C++ includes with and without .h [duplicate]

Answer 1

is the C++ standard library string header file containing std::string and its friends. is a different header, from the C standard library, which has functions for manipulating C strings (null-terminated strings) and other related functions.

The two are entirely different and unrelated. In C++ (as in C), a header file can have any extension. The C++ standard library headers have no extension; the C standard library headers have a .h extension. .hpp or .hxx are also common.