Protocol func returning Self

Question

I have a protocol P that returns a copy of the object:

protocol P {
    func copy() -> Self
}

and a class C that implements P:

Answer 1

I had a similar problem and came up with something that may be useful so I though i'd share it for future reference because this is one of the first places I found when searching for a solution.

As stated above, the problem is the ambiguity of the return type for the copy() function. This can be illustrated very clearly by separating the copy() -> C and copy() -> P functions:

So, assuming you define the protocol and class as follows:

protocol P
{
   func copy() -> P
}

class C:P  
{        
   func doCopy() -> C { return C() }       
   func copy() -> C   { return doCopy() }
   func copy() -> P   { return doCopy() }       
}

This compiles and produces the expected results when the type of the return value is explicit. Any time the compiler has to decide what the return type should be (on its own), it will find the situation ambiguous and fail for all concrete classes that implement the P protocol.

For example:

var aC:C = C()   // aC is of type C
var aP:P = aC    // aP is of type P (contains an instance of C)

var bC:C         // this to test assignment to a C type variable
var bP:P         //     "       "         "      P     "    "

bC = aC.copy()         // OK copy()->C is used

bP = aC.copy()         // Ambiguous. 
                       // compiler could use either functions
bP = (aC as P).copy()  // but this resolves the ambiguity.

bC = aP.copy()         // Fails, obvious type incompatibility
bP = aP.copy()         // OK copy()->P is used

In conclusion, this would work in situations where you're either, not using the base class's copy() function or you always have explicit type context.

I found that using the same function name as the concrete class made for unwieldy code everywhere, so I ended up using a different name for the protocol's copy() function.

The end result is more like:

protocol P
{
   func copyAsP() -> P
}

class C:P  
{
   func copy() -> C 
   { 
      // there usually is a lot more code around here... 
      return C() 
   }
   func copyAsP() -> P { return copy() }       
}

Of course my context and functions are completely different but in spirit of the question, I tried to stay as close to the example given as possible.