How to Use NSLog to Debug CGRect and CGPoint

December 19, 2013

CGPoint and CGRect are structures (versus objects) and therefore the old NSLog standby %@ will not work as expected.

Here is how each structure is defined:

struct CGPoint {
   CGFloat x;
   CGFloat y;
typedef struct CGPoint CGPoint;
struct CGRect {
   CGPoint origin;
   CGSize size;
typedef struct CGRect CGRect;

If you attempt NSLog to use the traditional ‘print object’ notation such as this:

// Print point structure using NSLog
CGPoint cgPoint = CGPointMake(1, 11);
NSLog(@"%@", cgPoint);

the compiler will generate a warning: Format specifies type ‘id’ but the argument has type ‘CGPoint’ (aka ‘struct CGPoint’)

Good news is, this is easy to fix:

NSLog(@"Point: %@", NSStringFromCGPoint(cgPoint));

The output of converting a CGPoint to an NSString looks as follows:

Point: {1, 11}

Output CGRect using NSLog

Likewise, CGRect is a structure and will also give NSLog troubles. The solution is similar:

// Print rect structure using NSLog
CGRect rect = CGRectMake(5, 5, 10, 10);
NSLog(@"Rect: %@", NSStringFromCGRect(rect));

And the output:

Rect: {{5, 5}, {10, 10}}

CGRect, CFDictionaryRef and NSLog

When working with CGRect structures, there is another solution, you can convert the rect to a CFDictionaryRef and print as shown below:

NSLog(@"CFDictionaryRef: %@", CGRectCreateDictionaryRepresentation(rect));

Which returns:

CFDictionaryRef: {
    Height = 10;
    Width = 10;
    X = 5;
    Y = 5;

If for some reason you need to keep the dictionary object around, here is how you can so if you are using ARC:

# Create reference to immutable CFDictionary
CFDictionaryRef currentListingRef = CGRectCreateDictionaryRepresentation(rect);
# Create Dictionary object managed by ARC
NSDictionary *dict = CFBridgingRelease(currentListingRef);

The code on line 5 moves the CGRef to and Objective-C object, and also hands the memory management over to ARC.