Mocking non-virtual methods in C++ without editing production code?

Question

Hello,

I am a fairly new software developer currently working adding unit tests to an existing C++ project that started years ago. Due to a non-technical reason, I'm not allowed to modify any existing code. The base class of all my modules has a bunch of methods for Setting/Getting data and communicating with other modules.

Since I just want to unit testing each individual module, I want to be able to use canned values for all my inter-module communication methods. I.e. for a method Ping() which checks if another module is active, I want to have it return true or false based on what kind of test I'm doing. I've been looking into Google Test and Google Mock, and it does support mocking non-virtual methods. However the approach described (http://code.google.com/p/googlemock/wiki/CookBook#Mocking_Nonvirtual_Methods) requires me to "templatize" the original methods to take in either real or mock objects. I can't go and templatize my methods in the base class due to the requirement mentioned earlier, so I need some other way of mocking these virtual methods

Basically, the methods I want to mock are in some base class, the modules I want to unit test and create mocks of are derived classes of that base class. There are intermediate modules in between my base Module class and the modules that I want to test.

I would appreciate any advise!

Thanks,

JW

EDIT: A more concrete examples

My base class is lets say rootModule, the module I want to test is leafModule. There is an intermediate module which inherits from rootModule, leafModule inherits from this intermediate module.

In my leafModule, I want to test the doStuff() method, which calls the non virtual GetStatus(moduleName) defined in the rootModule class. I need to somehow make GetStatus() to return a chosen canned value. Mocking is new to me, so is using mock objects even the right approach?

Answer 1

I would write a Perl/Ruby/Python script to read in the original source tree and write out a mocked source tree in a different directory. You don't have to fully parse C++ in order to replace a function definition.

Answer 2

One approach would be to specify different sources for testing. Say your production target uses rootModule.h and rootModule.cpp. Use different sources for your testing target. You can specify a different header by changing your include path, so that #include "rootModule.h" actually loads unittest/rootModule.h. Then mock rootModule to your heart's content.

Answer 3

There are some different ways of replacing non-virtual functions. One is to re-declare them and compile a new test executable for each different set of non-virtual functions you'd like to test. That's hardly scaleable.

A second option is to make them virtual for test. Most compilers allow you to define something on the command-line so compile your code with -DTEST_VIRTUAL=virtual or -DTEST_VIRTUAL to make them either virtual or normal depending on whether or not it's under test or not.

A third option which may be usable is to use a mocking framework that lets you mock non-virtual functions. I'm the author of HippoMocks (disclaimer with regard to neutrality and so on) and we've recently added the ability to mock plain C functions on X86 platforms. This can be extended to non-virtual member functions with a bit of work and would be what you're looking for. Keep in mind that, if your compiler can see both the use and the definition of a function at one time that it may inline it and that the mocking may fail. That holds in particular for functions that are defined in headers.

If regular C function mocking is sufficient for you, you can use it as it is now.

Answer 4

I suggest you take a look at the book Working Effectively with Legacy Code.

The book contains numerous advice and techniques (patterns) on how to break dependencies and make legacy code more suitable for unit test.