Objective-C uses dynamic binding, but how

dynamic-bindingobjective c

I know that Objective-C uses dynamic binding for all method calls. How is this implemented? Does objective-c "turn into C code" before compilation and just use (void*) pointers for everything?

Best Answer

Conceptually, what is going on is that there is a dispatcher library (commonly referred to as the Objective C runtime), and the compiler converts something like this:

[myObject myMethodWithArg:a andArg:b ];

into

//Not exactly correct, but close enough for this
objc_msgSend(myObject, "myMethodWithArg:andArg:", a, b);

And then the runtime deals with all the binding and dispatch, finds an appropriate function, and calls it with those args. Simplistically you can think of it sort of like a hash lookup; of course it is much more complicated that then in reality.

There are a lot more issues related to things like method signatures (C does not encode types so the runtime needs to deal with it).

Related Solutions

Objective-c – Best way to define private methods for a class in Objective-C

There isn't, as others have already said, such a thing as a private method in Objective-C. However, starting in Objective-C 2.0 (meaning Mac OS X Leopard, iPhone OS 2.0, and later) you can create a category with an empty name (i.e. @interface MyClass ()) called Class Extension. What's unique about a class extension is that the method implementations must go in the same @implementation MyClass as the public methods. So I structure my classes like this:

In the .h file:

@interface MyClass {
    // My Instance Variables
}

- (void)myPublicMethod;

@end

And in the .m file:

@interface MyClass()

- (void)myPrivateMethod;

@end

@implementation MyClass

- (void)myPublicMethod {
    // Implementation goes here
}

- (void)myPrivateMethod {
    // Implementation goes here
}

@end

I think the greatest advantage of this approach is that it allows you to group your method implementations by functionality, not by the (sometimes arbitrary) public/private distinction.

Objective-c – Shortcuts in Objective-C to concatenate NSStrings

An option:

[NSString stringWithFormat:@"%@/%@/%@", one, two, three];

Another option:

I'm guessing you're not happy with multiple appends (a+b+c+d), in which case you could do:

NSLog(@"%@", [Util append:one, @" ", two, nil]); // "one two"
NSLog(@"%@", [Util append:three, @"/", two, @"/", one, nil]); // three/two/one

using something like

+ (NSString *) append:(id) first, ...
{
    NSString * result = @"";
    id eachArg;
    va_list alist;
    if(first)
    {
        result = [result stringByAppendingString:first];
        va_start(alist, first);
        while (eachArg = va_arg(alist, id)) 
        result = [result stringByAppendingString:eachArg];
        va_end(alist);
    }
    return result;
}