116C  7e 00 00 00 00 00 00 00 94 43 03 10 28 4b 03 10 68 4b 03 10 e0 4b 03 10 f8 44 03 10 8b 00 00 00 00 00 00 00 94 43  ~.......”C..(K..hK..àK..øD..........”C
0x07471192  03 10 c8 4c 03 10 08 4d 03 10 80 4d 03 10 f8 44 03 10 95 00 00 00 00 00 00 00 94 43 03 10 4c 4e 03 10 78 4e 03 10  ..ÈL...M..€M..øD..........”C..LN..xN..
0x074711B8  e8 4e 03 10 f8 44 03 10 9d 00 00 00 00 00 00 00 94 43 03 10 dc 4f 03 10 20 50 03 10 a0 50 03 10 f8 44 03 10 a7 00  èN..øD..........”C..ÜO.. P.. P..øD..§.
0x074711DE  00 00 00 00 00 00 94 43 03 10 70 51 03 10 98 51 03 10 08 52 03 10 f8 44 03 10 af 00 00 00 00 00 00 00 94 43 03 10  ......”C..pQ..˜Q...R..øD..¯.......”C..
0x07471204  78 53 03 10 b0 53 03 10 28 54 03 10 f8 44 03 10 bb 00 00 00 00 00 00 00 94 43 03 10 0c 55 03 10 48 55 03 10 c0 55  xS..°S..(T..øD..».......”C...U..HU..ÀU
0x0747122A  03 10 f8 44 03 10 c3 00 00 00 00 00 00 00 94 43 03 10 dc 56 03 10 08 57 03 10 78 57 03 10 f8 44 03 10 cc 00 00 00  ..øD..Ã.......”C..ÜV...W..xW..øD..Ì...
0x07471250  00 00 00 00 94 43 03 10 6c 58 03 10 b8 58 03 10 38 59 03 10 f8 44 03 10 d2 00 00 00 00 00 00 00 94 43 03 10 30 5a  ....”C..lX..¸X..8Y..øD..Ò.......”C..0Z
0x07471276  03 10 60 5a 03 10 d0 5a 03 10 f8 44 03 10 de 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ..`Z..ÐZ..øD..Þ
0x0747110A  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

What the hell? I was 100% expecting there to be a string sitting pretty right there in my section. Back to the drawing board I guess… Let’s recap, the first member of that struct is a wchar_t *. Huh. The first member of the struct, is a wchar_t *. What does that even mean? It means, that it is a ‘pointer to a wchar_t’. A ‘pointer’ to a wchar_t! Oh right! A pointer! I remember from school that a pointer is just an address! So where we were expecting there to be some text sitting pretty, was garbage, Or so we thought. Wrong again. It is a POINTER! That means sitting in that location is an address! An address to where though? It should be an address to my string, but where on Earth (quite literally) would that address be? It has to be somewhere that is constant, right?

If we study the sections of a PE fie, there’s a section called ‘.rdata’. Microsoft defines this section as “Read-only initialized data”. Thinking back a moment these are magic strings, (heaven forbid), aka ‘static strings’, static is read-only. If I was to hazard a guess, it probably means that they live somewhere in that section, because the compiler has to put magic string somewhere… So that garbage number, is probably a pointer to a string somewhere in that ‘.rdata’ section. So, if we take that address, and adjust it for where the section data lies, we can find the “TestMethodInfo” string.

0x07462D94  54 00 65 00 73 00 74 00 4d 00 65 00 74 00 68 00 6f 00 64 00 49 00 6e 00 66 00 6f 00 00 00 00 00 00 00 00 00 44 00  T.e.s.t.M.e.t.h.o.d.I.n.f.o.........D.
0x07462DBA  75 00 6d 00 6d 00 79 00 41 00 73 00 73 00 65 00 72 00 74 00 00 00 00 00 00 00 00 00 00 00 43 50 50 55 6e 69 74 54  u.m.m.y.A.s.s.e.r.t...........CPPUnitT
0x07462DE0  65 73 74 49 6e 76 65 73 74 69 67 61 74 6f 72 54 65 73 74 3a 3a 6e 65 73 74 65 64 3a 3a 44 75 6d 6d 79 43 6c 61 73  estInvestigatorTest::nested::DummyClas
0x07462E06  73 3a 3a 5f 5f 47 65 74 54 65 73 74 4d 65 74 68 6f 64 49 6e 66 6f 5f 44 75 6d 6d 79 41 73 73 65 72 74 00 00 00 00  s::__GetTestMethodInfo_DummyAssert....
0x07462E2C  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 3f 5f 5f 47 65 74 54 65 73 74 4d 65 74 68 6f 64 49 6e  ....................?__GetTestMethodIn
0x07462E52  66 6f 5f 44 75 6d 6d 79 41 73 73 65 72 74 40 44 75 6d 6d 79 43 6c 61 73 73 40 6e 65 73 74 65 64 40 43 50 50 55 6e  fo_DummyAssert@DummyClass@nested@CPPUn
0x07462E78  69 74 54 65 73 74 49 6e 76 65 73 74 69 67 61 74 6f 72 54 65 73 74 40 40 53 47 50 42 55 4d 65 6d 62 65 72 4d 65 74  itTestInvestigatorTest@@SGPBUMemberMet
0x07462E9E  68 6f 64 49 6e 66 6f 40 43 70 70 55 6e 69 74 54 65 73 74 46 72 61 6d 65 77 6f 72 6b 40 56 69 73 75 61 6c 53 74 75  hodInfo@CppUnitTestFramework@VisualStu
0x07462EC4  64 69 6f 40 4d 69 63 72 6f 73 6f 66 74 40 40 58 5a 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  dio@Microsoft@@XZ

Voila! At last we finally found it. We found a ‘TestMethodInfo’ (if you’re wondering why it’s T.e.s.t.M.e.t.h.o.d.I.n.f.o…, it’s because it’s a wide character string so each character is 2 bytes. Unicode amirite?).

To recap, we’ve loaded the DLL into memory, mapped it and walked to the section. We taken the tag pointer, adjusted the address to find it in the ‘.rdata’ section, now we know that we’re looking at the MethodMetadata structure. So, we can take the original pointer, and cast that to a MethodMetadata object.

struct MethodMetadata
{
    const wchar_t *tag;
    const wchar_t *methodName;
    const unsigned char *helpMethodName;
    const unsigned char *helpMethodDecoratedName;
    const wchar_t *sourceFile;
    int lineNo;
};

Then, for each of the other members, which are pointers to strings in the .rdata section we can just adjust and capture the same way we did the tag. In fact, the compiler did us a favour, and laid them out so you can see them in the memory dump above. Next, we just advance our section pointer the distance of the size of a MethodMetadata, and we are able to get the next test name!!! (This is mostly true, I’ve glossed over some details of the other things that can be in the section)

I really hope you’re thinking “Damn! That was so much fun!” Because I definitely was! You can see now, that the steps to tie this into a test adapter aren’t too far away. I won’t get into that until a later post, but as for this post, we’ve uncovered how to discover what tests lie in a Microsoft C++ Unit Test Framework created test DLL. Wasn’t that just a blast digging into that?

I hope you will join me for the next part, where we figure out how to actually load and execute the tests from this metadata.

If you’re interested in seeing this code, my cobbled together project lives here. I apologize in advance for the code. I had a really bad habit of putting as many classes as I could in a single file. I don’t do this anymore. It was out of laziness.  The PeUtils.h/.cpp and CppUnitTestInvestigator.h/.cpp, in the CppUnitTestInvestigator project have the code for loading the DLL metadata.

“It’s still magic, even if you know how it’s done” — Terry Pratchett

Happy Coding!

