Conversation so far:
what is the use of include in c++
#include is a way of including a standard or user-defined file in the program and is mostly written at the beginning of any C/C++ program.
what kind of c/c++ files can use the #include directive
One type of file that can be included using #include are Header File or Standard files.
what does the header file or standard files do
Header File or Standard files contain C/C++ function declarations and macro definitions to be shared between several source files.

Question: what kind of function declarations
Self-contained rewrite: what kind of function declarations are contained in header file or standard files