Question

Consider the following code:

class C {
  public:
    int operator-(int x) {
        return 3-x;
    }
};

class wrapper {
  public:
    operator C() {
        static C z;
        return z;
    }
} wrap;

int main() {
    return wrap-3;
}

it gives this error on g++:

test.cpp: In function ‘int main()’:
test.cpp:17:17: error: no match for ‘operator-’ in ‘wrap - 3’

The conversion operator seems to be working because this version works:

class wrapper {
  public:
    operator int() {
        static int z=3;
        return z--;
    }
} wrap;

int main() {
    return wrap-3;
}

operator- also seems to be working because this code compiles:

class C {
  public:
    int operator-(int x) {
        return 3-x;
    }
};

int main() {
    C c
    return c-3;
}

What’s wrong with the combination of these two? Why can’t an operator be applied after implicit conversion? Are there any workarounds to this problem?

Answer 1

Implicit conversions aren’t performed on the first operand when a member function is matched. Just make your operator a non-member, perhaps a friend:

class C {
};

int operator-(C c, int x) {
    return 3-x;
}

From [over.match.oper]:

— If T1 is a complete class type, the set of member candidates is the result of the qualified lookup of T1::operator@ (13.3.1.1.1); otherwise, the set of member candidates is empty.