 

** As a disclaimer, I don’t own any of the code snips above, they are directly pulled from the VsCppUnit C++ Unit Testing Framework, which is Copyright (C) Microsoft Corporation **

 

References:
PE Format
Section Specifications
DUMPBIN

Template<T> vs. Generic<T>

The other day I was discussing the differences between C++ templates and C# generics. In my opinion, C++ templates reigns supreme. Not because I’m one of those guys with a “My compiler compiles your compiler.” shirt. But because in general abstractions in C++ tend to ‘cost’ lest than the same abstractions in C#. For instance, the foreach exists in both languages, but the iterator concept is far lighter weight then the IEnumerable concept. Both of these concepts make use of generics to be able to support iterating a collection of “things”.  Having one single ‘concept’, that works across a multitude of different containers, allowing for traversal. The iterator concept is a bit more in-depth, in that it allows for more than forward traversal unlike IEnumerable.

The discussion came down to “why?”, why I thought that the template system in C++ was better than that of C# generics. Why is right. At the time, my statement was rather off the cuff, and somewhat tongue-in-cheek. Granted I had my reasons, but not backed by research. Only backed by my overview knowledge of the operations of the two systems. So now that I’ve had time to mull it over, and solidify my arguments. I’ll explain my point-of-view, grounded in my initial thoughts.

Let me ask you something? When was the last time someone wrote Tetris “in” C# generics? No, I don’t mean “with” C# generics, I mean “in” C# generics.

Spoiler: the answer is never.

Why? Let’s explore reason numero uno, which admittedly wasn’t the first reason to my head at the time. The C++ template system is Turing complete. That’s right, it’s Turing complete. Wait what does that even mean? Well, the definition for Turing complete is along the lines of  “A system of data-manipulation, is said to be Turing complete, when it can simulate a Turing machine.” Thank you Wikipedia for that ever so enlightening statement. Basically, what this means is that the meta-language built into the C++ template system, is actually a full blown programming language. One where if you’re brave enough, or crazy enough, you can even implement Tetris. Remember, that the C++ template engine runs at compile time, so you guessed it — it’s compile time Tetris.

You don’t need to use the template system only for silly things like Tetris. You can also use it for actually useful things as well. Things like specific implementations of your generic algorithms based on traits about a type. Or how about even finding traits about a type?  You can use the template system, to choose different implementations based on the type you’re using, or rather traits about the group of types you’re using. As well as do computations at compile time.

template <int N>
struct sum
{
    static const int value = N + sum<N-1>::value;
};

template<>
struct sum<0>
{
    static const int value = 0;
};

int main()
{
    std::cout << sum<5>::value << "!\n";
}

Figure 1 – A simple example of a summation recursive template meta-program

All of this for “free” at compile time. When I say free, I mean run-time free. The C++ template engine does its work at compile time, and will make heavy use of the type system. Don’t get me wrong, it’s not all roses. If you make heavy heavy heavy use of the template system, you can have really long compile times and even bloat your binaries. I’ve seen code generation times upwards of 30 minutes. The code actually only took about 5 or so minutes to compile, but the code generation was ~30 minutes. Imagine writing all those classes, TGFCPPT (Thank God for CPP Templates).  So at this point, C++ templates have delivered a knock-out punch to C# generics. KO! Generics down for the count. However, I am still going to proceed with examining my initial reasoning, because I like to “beat dead horses” so to speak. I think it can be said that generics are like the template systems little brother. They look similar, but one is about half as useful and twice as fat.

So the second reason, and the first that came to my head (not surprisingly) was overhead. The C++ template system doesn’t have run-time costs, because it’s a compile time operation. Whereas the generic system must have some run-time overhead, just due to its nature.  I wouldn’t say I was stabbing in the dark, but I definitely didn’t have research to back up my claim. I had read about the generics system in a few blogs, I understood how it worked, sort of. Anyways, the claim was contended. Fair enough, I didn’t have the research in my back pocket to prove it. For most, it would’ve ended there. However, on my never ending quest to remove misinformation and search down the truth, I couldn’t let it go. So really, what is the cost of the C# generics vs. the C++ template engine?

First, lets look at how the C++ template engine actually works. To be honest, I couldn’t find all that much real information on how C++ templates work. So I will have to share my understanding, which admittedly is naive, but get’s the point across . When you have some code like the following.

template <typename T>
void print(T value)
{
    std::cout << value;
}

print(5);
print("Hello World");

The compiler will for all intents and purposes, generate two functions for you.

void print(int value)
{
    std::cout << value;
}

void print(const char *value)
{
   std::cout << value;
}

Now, there’s little value in this example. But as you can see, the compiler is basically saving you the keyboard time. The compiler will only generate those functions that are needed, so it doesn’t just go out and create functions for every type. Only the ones that need to be created. Given this happens compile time, by the time you’re running those template definitions are a thing of the past, and you’re running the code that was generated by the compiler. These functions each have their own unique addresses, and their own ability to be optimized.

Now, compare this to how C# generics work. Consider List<T>, you could have instantiation for List<int>, List<string>, even List<Object>. How does this work?

Well, for a minute we can brainstorm a couple of different ways this could be achieved.

  • We could do something similar to the C++ template engine, and for every unique instantiation we could generate IL. Meaning that List<int>, List<string>, and List<object> would each get their own set of instructions.  This could be referred to as code specialization.

Let’s be honest with ourselves, this isn’t a good approach here. It would lead to HUGE binaries, and would be hugely inefficient. Given that C# objects are references, the code would be duplicated for a large portion of instantiations.

  •  We could ignore the type of the target. Effectively erasing it from our implementation. I don’t care if it looks like a duck, or quacks like a duck, it’s a duck. Then we only need to maintain ONE version of the code, and at run-time, we just ignore the type so the List<int>, List<string>, and List<Object> all use the same instructions. They’re just ignorant to the type.

This has issues too, because since we’ve erased the type, we’ve lost that information about the objects. Then we need to do extra work, to make sure that List<Object> != List<string>. As a note, this is the approach JAVA takes to its generics. The benefit to this approach, is that we only have one master set of IL for the implementation. There’s not duplication, like the approach above. It also means, that you need to treat POD types, that are stack based, as heap values.

  • We could use one shared implementation like JAVA, but at the same time use run-time techniques to manage our types. So this way we’re respecting the types of the objects. So List<int>, List<string>, and List<Object> all would use the same code, except there would be some overhead to store the information about the types.

This last approach is the approach that C# takes for their generics. Now, this isn’t exactly 100% true, because I believe that C# generics use a mixture of code specialization, as well as code sharing. Because when we talk about “objects” vs. “primitives”, in C# one is heap allocated and referenced, and the other lives on the stack. Therefore, if you treated something like an int the same as a string, you would need to do what is called “boxing” and allocate that int on the heap. Which is mega overhead and not cheap. So there is a combination of code specialization and code sharing going on.

One of the goals of the design of generics, was that it should be as efficient as code that is hand specialized. Using these two techniques, the implementation is equally as efficient as hand specialized code, and much much more efficient that type erasure. This is due to the boxing thing I mentioned earlier. That’s a lofty goal, and I’ll say an admirable one, taking from the spirit of C++. However! There are definitely other costs that are paid, the cost of memory overhead for storing the vtables, and other RTTI data. Making generics, not exactly free. Clever techniques for loading this information saves execution time, but we still have to pay for the memory used. Contrary to popular belief, memory isn’t always free. Although, at the C# level, it can pretty much be assumed to be. Like C++ code generation, C# generics can also basically explode when being evaluated. In one blog that I read, the author mentions that a single instance of List<T> will explode out into 28 different types, where 5 unique List types could cost upwards of 70K in the working set.

