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[edit] Definition

#ifndef <what to check>

<if block>

[#else

<else block>]

#endif

Checks if what to check was previously defined in the code. If it isn't, the code in if block will be compiled. If it is, the code in else block will be compiled or when there is no else block, the compiler will continue after #endif.

It is the opposite of #ifdef.

Precompiler statements

#define • #ifdef • #ifndef • #endif • #else • #if