Question

I am doing an exercise to learn how to use selectors in Objective-C.
In this code I am trying to compare two strings:

int main (int argc, const char * argv[])
{
    @autoreleasepool
    {
        SEL selector= @selector(caseInsensitiveCompare:);
        NSString* str1=@"hello";
        NSString* str2=@"hello";
        id result=[str1 performSelector: selector withObject: str2];
        NSLog(@"%d",[result boolValue]);
    }
    return 0;
}

But it prints zero.Why?

Edit:
And if I change str2 to @”hell” I get an EXC_BAD_ACCESS.

Answer 1

The documentation for performSelector: states “For methods that return anything other than an object, use NSInvocation”. Since caseInsensitiveCompare: returns an NSInteger instead of an object you will need to create an NSInvocation, which is more involved.

NSInteger returnVal;
SEL selector= @selector(caseInsensitiveCompare:);
NSString* str1=@"hello";
NSString* str2=@"hello";

NSMethodSignature *sig = [NSString instanceMethodSignatureForSelector:selector];
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:sig];
[invocation setTarget:str1];
[invocation setSelector:selector];
[invocation setArgument:&str2 atIndex:2]; //Index 0 and 1 are for self and _cmd
[invocation invoke];//Call the selector
[invocation getReturnValue:&returnVal];

NSLog(@"%ld", returnVal);