#undef. McGraw-Hill The Complete Reference Fourth Edition

246

C + + : T h e C o m p l e t e R e f e r e n c e

If macro-name has been previously defined in a #define statement, the block of code will be compiled.

The general form of #ifndef is

#ifndef macro-name

statement sequence

#endif

If macro-name is currently undefined by a #define statement, the block of code is compiled.

Both #ifdef and #ifndef may use an #else or #elif statement. For example,

#include <stdio.h>

#define TED 10 int main(void)

{

#ifdef TED printf("Hi Ted\n");

#else printf("Hi anyone\n");

#endif

#ifndef RALPH printf("RALPH not defined\n");

#endif

} return 0; will print Hi Ted and RALPH not defined. However, if TED were not defined, Hi

anyone

would be displayed, followed by RALPH not defined.

You may nest #ifdefs and #ifndefs to at least eight levels in Standard C. Standard

C++ suggests that at least 256 levels of nesting be supported.

#undef

The #undef directive removes a previously defined definition of the macro name that follows it. That is, it "undefines" a macro. The general form for #undef is

#undef macro-name