Unit Testing
Related: C++ Unit Testing Functions
A goal of testing is to achieve complete code coverage, meaning all code is executed at least once. Minimally for a class, that means every public function is called at least once. Of course, the programmer of a class knows about a class' implementation and thus will want to also ensure that every private helper function is called, and that every line of code within every function is executed at least once, which may require multiple calls with different input values for a function with branches.
Children