Question

I’ve tried to use id to create duck typing in objective-c. The concept looks fine in theory but failed in practice. I was unable to use any parameters in my methods. The methods were called but parameters were wrong. I was getting BAD_ACESS for objects and random values for primitives. I’ve attached a simple example below.

The question:
Does any one knows why the methods parameters are wrong?
What is happening under the hood of the objective-c?

Note: I’m interest in the details. I know how to make the example below work.

An example:
I’ve created a simple class Test that is passed to an other class using property id test.

@implementation Test
- (void) aSampleMethodWithFloat:(float) f andInt: (int) i {
    NSLog(@"Parameters: %f, %i\n", f, i);
}
@end

Then in the class the following loop is executed:

for (int i=0; i < 10; ++i) {
    float f=i*0.1f;
    [tst aSampleMethodWithFloat:f andInt:i]; // warning no method found.
}

Here is the output that I’m getting. As you can see the method was called but the parameters were wrong.

Parameters: 0.000000, 0
Parameters: -0.000000, 1069128089
Parameters: -0.000000, 1070176665
Parameters: 2.000000, 1070805811
Parameters: -0.000000, 1071225241
Parameters: 0.000000, 1071644672
Parameters: 2.000000, 1071854387
Parameters: 36893488147419103232.000000, 1072064102
Parameters: -0.000000, 1072273817
Parameters: -36893488147419103232.000000, 1072483532

Update:

I’ve found out by accident that when I add a declaration of aSampleMethodWith... to the class with for loop the warning disappears and the method on the Test class is called correctly.

Update 2:
As pointed out by JeremyP the direct cause of the problem is that the floats are treated as doubles. But anyone knows why? (following the 5why principle 🙂 ).

According to @eman the call is translated to simple C function call and compiler directive to get the SEL. So the @selector gets confused. But why? The compiler have all necessary type informations in the first method call. Does any one knows a good source of information about the Objective-C internals I’ve search The Objective-C Programming Language but i didn’t find the answer.

Answer 1

By default floating point values are passed as doubles, not floats. The compiler does not know, at the point where [tst aSampleMethodWithFloat:f andInt:i]; occurs that it is only supposed to pass a float, so it promotes f to a double. This means that, in the method, when the compiler does know it is dealing with a float, f is the float formed by the first four bytes of the double passed to the method and i is an int formed from the second four bytes of the double passed.

You can fix this by either

• changing the first parameter of aSampleMethodWithFloat:andInt: to a double
• importing the interface declaration of Test into the file where you use it.

NB there is no gain except a small amount of space when using floats in C. You might as well use doubles everywhere.