testing - 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.