It’s not to say that C# generics don’t have some neat features. Specifically type constraints, and the ability to constrain types to interfaces. This is something that can be done with C++ templates (slightly easier post C++11), but the reality is that it’s not for the faint of heart. The second upside of this, is that the compiler errors you get when you violate a type constraint on a generic, they’re readable by a human. One of the downsides of C++ template usage and definitely of template meta-programming, is that the compile errors are pretty much non-coherent for anyone who isn’t a compiler writer or demigod.

In conclusion, I still think that C++ templates are far superior to C# generics. That said, they both have their benefits and costs. It is like anything, you need to understand the cost of something before you use it, and you should always pick the right tool for the job.

“Price is what you pay. Value is what you get.” — Warren Buffet

As always — keep calm and code on.

Happy Coding!

 

References:

[1] On generics and (some of) the associated overheads 

[2] Design and Implementation of Generics for the .NET Common Language Runtime

[3] Turing Completeness – Wikipedia

 

The Zero Cost Abstraction of Iteration

I’ve been hooked on C++ for about 10 years now. Since CMPUT101, I’ve been attracted to the syntax of the language, the simplicity of use, and later the driving ideals behind the language. C++’s author Bjarne Stroustrup, stands alone when it comes to preaching language ideals and pushing the language forward all while respecting these ideals. This is no easy task, especially when the language is designed by a committee. I’ve always been convinced one of the key reasons C++ is such a large success, and has remained so timeless; is the fundamental guiding principles behind the language. The principle that I believe reigns supreme is the ‘Zero Cost Abstraction’.

What is the ‘Zero Cost Abstraction’? Also called a ‘Zero Overhead Abstraction’. Well — it’s an abstraction, that costs nothing. Not like monetary currency; or a favour. Instead resource or runtime cost. Bjarne’s definition is “an abstraction that imposes no space or time overhead in excess of  what would be imposed by careful hand coding of a particular example of the abstraction.” So in other words, an abstraction that doesn’t use any more resources than if you carefully crafted the assembly yourself.

Well then, what is an ‘abstraction’? Well an abstraction is a concept that you can work with, that ignores the details of operation. A good metaphor for this is a steering wheel on a car. You turn the wheel left and right, to turn left and right, but you’re not concerned with the details of actually moving the wheels of the car on the road. In fact, you don’t want to be concerned with those details at the level of driving.  You want to be more focused on the bigger task at hand — driving the car.

So abstractions in computing allow us to work with concepts, craft algorithms and structure code, so that we don’t have to worry about the details. Programming languages themselves are abstractions, they allow us to work with logical operations, classes, English words, and common symbols to create instructions that cause computer hardware to do things. Languages like C and C++, are considered ‘close to the metal’, this means that they only thinly abstract the assembly that is run on the processor, only thinly abstract the management of memory (either directly to the MMU or through the OS), and only thinly abstract the other pieces of hardware. Languages like C# and JAVA (shudder), are higher level languages, meaning they have many layers between the code and the actual instructions that execute on the CPU. These higher level abstractions give way for more flexibility, but often have a cost. In the case of JAVA, the flexibility of portability, at the cost of run-time and space overhead. Garbage collection adds the flexibility of not having to worry about memory management, at the cost of run-time, and space overhead. Not to mention the cost to developers, as the become brainless in the areas of memory lifetime, and ownership rules.

[Coming Soon – a post on Virtual Machines]

So what does any of this have to do with the ‘Zero Overhead Abstraction’ principle? Well, obviously I have a story to go along with it. So some time ago I was working on my library [ASPeKT] and I was soliciting the help of the internet for advice on how to work better with the IL I was crafting. The comments I got, were, well, unexpected to say the least. The majority of the criticism I received was my use of direct for loops for iteration. As opposed to the cooler C# foreach loop or even better LINQ-to-everything!! Now, I can take critiquing of my code, after all that’s how you learn. So I pondered this for quite some time. I reviewed some of the pull requests which had transformed some of my boring for-loops, into beautifully crafted LINQ queries. I wasn’t buying it. I just couldn’t. It hid some of the intent of what I was trying to do, and call me old school, but I found it harder to read.

Enter foreach, this one I liked. C++11 introduced new semantics for it and I liked the clarity that it gave. When you’re iterating a collection using an indexer, the first thing you do is get the item at the index. The foreach, just abstracts this and your boundary checking in a nice syntax package. Color me smitten. Now, I didn’t actually refactor much in my library to use foreach. Honestly, I like it but I wrote the code already and it worked. However I started using foreach, pretty much everywhere I could, just like I did in C++ now. I know that C# probably had this before, and I obviously adopted it backwards, but I do a lot of shit backwards. So sue me. 

Fast forward a couple of months, I’m reading “Writing High-Performance .NET Code” by Ben Watson. He’s got a little section on loops for iteration. The section I think was on comparing C++ to C# for performance. When we’re talking speed of execution, less instructions is more. The less instructions you have to run, the faster your program will be. The fastest program you can write, is one that does nothing.

There was a snip on comparing a loop in C# to a loop in C++, and the generated ASM instructions. He was making the statement that C# doesn’t ALWAYS have much overhead over a language like C++. He showed how old skool for loop iteration, was virtually the same instructions for C++ and C#. But the foreach, it was something else. I was floored, I had never really thought about it, but at that moment I did. What does a foreach in C# actually do?

His example was something like this

[C++]
int sum2()
{
    int range[4] = { 1, 2 , 3 , 4};
    int sum=0;
    for(auto i=range; i<range+4; ++i)
    {
        sum+=*i;
    }
    return sum;
}

[ASM]
//-O2 Disabled so it wouldn't optimize the loop out
sum2(): # @sum2()
  push rbp
  mov rbp, rsp
  lea rax, [rbp - 16]
  mov rcx, qword ptr [.L_ZZ4sumvE5range]
  mov qword ptr [rbp - 16], rcx
  mov rcx, qword ptr [.L_ZZ4sumvE5range+8]
  mov qword ptr [rbp - 8], rcx
  mov dword ptr [rbp - 20], 0
  mov qword ptr [rbp - 32], rax
.LBB1_1: # =>This Inner Loop Header: Depth=1
  lea rax, [rbp - 16]
  mov rcx, qword ptr [rbp - 32]
  add rax, 16
  cmp rcx, rax
  jae .LBB1_4
  mov rax, qword ptr [rbp - 32]
  mov ecx, dword ptr [rax]
  add ecx, dword ptr [rbp - 20]
  mov dword ptr [rbp - 20], ecx
  mov rax, qword ptr [rbp - 32]
  add rax, 4
  mov qword ptr [rbp - 32], rax
  jmp .LBB1_1
.LBB1_4:
  mov eax, dword ptr [rbp - 20]
  pop rbp
  ret
.L_ZZ3sumvE5range:
  .long 1 # 0x1
  .long 2 # 0x2
  .long 3 # 0x3
  .long 4 # 0x4
[C#]
static class Program
{
     static int sum()
    {
          int[] range = new int[]{ 1, 2 , 3 , 4};
          int sum=0;
          for(var i=0; i<4; ++i)
          {
                 sum+=range[i];
           }
          return sum;
   }

}

[JIT ASM]
Program.sum()
    L0000: push ebp
    L0001: mov ebp, esp
    L0003: push edi
    L0004: push esi
    L0005: mov ecx, 0x717168e2
    L000a: mov edx, 0x4
    L000f: call 0x58f3200
    L0014: lea edi, [eax+0x8]
    L0017: mov esi, 0x251c084c
    L001c: movq xmm0, [esi]
    L0020: movq [edi], xmm0
    L0024: movq xmm0, [esi+0x8]
    L0029: movq [edi+0x8], xmm0
    L002e: xor esi, esi
    L0030: xor edx, edx
    L0032: mov ecx, [eax+0x4]
    L0035: cmp edx, ecx
    L0037: jae L0049
    L0039: add esi, [eax+edx*4+0x8]
    L003d: inc edx
    L003e: cmp edx, 0x4
    L0041: jl L0035
    L0043: mov eax, esi
    L0045: pop esi
    L0046: pop edi
    L0047: pop ebp
    L0048: ret
    L0049: call 0x73a52b10
    L004e: int3

As you can see 25 instructions in C++ vs.  30 instructions for C#. So your standard for loop in C++ and C# are running essentially the same instructions.

