If there is considerable logic in an appliation's main function, this can be difficult to unit test. The One Definition Rule prevents us from having the application's main and the unit test executable's main in the same executable. Furthermore, even if we could perform some trick to resolve the multiply defined symbol^[4], we would still need to figure out how to link the test executable with the application's main function to test the code.

When it becomes difficult to test code where it is currently, such as in the body of main, the simplest thing to do is to move the code somewhere else that makes it easy to test. Here is one way to accomplish this:

The delegating main looks like this:

extern int app_main(int argc, char* argv[]);

int main(int argc, char* argv[])
{
    return app_main(argc, argv);
}

If you don't already have a library for app_main, then create one and add all the other application code to the library as well. The build logic for making the executable will consist of compiling a single source file containing your delegating implementation of main and linking against a library containing the rest of the application. This will give you a starting point for writing unit tests for anything in your application.