An HXX file is a C++ header file. Header files in C++ (and 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 an interface, 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. Using .hxx can sometimes be a convention to distinguish between different types of header files or to indicate a specific purpose within a project. The compiler includes the contents of the header file into the source code file during the compilation process, allowing the program to access the declared elements. Header files promote code reusability and modularity, making it easier to manage and maintain large projects. They are crucial for creating well-structured and organized C++ programs.