A C++ source code file ('.cxx') contains human-readable source code written in the C++ programming language. These files are the building blocks of C++ applications and libraries. They contain declarations of classes, functions, variables, and other program elements, along with the implementation details that define their behavior. Before a C++ program can be executed, the '.cxx' file must be compiled by a C++ compiler (such as GCC, Clang, or Microsoft Visual C++ Compiler) into object code. The object code is then linked with other object files and libraries to create an executable program or a library. C++ is a powerful, general-purpose programming language widely used for developing operating systems, game engines, high-performance applications, and embedded systems. The '.cxx' extension is one of several common extensions used for C++ source files; others include '.cpp', '.cc', and '.C'. The specific extension used is often a matter of convention or project-specific requirements.