An HXX file is a C++ header file. Header files in C++ are used to declare functions, classes, variables, and other programming constructs that are intended to be used in multiple source code files. They act as interfaces, providing the necessary information for the compiler to understand how to use the declared entities. The HXX extension is less common than the more standard .h extension, but it serves the same purpose. When a C++ source file (typically with a .cpp extension) includes an HXX file using the #include directive, the compiler incorporates the declarations from the header file into the source file. This allows the source file to use the declared functions, classes, and variables without having to redefine them. Using header files promotes code reusability, modularity, and maintainability in C++ projects. They help to organize code into logical units and prevent code duplication. The HXX extension might be used to distinguish certain types of header files or to enforce specific coding conventions within a project.