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Compiler Warning (level 4) C4668

'symbol' is not defined as a preprocessor macro, replacing with '0' for 'directives'

A symbol that was not defined was used with a preprocessor directive. The symbol will evaluate to false. To define a symbol, you can use either the #define directive or /D compiler option.

This warning is off by default. See Compiler Warnings That Are Off by Default for more information.

The following sample generates C4668:

// C4668.cpp
// compile with: /W4
#include <stdio.h>

#pragma warning (default : 4668)   // turn warning on

int main() 
{
    #if q   // C4668, q is not defined
        printf_s("defined");
    #else
        printf_s("undefined");
    #endif
}
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