Mock non-virtual method C++ (gmock)

Question

I have class

class CSumWnd : public CBaseWnd
{

 private:
 bool MethodA()
}

Please can you help how to mock MethodA() without making virtual, I didn't understand the concept of hi-perf dependency injection

Answer 1

It means you will have to templatize your production code. Using your example:

CSumWind class definition:

class CSumWnd : public CBaseWnd
{

 private:
 bool MethodA()
};

Mocked CSumWnd class definition:

class MockCSumWnd : public CBaseWnd
{

 private:
 MOCK_METHOD(MethodA, bool());
};

Production class which have to be tested with mocked class CSumWind. Now it becomes templated to provide using CSumWind class in production code and MockCSumWnd class in tests.

template <class CSumWndClass>
class TestedClass {
//...
   void useSumWnd(const CSumWndClass &a);

private:
  CSumWndClass sumWnd;
};

Instantiation of TestedClass in production:

TestedClass <CSumWnd> obj;

Instantiation of TestedClass object in test executable:

TestedClass <MockCSumWnd> testObj;

Answer 2

If you don't want to change the existing code, here is a specific solution for VC++ I'm working on (https://github.com/mazong1123/injectorpp). The brief steps are:

1. Leverage DbgHelp.h to retrieve all methods' symbols and memory addresses of a class. Basically it retrieves meta info from .pdb file at runtime.
2. Compare the user input to-mock method symbol with step 1's output, get the to-mock method's memory address.
3. Leverage windows api WriteProcessMemory to change the entry bytes of the to-mock method, that is something similar as: __asm {move eax, 1; ret}.

Let's put key code here.

1. Retrieve methods' symbols and addresses of a class. Below is the key idea of the implementation. The full source code is availiable at https://github.com/mazong1123/injectorpp/blob/master/injectorpp/ClassResolver.cpp

   // Retrieve class symbol.
   if (SymGetTypeFromName(this->m_hProcess, modBase, className.c_str(), classSymbol) == FALSE)
   {
       throw;
   }
   
   // Get children of class - which are methods.
   DWORD numChildren = 0;
   if (SymGetTypeInfo(this->m_hProcess, classSymbol->ModBase, classSymbol->TypeIndex, TI_GET_CHILDRENCOUNT, &numChildren) == FALSE)
   {
       throw;
   }
   
   // Get methods info.
   if (SymGetTypeInfo(this->m_hProcess, classSymbol->ModBase, classSymbol->TypeIndex, TI_FINDCHILDREN, methods) == FALSE)
   {
       throw;
   }
   
   // Retrieve all methods.
   for (DWORD i = 0; i < numChildren; ++i)
   {
       ULONG curChild = methods->ChildId[i];
   
       // Resolve function.
       Function resolvedFunction;
       this->m_functionResolver->Resolve(classSymbol->ModBase, curChild, resolvedFunction);
   
       // Add the resolved function to the output.
       resolvedMethods.push_back(resolvedFunction);
   }
   

2. Step 2 is trival. It's only text comparing and processing.

3. How to inject the magic asm to change the method behavior: (The full source code is available at https://github.com/mazong1123/injectorpp/blob/master/injectorpp/BehaviorChanger.cpp)

   // A magic function to change the function behavior at runtime
   //
   // funcAddress - The address of the function to be changed from.
   // expectedReturnValue - The return value should be changed to.
   void BehaviorChanger::ChangeFunctionReturnValue(ULONG64 funcAddress, int expectedReturnValue)
   {
   
   
   // The purpose of this method is to change the return value
   // to what ever int value we expected.
   
   // Therefore, we just need to inject below asm to the header of specific function:
   //
   // mov eax, expectedValue
   // ret
   //
   // Above asm code tells the function to return expectedValue immediately.
   
   // Now let's prepare the asm command.
   byte asmCommand[6];
   
   // mov
   asmCommand[0] = 0xB8;
   
   // The value.
   asmCommand[1] = expectedReturnValue & 0xFF;
   asmCommand[2] = (expectedReturnValue >> 8) & 0xFF;
   asmCommand[3] = (expectedReturnValue >> 16) & 0xFF;
   asmCommand[4] = (expectedReturnValue >> 24) & 0xFF;
   
   // ret
   asmCommand[5] = 0xC3;
   
   WriteProcessMemory((HANDLE)-1, (void*)funcAddress, asmCommand, 6, 0);
   }