The foreach was a different story — he wouldn’t even show the instructions. He only showed the generated IL, and let me tell you, it was a lot.

In my own research, the ASM generator that I used, generated 150+ instructions, which didn’t seem like a lot. However, I did notice is that it didn’t generate ASM for the actually calls to MoveNext(), it just used the syntax ‘call’ MoveNext(). Which likely has a lot more instructions under the hood.

Let’s compare that to the pure C++, range foreach.

[C++]
int sum()
{
    int range[4] = { 1, 2 , 3 , 4};
    int sum=0;
    for(auto i : range)
    {
        sum+=i;
    }
    return sum;
}

[ASM]
sum(): # @sum()
  push rbp
  mov rbp, rsp
  mov rax, qword ptr [.L_ZZ3sumvE5range]
  mov qword ptr [rbp - 16], rax
  mov rax, qword ptr [.L_ZZ3sumvE5range+8]
  mov qword ptr [rbp - 8], rax
  mov dword ptr [rbp - 20], 0
  lea rax, [rbp - 16]
  mov qword ptr [rbp - 32], rax
  mov rax, qword ptr [rbp - 32]
  mov qword ptr [rbp - 40], rax
  mov rax, qword ptr [rbp - 32]
  add rax, 16
  mov qword ptr [rbp - 48], rax
.LBB0_1: # =>This Inner Loop Header: Depth=1
  mov rax, qword ptr [rbp - 40]
  cmp rax, qword ptr [rbp - 48]
  je .LBB0_4
  mov rax, qword ptr [rbp - 40]
  mov ecx, dword ptr [rax]
  mov dword ptr [rbp - 52], ecx
  mov ecx, dword ptr [rbp - 52]
  add ecx, dword ptr [rbp - 20]
  mov dword ptr [rbp - 20], ecx
  mov rax, qword ptr [rbp - 40]
  add rax, 4
  mov qword ptr [rbp - 40], rax
  jmp .LBB0_1
.LBB0_4:
  mov eax, dword ptr [rbp - 20]
  pop rbp
  ret

Hmmm…. 30 instructions. So as you can see, this is an illustration of a ‘Zero Cost Abstraction’. The code has been shrunk by quite a few characters. It becomes easier to read and reads with more intent, yet it doesn’t carry any overhead over the original loop.

So, what’s the development cost of writing your own foreach enumerable collection in C#. Implementing the IEnumerable concept.

Well — first you need to implement the IEnumerable interface, which returns an IEnumerator. The IEnumerator class is really the meat and potatoes. It contains your iteration state and a reference to your collection. You need to implement the MoveNext() call to increment the enumerator, which returns true or false based on whether it incremented. Then you need to implement Reset(), which resets the enumerator. You need to implement the ‘Current’ field to return the current object.

Vs.

What’s the development cost of making something enumerable in C++. Implementing the iterator concept.

You need to implement the iterator concept. Iterator concepts are described here, the minimum you need is the ForwardIterator. Which you need to be able to read and increment with multiple passes. The iterator concept models the pointer, so you need to implement operator++() and operator*(), and operator==(). Like the IEnumerator the iterator is the brains of iterator (fancy that), and contains some reference to the collection as well as iteration state.

In order for your collection to work with C++ foreach, you need to implement a begin() and end() method, which return an iterator.

begin() => returns an iterator to the first object
end() => returns an iterator of one past the last object

It’s as easy as that.

The moral of this story, just because an abstraction makes the code cleaner, doesn’t mean you’re not paying for it, someway. Of course, the flip-side is, if there is careful thought put into the abstraction — you can eat your cake and have it too.

“Small leaks sink big ships” – Benjamin Franklin

Happy Coding!

PL

Ye Olde Double Dispatch; The Invasive Visitor Pattern; and Pattern Matching.

As the title suggests we’re exploring The Ol’ Double Dispatch, Visitor Pattern and Pattern Matching. Given this topic can be language agnostic, I’ll choose C++ for examples. We all know it’s the language of languages. Kind of like Latin. It also gives me a good framework to work with because it’s close to the metal, which hopefully makes things less complicated, and way better than JAVA because, well… JAVA. Need I say more?

I’ll get to the back story, and an architecture choice which leads to a pervasive (invasive) design pattern, but first I’ll start with some history. Practitioners of C, would remember the paradigm of separating function from data, they remember this; because they have no other choice. In C, your tools are functions and structs, so that’s really all you got. Along the way some clever individuals invented something called virtual function dispatching. This allows you to have different function implementations based on the different type of data. This is a little something we like to call ‘polymorphism’ in the Computing world, you’ve probably heard of it. In fact, polymorphism is a fundamental pillar of Object Oriented programming. It is also, a form of what is called ‘single dispatch’.  Whereby you make a single function call and it’s dispatched to the proper implementation based on the data type.

By this point, you’re probably saying “You said you’d talk C++, but now you’re talking about C.” Well it’s important because it illustrates my point, and what would we be without C? Some type of cave person punching holes in cards? Give your head a shake. Anyways, the C++ compiler builds a virtual function table, and ensures that the correct function is called based on the type, all with the nice syntax of C++ to write it. So this example will be C++ syntax, to show virtual single dispatch.

// create a virtual base class
class base
{
public:
    virtual ~base() = default;
    virtual void do_something()
    {
        // do something
    }
};

// override the base class
class derived : public base
{
public:
   virtual void do_something() override
   {
       // do something derived
   }
}; 

int main()
{
     base b;
     derived d;
     base &a = b;
     base &c = d;
     
     // call the correct function, from the base, to the derived
     // base do_something
     a.do_something();

     // derived do_somethng
     c.do_something();
}

So Single Dispatch, virtual function table, and data / function segregation.

Now, there’s a second type of Single Dispatch, but it doesn’t exist in C (the latest C has it apparently), it’s called ‘function overloading’. In C++,  it’s done at compile time. Basically, the compiler picks the right function based on the type passed in. You know.. this.

void foo(int i)
{
    // do foo for int
}

void foo(double d)
{
    // do foo for double
}

int main()
{
   double pi{3.1415};
   int i{0};
   foo(i);
   foo(pi);
}

As I mentioned, this form of dispatching is only at compile time. The compiler has to know the type when selecting the proper overload. So something like this, won’t work.

class ibase
{
public:
    virtual ~ibase() = default;
    virtual void do_something() const = 0;
};

class derived1 : public ibase
{
public:
   virtual void do_something() const override
   {
   };
};

class derived2 : public ibase
{
public:
   virtual void do_something() const override
   {
   }
};

void foo(const derived1 &d)
{
    d.do_something();
}

void foo(const derived2 &d)
{
   d.do_something();
}


int main()
{
   // This won't compile, it's for illustration purpose
   derived1 d1;
   derived2 d2;
   ibase &b1 = d1;
   ibase &b2 = d2;
   foo(b1);
   foo(b2);
}

Now, I know what you’re thinking — ‘There’s no reason why you would ever do this. Just use polymorphism to solve this.’ You’re right! For this example.

