Why doesn't make_pair<string, string>() call the copy contructor, when given const string&? [duplicate]

Question

Both GCC and Clang refuse to compile this one:

#include <string>
#include <utility>

using namespace std;

int main() {
    const string s = "12345";
    const string& r = s;

    auto p = std::make_pair<string, string>(r, r);
}

GCC says:

error: cannot bind rvalue reference of type ‘std::__cxx11::basic_string<char>&&’ to lvalue of type ‘const std::string’ {aka ‘const std::__cxx11::basic_string<char>’}

While Clang says:

error: no matching function for call to 'make_pair'

Since I'm giving make_pair explicit types, why doesn't it construct new strings from const string&?

---

This one compiles:

auto p = std::make_pair<string, string>(string(r), string(r));

Answer 1

Assuming C++11 or later:

std::make_pair is not supposed to be used with explicitly-specified template arguments. They are intended to be deduced from the function arguments via forwarding references. The signature of std::make_pair is

template<class T1, class T2>
constexpr std::pair<V1, V2> make_pair(T1&& t, T2&& u);

This shows that T1 and T2 are used as forwarding references and therefore shouldn't be specified explicitly. (V1/V2 are computed from T1/T2 by decay.)

Explicitly specifying the template arguments breaks the forwarding behavior. With string as template argument you get string&& in the function parameter, which is a rvalue reference that doesn't accept lvalues. You would need to provide const string& as template argument for T to make T&& also const string&.

But don't do that and just write

auto p = std::make_pair(r, r);

as is intended usage.

---

Before C++11 there were no forwarding references and std::make_pair looked like this:

template <class T1, class T2>
std::pair<T1, T2> make_pair(T1 t, T2 u);

So the code from your question would compile prior to C++11. Still, specifying the template arguments was redundant then as well.