CODEWITHSUNDEEP
c++c++11operator-overloadingstdambiguous
Vincent
Vincent

Reputation: 60491

Ambiguous template binary operators?

Consider the following code

namespace std {
    struct type1 {/* Definition */};
    template <class T> constexpr T operator+(const type1& x1, const T& x);
    template <class T> constexpr T operator+(const T& x, const type1& x1);
}

namespace other {
    struct type2 {/* Definition */};
    template <class T> constexpr T operator+(const type2& x2, const T& x);
    template <class T> constexpr T operator+(const T& x, const type2& x2);
}

int main(int argc, char** argv) 
{
    std::type1 var1;
    other::type2 var2;
    auto x = var1 + var2; // What will happen here?
}

Upvotes: 3

Views: 204

Answers (3)

Dimitrios Bouzas
Dimitrios Bouzas

Reputation: 42939

According depended name look up (a.k.a Koenig Lookup) rules which are well defined. The compiler will lookup function names in the namespaces of the operator+ arguments in addition to the scopes and namespaces considered by the usual unqualified name lookup.

Thus, the compiler is going to add to the overload set both the matching overloaded operators (i.e., from namespaces std and other).

Consequently, you'll get an ambiguous call error.

Upvotes: 4

bashrc
bashrc

Reputation: 4835

You should get an ambiguous operator error on conforming compilers. Your program is ill formed since both versions of overload qualify as candidates.

Upvotes: 1

M.M
M.M

Reputation: 141633

This code causes undefined behaviour. It is not permitted to add your own things to namespace std;, other than explicit specializations of existing templates.

Upvotes: -1

Related Questions