doctest
Fastest feature-rich C++11/14/17/20 single-header testing framework
...It brings the ability of compiled languages such as D / Rust / Nim to have tests written directly in the production code thanks to a fast, transparent and flexible test runner with a clean interface. The framework is and will stay free but needs your support to sustain its development. There are lots of new features and maintenance to do. There are many C++ testing frameworks - Catch, Boost.Test, UnitTest++, cpputest, googletest and others. The key differences between it and other testing frameworks are that it is light and unintrusive. ...