How to give a warning when format specifiers are passed wrongly to a method

Question

Suppose i have a method that returns BOOL like :

+(BOOL) isNewCar {
    //returns YES or NO
}

and then I use this method in NSLog as :

NSLog("Is my car new, the answer is %@", [MyUtil isNewCar]);

Then i get compiler warning as :

If i ignore the warning and run, the app crashes. Now I have another method called printCar which accepts variable arguments just like NSLog whose implementation is :

 -(void) printCar:(NSString *)msg, ... {
     NSString * contents;
     va_list args;
     va_start(args, msg);
     contents = [[NSString alloc] initWithFormat:msg arguments:args];
     va_end(args);
     [self writeToFile:contents];
}

When i use the above method as :

[self printCar:@"The car is mine or not : %@", [MyUtil isNewCar]];

the compiler doesn't generate a warning. Also, the app crashes.

Question : Is there a way to generate a warning if wrong format specifiers are given to our custom method just as NSLog does??

Thanks in advance!

Answer 1

Yes , you need NS_FORMAT_FUNCTION macro to do that , if you press Command and then click on NSLog method , you can also see it

void NSLog(NSString *format, ...) NS_FORMAT_FUNCTION(1,2)

so your method

-(void) printCar:(NSString *)msg, ... 

becomes

-(void) printCar:(NSString *)msg, ... NS_FORMAT_FUNCTION(1,2)

you can also Command Click NS_FORMAT_FUNCTION to see more details about it