Each C++ source file needs to be compiled into an object file. The object files resulting from the compilation of multiple source files are then linked into an executable, a shared library, or a static library (the last of these being just an archive of object files). C++ source files generally have the .cpp, .cxx or .cc extension suffixes.

The first step that the compiler will do on a source file is run the preprocessor on it. Only source files are passed to the compiler (to preprocess and compile it). Header files aren’t passed to the compiler. Instead, they are included from source files.

For each C++ source file, the preprocessor will build a translation unit by inserting content in it when it finds an #include directive at the same time that it’ll be stripping code out of the source file and of the headers when it finds conditional compilation blocks whose directive evaluates to false. It’ll also do some other tasks like macro replacements.

Once the preprocessor finishes creating that (sometimes huge) translation unit, the compiler starts the compilation phase and produces the object file.

Source

1. https://www.toptal.com/c-plus-plus/c-plus-plus-understanding-compilation