B.2.124.6 Screening a Header File for Problems

What makes a header file precompilable? A header file is precompilable when it is interpreted consistently across different source files. Specifically, when it contains only complete declarations. That is, a declaration in any one file must stand alone as a valid declaration. Incomplete type declarations, such as struct S;, are valid declarations. The complete type declaration can appear in some other file. Consider these example header files:

file a.h
struct S {
#include "x.h" /* not allowed */
};

file b.h
struct T; // ok, complete declaration
struct S {
   int i;
[end of file, continued in another file] /* not allowed*/

A header file that is incorporated into a precompiled-header file must not violate the following. The results of compiling a program that violates any of these constraints is undefined.

• The header file must not use __DATE__ and __TIME__.

• The header file must not contain #pragma hdrstop.