objective-c++: is it possible to define a c++ class with a method which returns objective-c classes and uses covariant returns?

**Edit: this only happens with llvm; gcc supports this just fine.

Consider the following.

Objective-c classes A and B.

B开发者_如何学JAVA is a subclass of A.

We want a c++ hiearchy that looks like:

class X {
  //...
  public:
    virtual A* getFoo();
};
class Y : public X {
  //...
  public:
    B* getFoo();
};

However, if you do this, you'll get an error, as the Objective-c types confuse the c++ compiler:

error: virtual function 'getFoo' has a different return type ('Y *') than the function it overrides (which has return type 'X *')

I'm wondering if anyone has a workaround for this? (Obviously, long term, we'll be moving away from Objective-c classes, but that's not today).

P.S. This seems like the most similar question I could find, but I'm pretty sure it's a different problem.

This compiles and runs fine for me:

#import <Cocoa/Cocoa.h>

@interface A : NSObject

- (NSString*) bar;

@end

@implementation A

- (NSString*) bar
{
    return @"";
}

@end

@interface B : A
@end

@implementation B

- (NSString*) bar
{
    return @"!!!";
}

@end

class X {
  //...
  public:
    virtual A* getFoo() = 0;
};

class Y : public X {
  //...
  public:
    virtual B* getFoo() { return [B new]; }
};


int main (int argc, char const *argv[])
{
    X* x = new Y;
    NSLog(@"%@", [x->getFoo() bar]);  // >> !!!

    return 0;
}

Maybe your problem was that you didn't import B's header file into the file defining Y? You can't get covariance (in c++, at least) on incomplete classes, as the compiler needs to know that B inherits from A in order to compile Y.

Anyway, to answer your question, looks like it is possible to do this.