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Official Forum for Programming in Objective-C (the iPhone Programming Language) - Stephen Kochan
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Chapter 12 - The Preprocessor
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P252
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Topic: P252 (Read 1393 times)
bamboo
Newbie
Posts: 14
P252
«
on:
April 17, 2011, 11:29:21 AM »
"As an example of how this can be used, the following lines appear in the Foundation header file NSString.h:
#if MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_5
#define NSMaximumStringLength (INT_MAX-1)
#endif
This tests the value of the defined variable MAC_OS_X_VERSION_MIN_REQUIRED against the defined variable MAC_OS_X_VERSION_10_5. If the former is less than the latter, the #define that follows is processed; otherwise, it is skipped. "
I am reading Chapter 12 The Preprocessor today, and this sentence confuses me.
from the name MAC_OS_X_VERSION_MIN_REQUIRED, it seems like it is defined name using the #define statement, but since a defined name is not a variable, as it's mentioned previously.
can someone tell me what kind data type it is?
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skochan
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Posts: 3114
Re: P252
«
Reply #1 on:
September 09, 2011, 03:03:20 PM »
They are defined names and their values get substituted and then compared by the #if, which can evaluate numeric expressions. So there's a double processing (in a sense) of that line that's done by the preprocessor: first substitute the text defined for the two name, then evaluate the expression.
Cheers,
Steve
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