doctest stands out with its ability to write tests in a new and easier way, unlocking the potential for more thorough, up-to-date, and uniform testing.

ReSharper C++: Better Ways to Test C++ with doctest

by Viktor Kirilov

From the article:

What truly sets doctest apart is the ability to use it alongside your production code. This is made possible because of 5 main properties:

• lightning speed of compilation - see the benchmarks
• ability to remove everything testing-related from binaries for final production builds by defining a global preprocessor identifier
• easy integration with the main() function of applications - supporting 3 scenarios: only tests, only the program, and both
• ability to write tests across multiple shared objects and have them registered in a single place for uniform reporting
• transparency - no warnings or other artifacts

Going a step further – doctest can even be used as a general-purpose assert library within the production code itself - outside of a testing context!

Here is a direct question/answer thread of a user interested in mixing production code and tests.