Objective-C accessing float getters with variable names

Question

Let's say I have an NSArray called myArray of NSStrings (@"a0",@"a1",@"a2")

Then in a fast enumeration I loop into my array to build properties according to that NSStrings. I've got a problem accessing that properties.

I'm trying something like that :

@property (nonatomic) float a0propertyLow;
@property (nonatomic) float a0propertyHigh;
@property (nonatomic) float a1propertyLow;
@property (nonatomic) float a1propertyHigh;
..
.. etc.


for (NSString *aPos in myArray) {
    NSString *low = [NSString stringWithFormat:@"%@propertyLow",aPos];
    NSString *high = [NSString stringWithFormat:@"%@propertyHigh",aPos];
    SEL lowSel = NSSelectorFromString(low);
    SEL highSel = NSSelectorFromString(high);
    if ([self respondsToSelector:lowSel]&&[self respondsToSelector:highSel]) {
        id sumPartOne = [self performSelector:lowSel];
        id sumPartTwo = [self performSelector:highSel];
        float bla = (float)sumPartOne + (float)sumPartTwo;
    }
}

I know my code is wrong but I don't know how to make it work. My problem is that lowSel and highSel are getters which returns float but the perform selector method returns id which is ok for an object but not for floats.

So, how can I access my float getters with variable names ? I'm sure answer must be simple but it seems that my mind is looking for something complicated (and which obviously doesn't work) so I'm asking for help :)

Thank you very much for your help

Answer 1

You can't use performSelector: to call a method that returns a scalar value. The documentation for performSelector: clearly says what you have to do:

For methods that return anything other than an object, use NSInvocation.

An NSInvocation is a little more complex to set up but more flexible regarding arguments and return types.

In your case, it is probably easier to use Key-Value Coding instead:

[self valueForKey:low];

takes the return type into account and will automatically wrap the float in an NSNumber.

Answer 2

If you really need to use these getter methods, you can change your properties to double and use objc_msgSend_fpret():

#include <objc/runtime.h>
#include <objc/message.h>

double arg0 = objc_msgSend_fpret(self, lowSel);

If you can avoid getters (I know, that's not good practice, but anyway, it works for sure with the following method), and use the instance variables directly:

void *object_getIvarPtr(id obj, const char *name)
{
    if (!obj || !name)
    {
        return NULL;
    }

    Ivar ivar = object_getInstanceVariable(obj, name, NULL);

    if (!ivar)
    {
        return NULL;
    }

    return ((char *)obj + ivar_getOffset(ivar));
}

float arg0 = *(float *)object_getIvarPtr(self, [lowSel UTF8String]);

Hope this helps.