What type of file contains pre-made code that declares certain functions in C++?

A header file in C++ is a type of file that typically contains declarations of functions, classes, variables, and other constructs that can be shared across multiple source files. By including a header file in a source file using the #include directive, the compiler gains access to the declarations contained within that header, allowing the programmer to use the declared functions or classes without having to rewrite their definitions in every source file.

Header files are essential for code organization, as they help manage dependencies in a modular way. They usually have the extension .h or .hpp and are a key part of the C++ programming model, especially when working with large applications where multiple source files are compiled together.

The other options include source files, which contain the actual implementation of functions; executable files, which are the compiled result of C++ code that can be run on a machine; and library files, which might contain compiled code that can be linked to other programs at compile time or run time. While library files can contain a collection of functions, header files specifically serve the purpose of declaring functions and facilitating the interaction between different source files.