However, the point is, that if you combine the two examples, you get ‘Double Dispatch’.  Now, if C++ supported this, that would be grand. But it doesn’t, and the problem lies with what’s illustrated above. You can’t use a runtime type to do function overloading.

From this point forward, we’re going to use a image system as an example. In our image system, the image can be of different types, and we want to be able to apply some different action to the image; adjust colour, crop, scale, black & white, etc. If we sketch our example in our fairytale C++ language that supports runtime type function overloading. It would look like this.

struct rgb
{
public:
  unsigned char red, green, blue;
};

class iimage
{
public:
    virtual ~iimage()  = default;
    virtual int height() = 0;
    virtual int width() = 0;
    virtual rgb get_pixel_color(int x, int y) = 0;
    virtual void set_pixel_color(int x, int y, rgb color) = 0;
};

class bmp_image : iimage
{
  // any none shared functions  & data
  // implement shared functions
};

class jpg_image : iimage
{
  // any none shared functions & data
  // implement shared functions
};

class iaction
{
public:
    virtual ~iaction() = default;
    virtual void apply(bmp_image &image) = 0;
    virtual void apply(jpg_image &image) = 0;
};

class adjust_color_action : iaction
{
    virtual void apply(bmp_image &image) override
    {
       // some specific algorithm for bmp images
    }
    
    virtual void apply(jpg_image &image) override
    {
       // some specific algorithm for jpg images
    }
};

void apply_action(iimage &image, iaction &action)
{
    action.apply(image);
};

int main()
{
    iimage &image = generate_image();
    iaction &action = get_user_action();
    apply_action(image, action);
}

So as you can see, if this was to compile, we’ve done two things, we’ve separated our algorithms from our data; actions & images, and given ourselves the ability to apply different algorithms based on the different types of data. We’re using both types of single dispatch at once, a runtime polymorphic virtual function dispatch, when we call ‘apply’. Then a runtime function overload, with the argument passed in. That right there — is Double Dispatch.

Unfortunately for us. That runtime function overload, doesn’t exist in C++. So we need to somehow invert our call, so that the compiler can be sure of the type when it compiles.

Enter Visitor Pattern.

By inverting the dispatch of the action, onto the image base, we can cause the dynamic dispatch to the action, while knowing at compile time the type getting passed in. It looks like this.

struct rgb
{
public:
 unsigned char red, green, blue;
};

class iaction;

class iimage
{
public:
 virtual ~iimage() = default;
 virtual int height() = 0;
 virtual int width() = 0;
 virtual rgb get_pixel_color(int x, int y) = 0;
 virtual void set_pixel_color(int x, int y, rgb color) = 0;
 virtual void apply(iaction &action) = 0;
};

class bmp_image;
class jpg_image;

class iaction 
{ 
public: 
    virtual ~iaction() = default; 
    virtual void visit(bmp_image &image) = 0; 
    virtual void visit(jpg_image &image) = 0; 
};
class bmp_image : iimage
{
    // any none shared functions & data
    // implement shared functions
    virtual void apply(iaction &action) override
    {
         // the *this, tells the compiler, that when you
         // call apply on the bmp_image, it will call the
         // bmp_image overload of the iaction
         action.visit(*this);
    }
};

class jpg_image : iimage
{
    // any none shared functions & data
    // implement shared functions
    virtual void apply(iaction &action) override
    {
        action.visit(*this);
    }
};

class adjust_color_action : iaction
{
  virtual void visit(bmp_image &image) override
  {
    // some specific algorithm for bmp images
  }
 
  virtual void visit(jpg_image &image) override
  {
   // some specific algorithm for jpg images
  }
};

void apply_action(iimage &image, iaction &action)
{
    image.apply(action);
};

int main()
{
   iimage &image = generate_image();
   iaction &action = get_user_action();
   apply_action(image, action);
}

Okay — so as you can see, in this example we’re emulating Double Dispatch, by inverting our calls. Which uses first polymorphism to make the correct call, and then a compile time overloaded function. Voila! Double Dispatch, kind of. This pattern lends itself very nicely when you need to separate your function, from your data. It allows you to easily expand data function, without having to change code in the data. However, additional data becomes a bit of a pain, because you need to change all of your function classes to support the new data class.

Now, I mentioned earlier that there is an architecture that the Visitor Pattern becomes quite pervasive, to a point of being almost invasive. It’s the dreaded ‘Anemic Domain Model’ architecture / pattern! Gasp! What is the ‘Anemic Domain Model’? Well according to Wikipedia, it is “Anemic domain model is the use of a software domain model where the domain objects contain little or no business logic (validations, calculations, business rules etc.).”

[ See Post In Defense of The Anemic Data Model ]

If you’ve architected your system in this way. You might start to see where the visitor pattern starts to become pervasive. Just because you chose to decouple function from data, doesn’t mean you throw OO out the window completely. Right? The issue you face, is that if a component in your data model wants to become a hierarchy, how do you deal with it? You can’t add function to the data. Because that’s the point of your model. You need a pattern where you can apply a dynamic algorithm, to a dynamic piece of data.

Enter Visitor Pattern.

Because RESTful APIs lend themselves nicely to this pattern, let’s reimagine our image processing example as a REST api.

Let’s say we want to PUT to a transform, in our image collection. We want to be able to 1) have multiple transforms, and accept different kinds of images.

Our API is

PUT api/images/transform
{
   "action":"adjust_color",
   "image":"bin_data"
}

Now, imagine we had some kind of mechanism, that would dispatch us the deserialization of our JSON blob to

void on_put_images_tranform(iimage &image, iaction &action);

You could design things differently, where you didn’t need the visitor pattern. Although, I think this is a nice blend of OO, patterns, and separation of concerns. So it nicely illustrates my point.

HOWEVER! The visitor pattern isn’t cheap, you need multiple classes, you need multiple interfaces and it does obfuscate data away from function.

Enter “Pattern Matching”.

What is Pattern Matching, you’re asking? How does it relate to the Visitor Pattern? Pattern matching, in regards to programming languages, is for all intents and purposes a switch statement on steroids.

Your average switch looks like this:

enum states { stopped, running, error };

std::string to_string(states s)
{
    switch(s)
    {
    case states::stopped:
         return "stopped";
    case states::running:
         return "running";
    case states::error:
         return "error";
    default:
         return "unknown";
    } 
};

Imagine this guy, if you could apply that to types.

I need to switch languages here. C++ has a library, called Mach7 which is a pattern matching library, and it allows for functionality similar to what I’m going to describe. I just find that the C# syntax is cleaner and easier to read. As of C# 7 pattern matching, and the ability switch on object type, is a thing. The first time I saw it, I thought ‘Honestly, who would ever use that??’ Apparently, me.

I’m going to reimagine our example, in a hybrid language. Because I want to use the C# pattern matching syntax, but I want to stick with the original C++ example.

Instead of needing to perform the inversion, we can use a runtime detection of type to dispatch to the right call.

Like so

void apply_action(iimage &image, iaction &action)
{
    switch(image)
    { 
    case bmp_image bmp:
         action.apply(bmp);
    case jpeg_image jpg:
         action.apply(jpg);
    default:
       throw unsupported_object_exception();
    }
}

int main() 
{ 
    iimage &image = generate_image(); 
    iaction &action = get_user_action(); 
    apply_action(image, action); 
}

As you can see, we’re very close to the initial Double Dispatch described at the start of the post. We’ve come full circle!

To recap, Double Dispatch is a form of multiple dispatch, allowing us to dispatch polymorphic function on polymorphic data. The Visitor Pattern, is a pattern that allows us to emulate Double Dispatch in languages that have no support for it. The Visitor Pattern is especially good at allowing us to separate function from our data. It becomes very pervasive in a pattern where we keep our data and logic separate (an anti-pattern), but want to have data hierarchies. Pattern Matching is a language construct that allows us use a switch statement on steroids, to switch on object type. Giving us almost Double Dispatch.

