checking protocol types

[mongoose23]

Is there a way to check whether an object implements a certain protocol?

You can't seem to pass object references around by "protocol" in Obj-C, the way you can in Java.

That is, suppose I have a protocol:

@protocol TestProtocol

- (void)method1;
@end

I can't just go and create object points like:

TestProtocol *test;

can I? (I would like to do something like this)..

What's the best way to do multiple inheritance in ObjC?

[Kalimba]

You can "type" the id type to create a type that the compiler will understand implements your protocol, like this:

@protocol TestProtocol
- (void)method1;
@end

...

id<TestProtocol> anInstanceThatImplementsTestProtocol = ...;

You can use this same technique to type method parameters (the compiler will enforce this with type checking):

- (void) myMethod:(id<TestProtocol>)param
{
    // variables passed as 'param' must implement TestProtocol
}

In general, you can take any id type object, and ask if it implements a protocol:

id someUnTypedObject = ...;

if ( [someUnTypedObject conformsToProtocol:@protocol(TestProtocol)] )
{
    // yes it does, can call protocol methods on it
    [someUnTypedObject someProtocolMethod];
}

Finally, since a protocol can have @optional methods, you can check if an instance implements an optional method:

id someUnTypedObject = ...;

if ( [someUnTypedObject conformsToProtocol:@protocol(TestProtocol)] )
{
    // it conforms to the protocol
    if ( [someUnTypedObject respondsToSelector:@selector(someOptionalMethod)] )
    {
        // safe to send 'someOptionalMethod'
        [someUnTypedObject someOptionalMethod];
    }
}