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 to understand how to use the defined entities in other parts of the program. Header files are included in source code files (typically .cpp files) using the #include directive. This allows the compiler to access the declarations and definitions within the header file. The separation of declarations into header files promotes code reusability and modularity. By including a header file in multiple source files, the same declarations can be used across the entire project. This helps to avoid code duplication and ensures consistency in the program's structure. Header files often contain include guards, which are preprocessor directives that prevent the header file from being included multiple times in the same compilation unit. This is important because including the same header file multiple times can lead to compilation errors.