Happy Coding!

“The ultimate dream in life is to do what you love, and learn something from it” – Jennifer Love Hewitt

References

Virtual Functions in C [https://www.embedded.com/electronics-blogs/programming-pointers/4391967/Virtual-functions-in-C]

Double Dispatch with an Inverted Visitor Pattern [http://www.drdobbs.com/double-dispatch-with-an-inverted-visitor/184403497]

In Defense of The Anemic Data Model

According to Martin Fowler, who coined the term, this pattern is an ‘Anti-Pattern’, and it’s “contrary to the basic idea of Object Oriented design”. I’m not an Engineer (although I’m sure my parents wish I was), but engineering is about picking the best approach to solve the problem. The fact of the matter is, depending on the problem at hand, sometimes an ‘Anti-Pattern’ fits. This particular one, actually does serve some purpose, and I think the benefits outweigh the costs. So come with me for a moment, as I defend this pattern.

Firstly, a major argument against this architecture is that it “violates encapsulation”. It depends on your definition of the word ‘encapsulation’. If we refer to Wikipedia, we get two different definitions:

  • A language mechanism for restricting direct access to some of the object‘s components.[3][4]
  • A language construct that facilitates the bundling of data with the methods (or other functions) operating on that data.[5][6]

Some see the concept as either or, some see it as both. The reality is I could argue either. In fact, to me (and I borrowed this from Uncle Bob’s Clean Architecture) C has a very good mechanism for 1) bundling data and function 2) hiding direct access to functions of the object. That mechanism is the header / definition file pair. You expose everything you want the client to see via the header, and keep anything private in the compiled definition. Languages like C# and JAVA (shudder), expose ALL the functionality in one file, exposing to the client ALL the implementation details of the class. Isn’t that a violation of ‘encapsulation’? To me, having a class that stores data, and a class that stores function, then having a module that exposes them together. Is in fact, better encapsulation.

Another massive benefit of this pattern, is the ‘Separation of Concerns’. If you’re keeping all logic within a class, say you want to keep business logic, persistence logic, and presentation logic in the same class. You’ve now just created a coupling between your storage, your business logic, and your presentation layer. Trust me when I tell you, I’ve been to this Hell, and it isn’t a picnic. It’s an unmaintainable nightmare, the kind that Freddy himself couldn’t engineer. You can fight to use language features like partial classes to manage the distinction, but it only helps a little. You might argue that the idea is to only keep business logic with the class. Separate all else. Well what happens when the line between presentation and business logic become fuzzy? Well, people start coupling, and down the rabbit hole we go. This is illustrated in this statement by Fowler himself “The logic that should be in a domain object is domain logic – validations, calculations, business rules – whatever you like to call it. (There are cases when you make an argument for putting data source or presentation logic in a domain object, but that’s orthogonal to my view of anemia.)” [1] It isn’t orthogonal, in fact, it is the problem. You might be able to make the argument, but someone else might not. So now, you’ve got someone with a little less experience, and a little less insight, who sees this and replicates it, for the problem they’re solving. Next thing you know, you’re wondering how your UI layer now needs your Database layer to compile. Oh! What about code reviews, and proper discipline. We ALL know how software development works, you’ll spend hours in review, debating why this little piece of persistence logic fits, then why this little piece of UI logic fits. If you follow a clear cut pattern, don’t mix your logic with your data, you don’t have this issue, and it’s easier to catch in a code review, because there isn’t room for debate.

You can implement a very nice service layer, which works on your data model, AND use OO techniques. It is possible.

This pattern, keeping your data separate from your logic, finds massive benefit when you’re working with things like RESTful APIs or really any form of serialization / deserialization to POD data streams. This is due to the fact that serialization of function is difficult at best. Rehydration of complex type hierarchies isn’t child’s play, and doesn’t lend itself nicely to simple interfaces (see ODATA as an example). So you want the objects you pass back and forth to be light and airy. These objects are often referred to as DTO’s or ‘Data Transfer Objects’. In this pattern, you pass them between services, that can persist the object, do logic on the object, or display the object. You might decorate them to add functionality, but at the core, the object stands as data alone.

I’m not saying this ‘Anti-Pattern’ is a silver bullet, because the only silver bullet I believe in is Coors Light. I am however saying, if you’re running into the trap of a coupled nightmare, where one too many people made the argument that presentation and data source logic should be in your model, you might want to see if this architecture helps.

Happy Coding!

“There is nothing either good or bad but thinking makes it so.” – William Shakespeare

The Adventures of Dependency Injection and Inversion of Control.

I like to think my first words were ‘std::cout << “Hello World” << std::endl;’. The canonical C++ Hello World program. But alas, they weren’t. I cut my teeth on QBasic (Quick Beginners All purpose Symbolic Instruction Code, not to be confused with QuickBASIC). I did my time in QBasic, writing ‘viruses’ that would show download progress bars, and print ‘Ha Ha’ infinitely (sorry Dan).

Oh look, I’ve digressed; Back on topic — My first C++ experience was in CMPUT101. That’s when I fell in-love with the language. It was the :: that did it. Also, I always liked the simplicity of the language (Say what?). Anyways, as a young (and full of vigor, I might add) C++ software developer fresh out of school, I often found my code tangled and tightly coupled. Class design in C++ is hard. For me, often my class designs consisted of a lot of value members, and a lot of  concerns fused together.

A value member, is a instance of a class (or POD) as a member variable, of another class. In C++, value members are laid out in memory as they’re declared in the class.

class foo
{
public:
   int A;
   int B;
};

class bar
{
public:
   foo fooA;
   foo fooB;
};

If sizeof(int) == 4, then sizeof(foo) == 8 and sizeof(bar) == 16; Hopefully this makes sense.

[I’m getting there!]

So the thing about value members and call by value in C++, is that there is no polymorphism. You only get polymorphism, or virtual function calls, when you call on a reference or a pointer. Which means, when your class is full of value members, you’re EXTREMELY tightly coupled to the implementation of those members.

Here’s an example to illustrate what I’m talking about.

class file_to_database
{
private:
    file file_;
    database database_;
public:
    file_to_database(const std::string &filename, 
                    const std::string &connect_s)
    {
    }

    void parse()
    {
         std::string line;
         while(file_.read_line(line))
         {
              std::string tablename;
              std::string v1, v2;
              // do some parse work
              database_.insert(tablename, v1, v2);
         }
    }
};

int main(int argc, const char** argv)
{
    file_to_database ftdb(argv[0], argv[1]);
    ftdb.parse();
}

As you can see. We’re very very coupled to both the implementation of the file and database.

When you start delivering code for realz, and have to debug in the field. That’s about the time you remember back to school, and this thing called a ‘Unit Test’. Then it dawns on you, *that might not have just been a ploy to get me to do more homework, but actually useful!!!* So you think to yourself, *Self — I should probably unit test my code*. So you trudge down that road. Hi! Ho! Hi! Ho!

There’s only one problem with that. Everything in your class is a value member. Meaning no polymorphism. Meaning you’re stuck with the implementation you wrote, coupled to a database and all. *Shit.*

I headed on a path of discovery. I was going to solve this problem. How can I make my classes more testable and less coupled to implementation details?

If I boil down my research (and years of experience after). It comes down to 3 rules.

  1. Favour composition over inheritance – inheritance is the tightest coupling you can get. Unless you need it, don’t use it.
  2. Develop to an interface – use well designed interfaces to separate implementation from algorithms.
  3. Use Dependency Injection – dun. dun. dun.

Let’s see where I went wrong.

