What does the `#include` directive do in a C++ program?

The #include directive in a C++ program serves the purpose of including the content from another file or library into the current program. This is especially useful for bringing in standard libraries, user-defined libraries, or header files that contain declarations and definitions necessary for the program to function correctly.

When you use #include, it essentially copies the content of the specified file and pastes it into the location of the directive. This allows the program to utilize functions, classes, and variables defined in that file without having to rewrite them in the program. For example, including the <iostream> header allows you to use input-output functionality like std::cout and std::cin.

While defining variables, initializing the main function, or declaring data types are important concepts in C++, they are not what the #include directive does. Instead, the directive focuses on providing access to additional code and resources, enabling better modular programming and code reuse.