Question

Referring to this web site http://www.cplusplus.com/reference/std/utility/make_pair/

The std::make_pair has this signature (and possible implementation):

template <class T1,class T2>
pair<T1,T2> make_pair (T1 x, T2 y)
{
    return ( pair<T1,T2>(x,y) );
}

I am wondering why std::make_pair has input parameter by value and not by const references?

Is there any particular reason for this?

Answer 1

It originally was taking the parameters by const reference, but that introduced some unexpected problems. It was changed to pass by value after a defect report:

http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#181

It is expected that the compiler will inline the function and be able to optimize away the parameter passing most of the time.