Macro compiler error | 易学教程

问题

I tried making a quick macro for creating and showing a simple "ok" dialog box in iOS:

#define ALERT_DIALOG(title,message) \
do\
{\
    UIAlertView *alert_Dialog = [[UIAlertView alloc] initWithTitle:(title) message:(message) delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];\
    [alert_Dialog show];\
} while ( 0 )

If I try to use it in my code:

ALERT_DIALOG(@"Warning", @"Message");

I get the error:

Parse Issue. Expected']'

And the error seems to be pointing at the second @ right before "Message".

However, if I simply copy paste the macro I do not get this error:

NSString *title = @"Warning";
NSString *message = @"Message";
do
{
    UIAlertView *alert_Dialog = [[UIAlertView alloc] initWithTitle:(title) message:(message) delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];
    [alert_Dialog show];
} while ( 0 );

Is there something against using Objective-c constructs in macros? Or did I just do something rather stupid that I can't find

回答1:

The problem with your macro is that both occurrences of message in

... [[UIAlertView alloc] initWithTitle:(title) message:(message) ...

are replaced by @"Message", resulting in

.... [[UIAlertView alloc] initWithTitle:(@"Warning") @"Message":(@"Message") ...

and that causes the syntax error.

I don't think that it is really worth defining this as a macro, but if you do, you have to use macro arguments which do not occur at places where they should not be expanded, e.g.

#define ALERT_DIALOG(__title__,__message__) \
do\
{\
    UIAlertView *alert_Dialog = [[UIAlertView alloc] initWithTitle:(__title__) message:(__message__) delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];\
    [alert_Dialog show];\
} while ( 0 )

or similar.

回答2:

Instead of declaring it as a macro, you can declare it as a C function instead:

void ALERT_DIALOG(NSString *title, NSString *message) {
    UIAlertView *alert_Dialog = [[UIAlertView alloc] initWithTitle:(title) message:(message) delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];\
    [alert_Dialog show];
}

来源：https://stackoverflow.com/questions/14202963/macro-compiler-error

标签

objective-c

ios

preprocessor-directive