C2440 error when using typecasting operator overload with Visual Studio 10

Question

I'm hitting a C2440 error:

C:\code>cl test.cpp /EHsc                       

Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01 for 80x86                 
Copyright (C) Microsoft Corporation.  All rights reserved.                                      

test.cpp                                                                                        
test.cpp(18) : error C2440: 'initializing' : cannot convert from 'D<TYPE,SELECTOR>' to 'int'
    with
    [
        TYPE=int,
        SELECTOR=int
    ]
    Ambiguous user-defined-conversion

When I try to compile the following test code with visual studio 10, it seems to be working fine with clang -Weverything and armcc.

Is there a particular reason this isn't working, and is there a workaround aside from "don't do that"?

#include <iostream>

template <typename TYPE, typename SELECTOR = int> struct D  {
    TYPE x;
    D() : x(2) {}
    D(TYPE X) : x(X) {}

    operator const TYPE&() const { return x; }
};
template <typename TYPE> struct D<TYPE, int> : public D<TYPE, char> {
    D() : D<TYPE, char>(3) {}
    operator TYPE&() { return D<TYPE, char>::x; }
};

int main() {

    D<int, int> test3;
    int t3 = test3;

    D<int, char> test2;
    int t2 = test2;

    std::cout << +t3 << " " << +t2 << "\n";  // Expected output "3 2" (which we get from clang)
    return 0;
}

EDIT: I'm aware that the D<TYPE, int> case is providing an lvalue, and D<TYPE, non-int> case is providing only an rvalue. That is quite intentional.

Answer 1

Ambiguous user-defined-conversion

Ambiguous is the keyword.

D<int, int> test3;
int t3 = test3;

Here the compiler doesn't know wether to use const int &D<int, char>::operator const int &() or int &D<int, int>::operator int &().

The problem is that t3 is int and not int& or const int&. If you actually work with references everything works fine (or at least as expected):

D<int, int> test3;
int& t31 = test3; //OK
const int& t32 = test3; //OK

D<int, char> test2;
int& t21 = test2; //Doesn't work
const int& t22 = test2; //OK

The problem is for the compiler to decide wether to use the const version of the base class or the non-const version of the inheriting class.

I would recommend you overload the const version of the operator in the inheriting class, so that the compiler can choose which one to use from this class only:

#include <iostream>

template <typename TYPE, typename SELECTOR = int> struct D {
    TYPE x;
    D() : x(2) {}
    D(TYPE X) : x(X) {}

    operator const TYPE&() const { return x; }
};
template <typename TYPE> struct D<TYPE, int> : public D<TYPE, char> {
    D() : D<TYPE, char>(3) {}

    operator const TYPE&() const { return D<TYPE, char>::x; }
    operator TYPE&() { return D<TYPE, char>::x; }
};

int main() {

    D<int, int> test3;
    int t3 = test3; //OK
    int& t31 = test3; //OK
    const int& t32 = test3; //OK

    D<int, char> test2;
    int t2 = test2; //OK
    //int& t21 = test2; //Doesn't work
    const int& t22 = test2; //OK

    std::cout << +t3 << " " << +t2 << "\n";  // Expected output "3 2" (which we get from clang)
    return 0;
}

---

Other ways to address this problem:

You can now manually cast to int&

D<int, int> test3;
int t3 = (int&)test3;

or manually cast to const int&

D<int, int> test3;
int t3 = (const int&)test3;

or explicitly call the functions

D<int, int> test3;
int t3 = test3.operator int &();

or

D<int, int> test3;
int t3 = test3.operator const int &();

or you could make both function const

template <typename TYPE> struct D<TYPE, int> : public D<TYPE, char> {
    D() : D<TYPE, char>(3) {}
    operator const TYPE&() const { return D<TYPE, char>::x; }
};

Or you could just remove the redifinition of the function (As TYPE doesn't change).

template <typename TYPE, typename SELECTOR = int> struct D {
    TYPE x;
    D() : x(2) {}
    D(TYPE X) : x(X) {}

    operator const TYPE&() const { return x; }
};

template <typename TYPE> struct D<TYPE, int> : public D<TYPE, char> {
    D() : D<TYPE, char>(3) {}
};

Yet I guess the latter 2 variants are not what you want because of the const.

Answer 2

There are two function of operator TYPE&():

public D<int, int>::operator int&() 
private D<int, char>::operator const int&() const

possible to call it explicit:

int t3 = test3.::D<int, int>::operator int&();

concerning gcc is allow to call (if puplic inheritance):

int t3 = test3.::D<int, char>::operator const int&();

but for msvc isn't

gcc allow both variant and seems it is right:

D<int, int> test3;
const D<int, int> test4;

int t3 = test3; // called D<int, int>::operator int&();
int t4 = test4; // called D<int, char>::operator const int&() const;

msvc allow only second variant.

msvc does not considers that test3 is not const and does not select const and non const function.

Seems it is msvc bug.