Question

Imagine I have two protocols:

@protocol A
@end

and

@protocol B <A> // Protocol B conforms to protocol A.
@end

And also two variables:

id<A> myVar = nil;

and

id<B> otherVar = //correctly initialized to some class that conforms to <B>;

Then, why can’t I assign ‘otherVar’ to ‘myVar’?

myVar = otherVar; //Warning, sending id<B> to parameter of incompatible type id<A>

Thanks!

Answer 1

Is the protocol’s (B) declaration (not just its forward declaration) visible? And does the declaration precede myVar = otherVar;?

When the declaration order is correct, clang did not complain.

To illustrate:

@protocol A
@end

@protocol B; // << forward B

void fn() {
    id<A> myVar = nil;
    id<B> otherVar = nil;
    myVar = otherVar; // << warning
}

// declaration follows use, or is not visible:    
@protocol B <A>
@end

whereas the properly ordered version produces no warning:

@protocol A
@end

@protocol B <A>
@end

void fn() {
    id<A> myVar = nil;
    id<B> otherVar = nil;
    myVar = otherVar;
}