Question

I have a NSMutableDictionary holding EXIF metadata from a picture.
An example:

const CFStringRef kCGImagePropertyExifExposureTime;

Instead of accessing every key individually, I just want write the whole dictionary content into a label.
When I want to write this data into the console I would just use:

NSLog(@"EXIF Dic Properties: %@",EXIFDictionary );

That works fine, but if I use:

NSString *EXIFString = [NSString stringWithFormat:(@"EXIF Properties: %@", EXIFDictionary)];

I get warnings that the result is not a string literally and if I try to use that string to set my label.text, the program crashes.

Any idea where my error is?

Answer 1

[NSString stringWithFormat:(@"EXIF Properties: %@", EXIFDictionary)] is not, as you may think, a method with two arguments. It’s a method with one argument. That one argument is (@"EXIF Properties: %@", EXIFDictionary), which uses the comma operator and ends up returning EXIFDictionary. So in essence you have

[NSString stringWithFormat:EXIFDictionary]

which is obviously wrong. This is also why you’re getting a warning. That warning tells you that the format argument is not a string literal, because using variables as format strings is a common source of bugs. But more importantly here, that argument isn’t even a string at all, and so it crashes.

Remove the parentheses and everything will be fine. That will look like

[NSString stringWithFormat:@"EXIF Properties: %@", EXIFDictionary];