This appendix describes the C++ compiler pragmas. A pragma is a compiler directive that allows you to provide additional information to the compiler. This information can change compilation details that are not otherwise under your control. For example, the pack pragma affects the layout of data within a structure. Compiler pragmas are also called directives.
The preprocessor keyword pragma is part of the C++ standard, but the form, content, and meaning of pragmas is different for every compiler. No pragmas are defined by the C++ standard.
Note - Code that depends on pragmas is not portable.