fake/mock nonvirtual C++ methods

Question

It known that in C++ mocking/faking nonvirtual methods for testing is hard. For example, cookbook of googlemock has two suggestion - both mean to modify original source code in some way (templating and rewriting as interface).

It appear this is very bad problem for C++ code. How can be done best if you can't modify original code that needs to be faked/mocked? Duplicating whole code/class (with it whole base class hierarchy??)

Answer 1

One way that we sometimes use is to split the original .cpp file into at least two parts.

Then the test apparatus can supply its own implementations; effectively using the linker to do the dirty work for us.

This is called the "Link Seam" in some circles.

Answer 2

I followed the Link Seam link from sdg's answer. There I read about different types of seams, but I was most impressed by Preprocessing Seams. This made me think about exploiting further the preprocessor. It turned out that it is possible to mock any external dependency without actually changing the calling code.

To do this, you have to compile the calling source file with a substitute dependency definition. Here is an example how to do it.

dependency.h

#ifndef DEPENDENCY_H
#define DEPENDENCY_H

class Dependency
{
public:
    //...
    int foo();
    //...
};

#endif // DEPENDENCY_H

caller.cpp

#include "dependency.h"

int bar(Dependency& dependency)
{
    return dependency.foo() * 2;
}

test.cpp

#include <assert.h>

// block original definition
#define DEPENDENCY_H

// substitute definition
class Dependency
{
public:
    int foo() { return 21; }
};

// include code under test
#include "caller.cpp"

// the test
void test_bar()
{
    Dependency mockDependency;

    int r = bar(mockDependency);

    assert(r == 42);
}

Notice that the mock does not need to implement complete Dependency, just the minimum (used by caller.cpp) so the test can compile and execute. This way you can mock non-virtual, static, global functions or almost any dependency without changing the productive code. Another reason I like this approach is that everything related to the test is in one place. You don't have to tweak compiler and linker configurations here and there.

I have applied this technique successfully on a real world project with big fat dependencies. I have described it in more detail in Include mock.

Answer 3

Code has to be written to be testable, by whatever test techniques you use. If you want to test using mocks, that means some form of dependency injection.

Non-virtual calls with no dependence on a template parameter pose the same problem as final and static methods in Java[*] - the code under test has explicitly said, "I want to call this code, not some unknown bit of code that's dependent in some way on an argument". You, the tester, want it to call different code under test from what it normally calls. If you can't change the code under test then you, the tester, will lose that argument. You might as well ask how to introduce a test version of line 4 of a 10-line function without changing the code under test.

If the class to be mocked is in a different TU from the class under test, you can write a mock with the same name as the original and link that instead. Whether you can generate that mock using your mocking framework in the normal way, I'm not so sure.

If you like, I suppose it's a "very bad problem for C++" that it's possible to write code that's hard to test. It shares this "problem" with a great number of other languages...

[*] My Java knowledge is quite low-power. There may be some clever way of mocking such methods in Java, which aren't applicable to C++. If so, please disregard them in order to see the analogy ;-)

Answer 4

I think it is not possible to do it with standard C++ right now (but lets hope that a powerful compile-time reflection will come to C++ soon...). However, there are a number of options for doing so.

You might have a look at Injector++. It is Windows only right now, but plans to add support for Linux & Mac.

Another option is CppFreeMock, which seems to work with GCC, but has no recent activities.

HippoMocks also provide such ability, but only for free functions. It doesn't support it for class member functions.

I'm not completely sure, but it seems that all the above achieve this with overwriting the target function at runtime so that it jumps to the faked function.

The there is C-Mock, which is an extension to Google Mock allowing you to mock non-virtual functions by redefining them, and relying on the fact that original functions are in dynamic libraries. It is limited to GNU/Linux platform.

Finally, you might also try PowerFake (for which, I'm the author) as introduced here.

It is not a mocking framework (currently) and it provides the possibility for replacing production functions with test ones. I hope to be able to integrate it to one or more mocking frameworks; if not, it'll become one.

Update: It has an integration with FakeIt.

Update 2: Added support for Google Mock

It also overrides the original function during linking (so, it won't work if a function is called in the same translation unit in which it is defined), but uses a different trick than C-Mock as it uses GNU ld's --wrap option. It also needs some changes to your build system for tests, but doesn't affect the main code in any way (except if you are forced to put a function in a separate .cpp file); but support for easily integrating it into CMake projects is provided.

But, it is currently limited to GCC/GNU ld (works also with MinGW).

Update: It supports GCC & Clang compilers, and GNU ld & LLVM lld linkers (or any compatible linker).

Answer 5

@zaharpopov you can use Typemock IsolatorPP to create mocks of non-virtual class and methods without changing your code (or legacy code). for example if you have a non-virtual class called MyClass:

class MyClass
{
 public:
   int GetResult() { return -1; }
}

you can mock it with typemock like so:

MyClass* fakeMyClass = FAKE<MyClass>();
WHEN_CALLED(fakeMyClass->GetResult()).Return(10);

By the way the classes or methods that you want to test can also be private as typemock can mock them too, for example:

class MyClass
{
private:
   int PrivateMethod() { return -1; }
}


MyClass* myClass =  new MyClass();

PRIVATE_WHEN_CALLED(myClass, PrivateMethod).Return(1);

for more information go here.

Answer 6

You very specifically say "if you can't modify original code", which the techniques you mention in your question (and all the other current "answers") do.

Without changing that source, you can still generally (for common OSes/tools) preload an object that defines its own version of the function(s) you wish to intercept. They can even call the original functions afterwards. I provided an example of doing this in (my) question Good Linux TCP/IP monitoring tools that don't need root access?.

Answer 7

That is easier then you think. Just pass the constructed object to the constructor of the class you are testing. In the class store the reference to that object. Then it is easy to use mock classes.

EDIT :

The object that you are passing to the constructor needs an interface, and that class store just the reference to the interface.

---

struct Abase
{
  virtual ~Abase(){}
  virtual void foo() = 0;
};

struct Aimp : public Abase
{
  virtual ~Aimp(){}
  virtual void foo(){/*do stuff*/}
};

struct B
{
  B( Aimp &objA ) : obja( objA )
  {
  }

  void boo()
  { 
    objA.foo();
  }

  Aimp &obja;
};


int main()
{
//...
Aimp realObjA;
B objB( realObjA );
// ...
}

In the test, you can pass the mock object easy.

Answer 8

I used to create an interface for the parts I needed to mock. Then I simply created a stub class that derived from this interface and passed this instance to my classes under test. Yes, it is a lot of hard work, but I found it worth it for some circumstances.

Oh, by interface I mean a struct with only pure virtual methods. Nothing else!