Precompiled Header - Rationale

Rationale

In the C and C++ programming languages, a header file is a file whose text may be automatically included in another source file by the C preprocessor by the use of a preprocessor directive in the source file.

Header files can sometimes contain very large amounts of source code (for instance, the header files windows.h and Cocoa/Cocoa.h on Microsoft Windows and Mac OS X, respectively). This is especially true with the advent of large "header" libraries that make extensive use of templates, like the Eigen math library and Boost C++ libraries. They are written almost entirely as header files that the user #includes, rather than being linked at runtime. Thus, each time the user compiles their program, the user is essentially recompiling numerous header libraries as well that would ordinarily have been precompiled into shared objects or dynamic link libraries.

To reduce compilation times, some compilers allow header files to be compiled into a form that is faster for the compiler to process. This intermediate form is known as a precompiled header, and is commonly held in a file named with the extension .pch or similar, such as .gch under the GNU Compiler Collection.

A related feature is the prefix header, which is a file that is automatically included by the compiler without requiring the use of any compiler directives. Prefix headers are commonly precompiled, but not all precompiled headers are prefix headers.