Question

The following code is giving me a compilation error. Can anyone please tell me why?

class mytype {
public:
    int value;
    mytype(int a) {
        value = a;
    }
    friend ostream& operator<<(ostream& stream, const mytype& a) {
        stream << a.value;//works
        return stream;
    }
    friend ostringstream& operator<<(ostringstream& stream, const mytype& a) {
        stream << (a.value);//compilation error
        return stream;
    }
};

Error:

error C2027: use of undefined type
‘std::basic_ostringstream<_Elem,_Traits,_Alloc>’

Upon fixing that:

error C2666: ‘operator <<‘ : 18 overloads have similar conversions

Final fix:

Declare constructor as explicit. Works on MSVC then.

I wonder why.

Answer 1

There is an overload ambiguity in the call to operator<< in the ostringstream overload.

stream << a.value;

There are a number of operator<< overloads that are members of the ostream class, which is a base class of ostringstream. One of these is declared as:

ostream& ostream::operator<<(int);

This overload is an exact match for the right-hand side (a.value is an int) but requires a derived-to-base conversion on the left-hand side (stream is an ostringstream, which is derived from ostream).

However, there is also your ostringstream overload:

ostringstream& operator<<(ostringstream&, const mytype&);

This overload is an exact match for the left-hand side (stream is an ostringstream), and a user-defined converting constructor (your mytype(int) constructor) can be used to convert a.value (an int) to a mytype.

Since one overload matches the first argument better and the other overload matches the second argument better, there is an ambiguity. You can fix this either by:

• Explicitly converting the left-hand side to an ostream (using (ostream&)stream = a.value;), or
• Remove the user-defined conversion by making the constructor explicit.