The .hpp file extension is commonly used for C++ Header Files, which are essential components in C++ programming projects. Header files typically contain declarations of functions, classes, variables, and constants that are intended to be shared across multiple source (.cpp) files. They do not usually contain the actual implementation (definitions) of these elements, although inline functions and template definitions are often placed entirely within header files. The primary purpose of using header files is to allow the compiler to know the structure and interface of the code before it encounters the actual implementation during the linking phase. This separation of declaration and definition aids in modular programming, reduces compilation times for large projects, and prevents redefinition errors. While the standard convention often uses the .h extension for C-style headers or generic C++ headers, the .hpp extension is frequently adopted by developers and some build systems specifically to denote a C++-only header file, sometimes to avoid potential conflicts or ambiguity when mixing C and C++ codebases. They are plain text files readable by any text editor, but are specifically designed to be processed by the C++ preprocessor and compiler.