Sams Teach Yourself C++ in 24 Hours, Third Edition

The Preprocessor and the Compiler

Every time you run your compiler, your preprocessor runs first. The preprocessor looks for preprocessor instructions, each of which begins with a pound symbol (#). The effect of each of these instructions is a change to the text of the source code. The result is a new source code file: a temporary file that you normally don't see, but that you can instruct the compiler to save for later examination.

The compiler does not use your original source code file; it reads the output of the preprocessor and compiles that file. #include instructs the preprocessor to find the file whose name follows the #include directive and to write it into the intermediate file at that location. It is as if you had typed that entire ...