Why does defined(X) not work in a preprocessor definition without a space?
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by Devin
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Published on 2010-03-19T21:20:48Z
Indexed on
2010/03/19
21:31 UTC
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A preprocessor definition that includes defined(X)
will never evaluate to true, but (defined X)
will. This occurs in MSVC9; I have not tested other preprocessors. A simple example:
#define FEATURE0 1
#define FEATURE1 0
#define FEATURE2 1
#define FEATURE3 (FEATURE0 && !FEATURE1 && (defined(FEATURE2)))
#define FEATURE4 (FEATURE0 && !FEATURE1 && (defined FEATURE2))
#define FEATURE5 (FEATURE0 && !FEATURE1 && (defined (FEATURE2)))
#if FEATURE3
#pragma message("FEATURE3 Enabled")
#elif (FEATURE0 && !FEATURE1 && (defined(FEATURE2)))
#pragma message("FEATURE3 Enabled (Fallback)")
#endif
#if FEATURE4
#pragma message("FEATURE4 Enabled")
#elif (FEATURE0 && !FEATURE1 && (defined FEATURE2))
#pragma message("FEATURE4 Enabled (Fallback)")
#endif
#if FEATURE5
#pragma message("FEATURE5 Enabled")
#elif (FEATURE0 && !FEATURE1 && (defined (FEATURE2)))
#pragma message("FEATURE5 Enabled (Fallback)")
#endif
The output from the compiler is:
1>FEATURE3 Enabled (Fallback)
1>FEATURE4 Enabled
1>FEATURE5 Enabled
Working cases: defined (X)
, defined( X )
, and defined X
.
Broken case: defined(X)
Why is defined
evaluated differently when part of a definition, as in the #if
cases in the example, compared to direct evaluation, as in the #elif
cases in the example?
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