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Why is my objective c method giving me "error: object cannot be used as parameter to method",

I have looked at this over and over and I see no problem. This is probably obvious and I am probably an idiot and I apologize in advance for that.

In my interface, I:

@interface PolygonShape: NSObject 
{
    int numberOfSides;
    int minimumNumberOfSides;
    int maximumNumberOfSides;

}

@property int numberOfSides, minimumNumberOfSides, maximumNumberOfSides;

// class methods
+ (float) getAngleInDegrees: (PolygonShape *) polyshape;
+ (float) getAngleInRadians: (PolygonShape *) polyshape;
+ (NSString) getName: (PolygonShape *) polyshape;

// instance methods
- (id) init;
- (id) initWithNumberOfSides: (int) sides minimumNumberOfSides: (int) min 
       maximumNumberOfSides: (int) max;
@end

The part in the implementation that I am getting errors relates to the getName method:

@implentation ...

+ (NSString) getName: (PolygonShape *) polyshape
{
// here is where I get the "error: can not use an object as parameter to a method"
    int sides = [polyshape numberOfSides];
    NSString * s = [NSString init];

    switch (sides) {
        case 3:
            s = "@Triangle";
// there also an "assignment from incompatible pointer type" warning ... but its secondary
            break;
        case 4:
            return "@Square";
            break;

        default:
            break;
    }
}

The thing that turns me on is that the class methods work great:

+ (float)getAngleInDegrees:(PolygonShape *) polyshape;
+ (float)getAngleInRadians:(PolygonShape *) polyshape;
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3 answers


Your getName: method should return (NSString *), not (NSString). I am guessing this is a bug; if yes, then yes, the error message could be more informative.

In fact, in Objective-C you will never see objects passed without them * behind them, not as return values, not as parameters, not as local variables, not as member variables.



By the way, the warning you are talking about is because you have a typo by mixing "@foo" with @ "foo". The latter is an Objectice-C string literal, the former is a C string literal whose first character is @.

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In addition to the other answers you are using [NSString init]

where you should be using [[NSString alloc] init]

. However, this will only contain an empty string, so you would probably be better off initializing s with either @""

or nil

.



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I think the error in this case is a bit misleading. In Objective-C, it is usually not possible to pass an object by value (including return values). In this case, you are specifying the return value as NSString

, not NSString*

. The declaration must be:

+ (NSString*)getName:(PolygonShape *) polyshape

not

+ (NSString)getName:(PolygonShape *) polyshape
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