  1. Check. My classes were generally composed of other types, as value members.
  2. No polymorphism, no interfaces.
  3. What is this even??

Now, at this point I was part of the all mighty C++ development crew!! The language coursed through my veins. Oooh Rah. I had a major hate-on for Enterprise Developers and any type of managed languages. (I still hate JAVA, so if you’re a JAVA developer stop reading right now and go learn a real programming language. Just kidding! 🙂 No but seriously, my language compiles your language.) However, I had heard about this thing called ‘Dependency Injection’ from the Enterprise team at work. I also had read about it on the internet. I took a look at some ‘Dependency Injection’ code examples on the internet. Uhhh. Okay Mr.C-Sharp, where the heck does anything begin, who owns what? Wow. I’m going to stay as far from ‘Dependency Injection’ as I can.

By this time, I had solved my unit testing woes. I started using reference members to pure virtual classes (interfaces). Lastly I started taking all the implementations of the classes in my constructors. This allowed me to write tests against my classes, make sure they functioned correctly using mock implementations. Then in production, supply the real-world implementation. I could now mock out my databases and files, supply those in the constructor and test my classes. This also, really simplified my ownership models. I won. Check and mate, Mr. C-Sharp. No ‘Dependency Injection’ here.

  1. Check.
  2. Check.
  3. Hell no. Who needs that? I’ll control where things are created, destroyed and owned. Thank you very much.

It didn’t matter, because I was now cooking with dynamite, I could unit test! Hizuh! I don’t need your silly ‘Dependency Injection’ where I have no idea who made what, when or who owns anything.

Here’s the example above, re-imagined to suit my testing needs.

class ifile
{
public:
   virtual ~ifile() = default;
   virtual bool read_line(std::string &line)=0;
};

class idatabase
{
public:
   virtual ~idatabase() = default;
   virtual void insert(const std::string &tablename,
                       const std::string &value1,
                       const std::string &value2)=0;
};

class file_to_database
{
private:
     ifile &file_;
     idatabase &database_;
public:
     file_to_database(ifile &file, idatabase &database)
     :file_(file), database_(database)
     {
     }

     void parse()
     {
         std::string line;
         while(file_.read_line(line))
         {
              std::string tablename;
              std::string v1, v2;
              // do some parse work
              database_.insert(tablename, v1, v2);
         }
     }
};
class file : public ifile
{
public:
    file(const std::string &filename)
    {
    }
    virtual bool read_line(std::string &line) override
    {
        // implementation
    }
};

class database : public idatabase
{
public:
   database(const std::string &connect_s)
   {
   }
   virtual void insert(const std::string &tablename,
                       const std::string &value1,
                       const std::string &value2)
   {
        // implement
   }
};

int main(int argc, const char** argv)
{
    file f(argv[0]);
    database(argv[1]);

    file_to_database ftdb(file, database);
    ftdb.parse();
}

As you can see, this refactor takes the parsing code from near impossible to test without a file and a database. To extremely easy to test, by mocking out our inputs (ifile) and our outputs (idatabase). The awesome thing, I didn’t even touch the parsing algorithm. Just a couple smart interfaces, and introduce the dependencies in the constructor. Bingo. Bango.

So — a little while later, I’m attending CPPCON and I’m having a couple wobblies with a friend. He’s a C++ Guru and did some time at a C# shop. We got onto this topic. I tell him “Man, have you heard of this ‘Dependency Injection’ thing? What a nightmare. How do you even know when things are constructed? Who even owns what?”

He pauses. Then he delivers a mind-bottling statement. “Dependency Injection IS NOT Inversion of Control.” I said, “What?!?” He pauses again. “Dependency Injection IS NOT Inversion of Control. Dependency Injection is simple — just make your classes take what they depend on in the constructor…. When you use interfaces it makes it so you can replace the implementation of the dependencies, without modifying the class.” I think I almost fell off my stool (and it wasn’t the amount of wobblies I had). “No. No. No. Dependency Injection is this crazy-ass thing that obfuscates where objects get created and requires that everything on the planet be an interface, even simple things.” I cried.

He smiled, took a sip of his water — “You’re wrong. That’s an Inversion of Control container.” I take a sip of my beer. “Well. What’s an Inversion of Control container?”. He looked at me smugly, “What you just described. That is.”

It was this moment, when I learned the difference between ‘Dependency Injection’ and ‘Inversion of Control’. Dependency Injection is that simple, make your classes take their dependencies in the constructor. If those dependencies happen to be interfaces, you can swap out implementations as needed and your class won’t change. This is sometimes referred to as the ‘Hollywood Principle’ — Don’t call us. We’ll call you.

Inversion of Control, is succeeding control of object creation, and ownership to a framework. It utilizes ‘Dependency Injection’ in order to build up objects in a hierarchical fashion.  In my opinion, these were frameworks created because some developer had nothing better to do with his/her time. Enterprise developers, amirite!?! 😀 But in all seriousness, they really obfuscate ownership and creation. Which is my opinion are fundamental to software design. This is done in the name of modularity, so I understand. However, I’m not a fan. They do have their usage in large applications that need extremely customizable and modular foundations. Although, I’d argue these are few and far between.

Lastly, neither of these are to be confused with ‘Dependency Inversion’. Sometimes, people will confuse ‘Inversion of Control’ and ‘Dependency Injection’ and call it by some ugly step child name of ‘Dependency Inversion’. ‘Inversion of Control’, uses ‘Dependency Inversion’, so does ‘Dependency Injection’. Essentially, ‘Dependency Inversion’ is ensuring that your higher level components, don’t depend on your lower level components. The term though, is generally used to refer to architecture design, at the module level. By making both high and low level modules depend on a shared interface i.e. one caller and one implementer, the direct dependency is now ‘inverted’.

In summary, ‘Dependency Injection’ is the act of taking all class dependencies via the constructor. ‘Inversion of Control’ also called ‘Inversion of Sanity’, is the act of allowing a framework to control all of the important decisions of your program, and you pretending to control it via XML. ‘Dependency Inversion’ is neither of these, but an architectural pattern that helps to decouple modules within a system, by having high and low level components work through interfaces.

Happy Coding!

“Be yourself, because everyone else is already taken.” — Oscar Wilde

 

The Wedding Cake Architecture

First, let me start by stating that I’m not claiming to have invented this. Not even close. I ‘borrowed’ the idea, from “Uncle Bob”. Interestingly enough, I stumbled upon it (kinda), before I ever read about it. Here’s the story.

I had had my doubts about frameworks and ORMs like Entity Framework for some time. What struck me as odd was a lot of the examples show direct usage of the table classes (classes that represent directly the data in the tables) at the API or UI level. Now, I understand the need for this in a blog post or an example for simplicity. What ends up happening though, is this becomes the foundation for reality.  Then once you’ve exposed classes that directly represent your table structure to an API, be it REST or WCF, you’ve unintentionally (or intentionally) coupled your API or UI to the database.  This means that often if you want to make a change to your table structure or persistence medium, you better be willing to shell out the time to fix the broken API clients or UI.

Armed with that thought, I went out to create an architecture that didn’t have this problem. I was dead-set on architecting a solution that ensured decoupling of the database and the clients of the API layer. I was going to develop an API that had a fluent business interface, and ensured that the persistence happened behind the curtains of the API.

The original design principle was dead simple. ‘Keep the persistence out of the API’. So there was a Business Logic Layer (BLL) which exposed a consumable API and housed the business and translation logic. Then a Data Access Layer (DAL) which housed the logic of storing the table classes. Classic separation of concerns. Classic.

OriginalArch

I went about my business of implementation. Then next thing I know!? This architecture worked, it performed, it was unit testable (mostly) and it was easy to understand! HooRay! Kinda.

