Why does adding and subtracting two #define'd macros not work on iPhone / Objective-c

In my iPhone app, I have this in my .h file

#define ORIENTATION_IS_PORTRAIT UIDeviceOrientationIsPortrait([[UIDevice currentDevice] orientation])
#define FULLSCREEN_开发者_高级运维MODE         ORIENTATION_IS_PORTRAIT?460:300
#define AD_HEIGHT               ORIENTATION_IS_PORTRAIT?50:32
#define FULLSCREEN_MODE_WITH_AD FULLSCREEN_MODE - AD_HEIGHT //doesn't work

To sum it up, I have a macro to detect the orientation. I have another for the height of my UIWebView when it is in full screen mode (activated by a button press). I need to keep the ad at the bottom. However, to get the height of the UIWebView minus the ad height will only return the AD_HEIGHT.

For addition, (FULLSCREEN_MODE + AD_HEIGHT) returns the FULLSCREEN_MODE only.

Also, when using constants, where exactly do I put them. I searched and could only find answers like "in the .m file", but where??

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What you want is to have it like this:

#define FULLSCREEN_MODE         (ORIENTATION_IS_PORTRAIT?460:300)
#define AD_HEIGHT               (ORIENTATION_IS_PORTRAIT?50:32)
#define FULLSCREEN_MODE_WITH_AD (FULLSCREEN_MODE - AD_HEIGHT)

For more complex expressions, I'd suggest you wrap that in do { ... } while(0) instead of just grouping them together.

What's happening here is it's being parsed as:

UIDeviceOrientationIsPortrait([[UIDevice currentDevice] orientation]) ? 460 : 300 - UIDeviceOrientationIsPortrait([[UIDevice currentDevice] orientation]) ? 50 : 32

which is not the math you want.

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Surround every macro with () - it's a good habit and will save you some unexpected behavior.

example:

#define FULLSCREEN_MODE         (ORIENTATION_IS_PORTRAIT?460:300)

The preprocessor does a direct replacement of one piece of text for another. If you have order of operations issues, the parentheses will save you.

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Alternately, avoid the preprocessor and it's confusion altogether, and use inline functions.

inline BOOL orientationIsPortait() {
    return UIDeviceOrientationIsPortrait([[UIDevice currentDevice] orientation]);
}


inline int fullscreenMode() {
    return orientationIsPortait() ? 460 : 300;
}

... etc

These would go outside of your @implementation block.

For that matter, I'd bet you don't run these function but once in a great while, so thinking about optimizations like macros or inlining is highly premature, and probably never important.

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Avoid macros. Define functions. You can declare them inline.