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Asked: 2026-06-05T15:24:28+00:00

When using a statement like #if TRUE , what should I expect to happen?

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When using a statement like #if TRUE, what should I expect to happen? An explanation would be very appreciated! I understand how #if 1 works, but it gives a completely different result in my code than using #if TRUE. I understand #if is a conditional directive, and what that implies; it’s just the TRUE (or FALSE) part that I don’t understand. It seems as though using it in this way never executes code following the statement. Here is an example:

#if TRUE
     cout << "true" << endl;
#endif

#if FALSE
     cout << "false" << endl;
#endif

I never seem to see "true" or "false" printed to screen and using Visual Studio, the inner statement is automatically grayed out.

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1 Answer

  1. Editorial Team

    The preprocessor will include/exclude the contents of an #if #endif block depending on wether the expression after #if evaluates to true or false.

    #if TRUE will only evaluate to true if

    • the macro TRUE is defined
    • the value of TRUE != 0

    In your example neither TRUE nor FALSE are defined so both blocks are false and excluded.

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