To exemplify what I mean, here’s a canonical example of a bank transaction.

// Exposed via BLL DLL
public enum EntryTypes { Debit, Credit }

public class Transaction
{
    public Guid Id { get; }
    public Guid Account { get; }
    public DateTimeOffset Time { get; }
    public EntryTypes Type { get; }
    public double Value { get; }
}

public interface ITransactionManager
{
    Transaction Deposit(Guid account, double value);
    Transaction Withdraw(Guid account, double value);
}

public class TransactionManager : ITransactionManager
{
    IPersistence Persistence { get; }

   public Transaction Deposit(Guid account, double value)
   {
       // create a new transaction
       var transaction = new Transaction();
    
       // ...
       // translate the transaction to a transaction entry
    
       var entry = new TransactionEntry();
       // ... copy the fields
    
       Persistence.CreateTransactionEntry(entry);
       return transaction;
    }

    public Transaction Withdraw(Guid account, double value)
    {
       var account = Persistence.ReadAccountEntry(account);
       if( account.Balance < value )
            throw new InsufficientFundsException();
       // create a new transaction
       var transaction = new Transaction();
       // ...
       // translate the transaction to a transaction entry
       var entry = new TransactionEntry();
       // ... copy the fields
       Persistence.CreateTransactionEntry(entry);
       return transaction;
     }
}

// Exposed via DAL DLL
[Table("Transactions")]
public class TransactionEntry
{
    public Guid Id { get; }
    public Guid Account { get; }
    public DateTimeOffset Time { get; }
    public EntryTypes Type { get; }
    public double Value { get; }
}

[Table("Accounts")]
public class AccountEntry
{
    public Guid Id { get; set; }
    public string Owner { get; set; }
    public double Balance { get; set; }
}

public interface IPersistence
{
    void CreateTransactionEntry(TransactionEntry entry);
    AccountEntry ReadAccount(Guid accountId);
}

public Database : IPersistence
{
    public void CreateTransactionEntry(TransactionEntry entry)
    {
        // Code to store the table entry into a database.
    }
    // ...
}

Now, this example is contrived and won’t compile in any language but it illustrates my point. You can see from the client API, there is no mention of persistence. This means that the clients of the API don’t need to deal in classes that directly represent your tables (or know about them). I’ve intentionally let the two classes have identical fields, because this is just the reality for now. It leaves us the ability to change it in the future. If we ever need to change a table data type, add a column or change where the values get stored.

But there’s a problem with this architecture, it’s subtle (at least it was for me, experienced people are probably screaming ‘YOU IDIOT’) but it’s there. The fact that the BLL even knows about the table classes is a problem. There are a few things wrong:

  1. I used Entity Framework in this example. So I’ve basically just enforced my business logic to use Entity Framework. This isn’t really a bad thing, Entity is widely used, supported by Microsoft and easy to use (IMO). However, if you ever wanted to depart from Entity Framework, you’re going to have a bad time.
  2. I’ve coupled the Business Logic to the table columns (yet again). In this example, if you ever wanted to have different storage implementations, they better represent the data the same way across the implementations (or build a translation shim).
  3. I broke the Single Responsibility Principle on my BLL. Translating objects from API objects to database objects isn’t Business Logic so it’s got no ‘business’ being there.

Now, the saving grace of this architecture is that I did one thing right. I kept those details, the persistence details, away from the clients of the API. So even if I had released the API and had clients consuming, I wouldn’t break clients fixing my mistake.

As the architecture began to grow this oversight became more apparent. Which is always the case. I started to realize that things like porting to a different storage medium, or changing ORMs would pose a huge undertaking.

Another problem, since the only downstream interface coming out of the BLL utilized the table classes, all my tests were directly coupled to the table entries. I had a huge problem.

At this point I had already started reading “Clean Architecture” by Robert ‘Uncle Bob’ Martin. I started to understand where I had gone wrong, and those three issues noted above started to become clear, and so did a solution — Clean Architecture. Like I said, I stumbled across this architecture. More like tripped, fell, and face planted right into it.

Uncle Bob talks about a 4 ringed solution. In the centre ring, we have ‘Entities’ these are your ‘Enterprise Business Rules’. In the next ring, you have ‘Use Cases’ these are your ‘Application Business Rules’. Next, ‘Interface Adapters’ are your controllers and presenters. Then finally, ‘Frameworks and Drivers’ the external interfaces. He talks about never letting an inside ring, know about things in an outside ring. Can you see now my blunder?

Okay — so what would the point of this blog post be, if I didn’t have a little perspective on that architecture? Otherwise, I could just say ‘go read the book’.  Disclaimer: I’m not knocking Uncle Bob. I see the value in his architecture, and he’s got many many more years on me.

I think it can be simplified… I gave it a KISS. My interpretation is the ‘Wedding Cake Architecture’ and it looks like this.

WeddingCakeArch

It’s very very very heavily influenced by the Clean Architecture. Except that I’ve shaved out a layer. I’ve also intentionally drawn the layers with differing thicknesses, this is to illustrate the weighted dispersal of the code. You’ll have less code in the Business Models module, than you will down in the everything else module. You personally might not have much code in those layers, but if you imagine the sheer amount of coding behind Entity Framework, or a different ORM, or the ASP.NET Web API, you’ll see why that layer is so thick.

Here’s the example, reimagined:

// Exposed via BLL DLL
public enum EntryTypes { Debit, Credit }

public class Transaction
{
    public Guid Id { get; }
    public Guid Account { get; }
    public DateTimeOffset Time { get; }
    public EntryTypes Type { get; }
    public double Value { get; }
}

public interface ITransactionManager
{
    Transaction Deposit(Guid account, double value);
    Transaction Withdraw(Guid account, double value);
}

public class TransactionManager : ITransactionManager
{
    IPersistence Persistence { get; }

   public Transaction Deposit(Guid account, double value)
   {
       // create a new transaction
       var transaction = new Transaction();
       // setup transaction
       Persistence.CreateTransaction(transaction);
       return transaction;
    }

    public Transaction Withdraw(Guid account, double value)
    {
       var account = Persistence.ReadAccount(account);
       if( account.Balance < value )
            throw new InsufficientFundsException();
       // create a new transaction
       var transaction = new Transaction();
       // ...
       
       Persistence.CreateTransaction(transaction);
       return transaction;
     }
}

// Exposed via DAL DLL
[Table("Transactions")]
internal class TransactionEntry
{
    public Guid Id { get; }
    public Guid Account { get; }
    public DateTimeOffset Time { get; }
    public EntryTypes Type { get; }
    public double Value { get; }
}

[Table("Accounts")]
internal class AccountEntry
{
    public Guid Id { get; set; }
    public string Owner { get; set; }
    public double Balance { get; set; }
}

public interface IPersistence
{
    void CreateTransaction(Transaction entry);
    Account ReadAccount(Guid accountId);
}

public Database : IPersistence
{
    public void CreateTransaction(Transaction entry)
    {
        // translate the transaction to a transaction entry 
        var entry = new TransactionEntry(); 
        // ... copy the fields
        // Code to store the table entry into a database.
    }
    // ...
}

 

Now, the Business Logic Layer has zero knowledge of anything other than it’s own logic. So lets look at the benefits:

  1. Any clients of the API won’t know about the persistence (so I meet my design principle #1).
  2. We haven’t coupled our Business Logic to our persistence. So we have the flexibility to change our storage medium, or have different store implementations.
  3. Our Unit Tests will now be flexible to any of those changes. Not like before where the unit tests were directly coupled to the table classes. Before we had to use them to check proper output from the BLL.

Overall, I learned a lot from this little venture down Architecture Alley. I hope you can learn something from tale of the adventure too.

Happy Coding!

PL