Basic Objective-C Pointer Question

Question

#import <Foundation/Foundation.h>

BOOL areIntsDifferent( int thing1, int thing2 ) {
    if (thing1 == thing2) {
        return (NO);
    } else {
        return (YES);
    }
}

NSString *boolString (BOOL yesNo) {
    if (yesNo == NO) {
        return( @"NO" );
    } else {
        return( @"YES" );
    }
}

int main (int argc, const char * argv[]) {
    BOOL areTheyDifferent;

    areTheyDifferent = areIntsDifferent (5,5);

    NSLog(@"are %d and %d different? %@", 5, 5, boolString(areTheyDifferent));

    areTheyDifferent = areIntsDifferent (23,42);

    NSLog(@"are %d and %d different? %@", 23, 42, boolString(areTheyDifferent));

    return (0);
}

This is the entire program. It's not terribly complex but it illuminates a common problem that I keep running into - understanding pointers. In this situation, is the return value of the boolString function a pointer because there is no memory allocated to hold a string? And if so, why isn't the areIntsDifferent return value a pointer to a BOOL value? Could I rewrite this program so that the return value of boolString was a string and not a pointer to a string? If so, HOW? I tried to eliminate the * but then I get a compiler error.

Answer 1

When dealing with instances of Objective-C objects you will only ever be dealing with pointers which reference the instance.

In light of that fact...

Is the return value of the boolString function a pointer because there is no memory allocated to hold a string?

In the case of your boolString function the literals @"YES" and @"NO" actually result in instances of objects which can be returned as pointers to NSString objects. Given the above comment about instances only being usable via pointers, it returns an NSString* because that is what @"YES" and @"NO" are, and is the only way that it CAN return a string.

Why isn't the areIntsDifferent return value a pointer to a BOOL value?

BOOL is a value type and can therefore be returned directly from methods/functions and passed directly as parameters.

Could I rewrite this program so that the return value of boolString was a string and not a pointer to a string?

No, for the reasons already stated regarding Objective-C objects.

Answer 2

NSString is an object. In Obj-C, objects are always passed around by pointer, never by copying their actual memory.

A BOOL is a primitive. Unlike objects, primitives are passed by value.

Part of the reason for this dichotomy is that primitives are small but objects can be very large (like a UIImage), so passing objects by value would waste a lot of memory. Another reason is that passing objects by pointer allows collections to hold objects of a mixture of different types. E.g. an NSArray can hold a UIImage right next to an NSString and an NSNumber. This is easy for Apple to implement because all of the pointers are the same # of bytes in memory. If the objects were dealt with by value rather than pointer, mixing types in a collection would be very difficult because you would have items of different sizes rather than pointers of uniform size.