A .hh file is a header file commonly used in C++ programming. Header files contain declarations of functions, classes, variables, and other programming constructs. They serve as an interface between different parts of a C++ program, allowing code in one file to access and use code defined in another. The primary purpose of a header file is to provide the compiler with the necessary information about the structure and functionality of code defined elsewhere, without requiring the compiler to process the entire source code. This separation of interface and implementation promotes modularity, code reuse, and faster compilation times. Header files are typically included in source code files using the #include directive. They often contain preprocessor directives, such as #ifndef, #define, and #endif, to prevent multiple inclusions of the same header file, which can lead to compilation errors. While .h is also a common extension for C++ header files, .hh is often used to distinguish C++ header files from C header files, especially in larger projects.