more than one instance of function template matches the argument list

Question

#include <iostream>
using std::cout;
using std::endl;

#include <string>
using std::string;

template <class T>
T max(T const& t1, T const& t2) {
    return t1 < t2 ? t2 : t1;
}

int main() {
    cout << max(1, 2) << endl;
    cout << max<double>(1.2, 2) << endl;
    string s1 = "hello";
    string s2 = "world";
 -->   cout << max(s1, s1) << endl;
}

On the line with arrow, it complains about: "more than one instance of function template "max" matches the argument list: -- function template "const _Tp &std::__1::max(const _Tp &__a, const _Tp &__b)"-- function template "T max(const T &t1, const T &t2)" -- argument types are: (std::__1::string, std::__1::string)"

I am confused since both of them are string, and not sure what else the template can match of.

Collision with std::max? What happens if you change the name? — Dave S

Guillaume Racicot Guillaume Racicot · Accepted Answer · 2020-01-12T03:52:12

The max function is in conflict with the std::max function found via ADL. To fix that, you can replace the call by a qualified lookup:

cout << ::max(s1, s1) << endl;

You can also surround the function name with parenthesis to disable ADL:

cout << (max)(s1, s1) << endl;

Integers have no namespaces so they are not subject to ADL, hence why you haven't seen the problem with those.

more than one instance of function template matches the argument list

1 Answers