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I have three questions surrounding what I think is the topic of structs in obj-c

1) Why is it that I often (or always) cannot take a member var that is a CGPoint or a CGRect and set the values one by one? I find I have to do:

CGPoint point;
point.x = someValue;
point.y = someOtherValue;
obj.myPoint = point;

instead of simply obj.myPoint.x = someValue etc.

2) Is this behavior that is consistent across all structs in obj-c?

3) Is there an easy way to add two CGPoints? It seems like there should already be, but I couldn't find one. I thought it'd be cumbersome if I'd have to use a temporary CGPoint to accumulate values between two CGPoints before setting the dest var to the temp var (because of not being able to just do pointA.x += pointB.x (same for y).

Joey
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2 Answers2

11

1) From @sb in an answer to Cocoa Objective-c Property C structure assign fails

That won't accomplish anything, because [t member] returns a struct, which is an "r-value", ie. a value that's only valid for use on the right-hand side of an assignment. It has a value, but it's meaningless to try to change that value.

Basically you just have to live with the fact that you can't set the fields of struct directly when returned from a function.

2) Yes

3) Unfortunately I don't think there is a built-in convenience method for adding two CGPoint. If you find your self doing this frequently you can make your own:

CGPoint CGPointAdd(CGPoint p1, CGPoint p2)
{
    return CGPointMake(p1.x + p2.x, p1.y + p2.y);
}

and then use it like:

obj.pointA = CGPointAdd(obj.pointA, pointB);

not as elegant as obj.pointA.X += ... but sometimes life isn't fair.

Community
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Robert Höglund
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0

This is perfectly normal. The 'obj' owns that var and has getters and setters, you can not modify parts of that variable.

The best thing to do is copy the struct and modify whatever you need.

Also note you can use the CGPointMake(x, y) function (and the same for all CG structs), which is much easier.

To update this is easiest:

CGPoint point = obj.myPoint;
point.x += 10.0f;
obj.myPoint = point;

Obj-C 2.0 hides the getters and setters which looks like this:

CGPoint point = [obj myPoint];
point.x += 10.0f;
[obj setMyPoint:point];
v01d
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