c++ - How to mock library calls?

Question

New to cpp (Java guy).

I have 3rd party library that has method sendMail(txt). I don't want to test the library. i want to test my own method, so in order to do this , i need to mock the library calls .

My own method is looking like this:

#include "mailsender.h"

int run(txt){
    analysis(txt);
    ...
    ...
    int status =  sendMail(txt);//sendMail is a 3rd party library call. i need to mock it.its not part of the unit test
    return status;
}

In Java the mailsender was interface and it was injected to my class, so in case of test i inject mock. What is a good practice in cpp to mock library calls? I can wrap the 3rd party library call in a class and inject this class, but i am looking for something simpler and for the common practice (maybe ifndf).

I am familiar with googlemock. googlemock allow me to mock classes . i am not aware to option how to mock a call in my tested method.

Answer 1

So I assume you have a 'global' function that is implemented in a library that you both include a header file for (to get the definition) and link (to get the implementation).

You obviously need to replace the implementation of the library with your own - one that does "nothing", so you can do this in 2 ways:

• you replace the .dll (or .so) with your own implementation that has all the methods the 3rd party library exposes. This is easy once you've written a new version of all the 3rd party lib functions, but writing them all out can be a pain.
• you remove the library temporarily, and replace the calls you make to that in a .cpp source file that implements those functions. So you'd create your own sendMail() function in a .cpp file and include this into the program instead of the mailsender.h include.

The latter is easier, but you might also have to modify your program to not link with the 3rd party lib. This can also require changing the #include as well, as some compilers (eg VC++) allow you to embed linker directives in the source. If your does this, then you won't be able to stop the linker from including the 3rd party lib.

The other option is to modify your code to use a different call to the sendMail call, eg test__sendMail() that you implement yourself. Wrap this is a macro to conditionally include your, or the real, function call depending on your build options.

If this was a c++ library then you'd probably be able to use a mocking framework like you're used to, but it sounds like its a C library, and they simply provide a list of functions that you use directly in your code. You could wrap the library in your own class and use that instead of calling the 3rd party lib functions directly.

There is a list of C mocking frameworks.