### Precompiled Header Files

A precompiled header is an image of the compiler’s symbol table. Create a precompiled header file for commonly included files. You can also use a precompiled header file to temporarily change header files that do not normally change otherwise (for example, OS ABI headers or standard ANSI library header files). Then replace the original header files with the precompiled header file to significantly improve compile time.

A precompiled header cannot do any of the following:

- Define non-inline functions
- Define global data
- Instantiate template data
- Instantiate non-inline functions

You must include precompiled headers before defining or declaring other objects. You can only use one precompiled header file in a translation unit.

See also “precompile_target”.