Behaviour of the #ifndef directive is the same in both C and C++.

Purpose

The #ifndef directive is one of five preprocessor selection statements allowing selection of alternative sections of code for compilation. The other four selection statements are: #ifdef, #if, #elif, and #else.

Format

All preprocessor directives begin with the # symbol. It must be the first character on the line or the first character on the line following optional white space.

Spaces or tabs are permitted between the # and ifndef, but not escape characters or other symbols or macros. The preprocessor removes white space and concatenates the # and ifndef together.

The following are valid uses:

#ifndef my_macro
# ifndef my_macro;
# ifndef my_macro
# /* comments are white space */ ifndef my_macro

The following are invalid uses:

// #\ is not a valid preprocessor directive
# \t ifndef my_macro
// #" is not a valid preprocessor directive
# "" ifndef my_macro

Use

If the macro name does not exist, then the statements following #ifndef until the end of the block (#endif, #else or #elif) are compiled.

If the macro name exists, then the statements following #ifndef until the end of the block (#endif, #else or #elif) are skipped (not compiled).

Since the code is conditionally compiled, it is possible the code never gets compiled and, as a consequence, any errors in it are never caught or noticed.

The simplest preprocessor selection block consists of just an #ifndef and a terminating #endif statement.

More complex selection blocks consist of an #ifndef followed by one or more #elif and / or a final #else statement.

Any valid preprocessor statements, including other #ifndef statements, may be part of the code in the selection block. There is no limit on the level of nesting of selection statements.

The following:

#ifndef my_macro

is equivalent to:

#if !defined my_macro

This directive is most commonly used to prevent multiple inclusion of header files:

#ifndef my_header
#define my_header
.
.
.
// contents of the header file
.
.
.
#endif

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