A .hpp file is a header file used in C++ programming. It typically contains declarations of functions, classes, structures, enumerations, and other programming constructs. These declarations provide the compiler with the necessary information about the interface of these elements, allowing them to be used in other source code files. Header files are included in source code files using the #include directive. The primary purpose of a header file is to promote code reusability and modularity by separating the interface of a component from its implementation. This allows developers to modify the implementation without affecting the code that uses the interface, as long as the interface remains unchanged. Header files often contain include guards (#ifndef, #define, #endif) to prevent multiple inclusions of the same header file, which can lead to compilation errors. They are essential for organizing and managing large C++ projects.