When you compile source files, the included header files are also compiled. If a header file is included in more than one source file, it is recompiled when each source file is compiled. Also, you might include header files that introduce many lines of code, but the primary source files that include them are relatively small. Therefore, it is often desirable to avoid recompiling a set of header files by precompiling them. These are referred to as PCH files.

By default, when the compiler creates a PCH file, it:

The compiler can precompile header files automatically, or enable you to control the precompilation.