Summary

Both classical polymorphism and generic programming deal with the essential problem of parameterizing the behavior of an algorithm: for example, writing a search function that works with any arbitrary matching operation.

Classical polymorphism tackles that problem by specifying an abstract base class with a closed set of virtual member functions, and writing polymorphic functions that accept pointers or references to instances of concrete classes inheriting from that base class.

Generic programming tackles the same problem by specifying a concept with a closed set of requirements, and instantiating function templates with concrete classes modeling that concept.

Classical polymorphism has trouble with higher-level parameterizations (for example, manipulating function objects of any signature) and with relationships between types (for example, manipulating the elements of an arbitrary container). Therefore, the Standard Template Library uses a great deal of template-based generic programming, and hardly any classical polymorphism.

When you use generic programming, it will help if you keep in mind the conceptual requirements of your types, or even write them down explicitly; but as of C++17, the compiler cannot directly help you check those requirements.