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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?
259
std::make_pairCreates a std::pair object, deducing the target type from the types of arguments.
We pass by const reference to avoid making a copy of the object. When you pass a const reference, you pass a pointer (references are pointers with extra sugar to make them taste less bitter).
The make_pair() function, which comes under the Standard Template Library of C++, is mainly used to construct a pair object with two elements. In other words, it is a function that creates a value pair without writing the types explicitly.
The important difference is that when passing by const reference, no new object is created. In the function body, the parameter is effectively an alias for the object passed in. Because the reference is a const reference the function body cannot directly change the value of that object.
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.
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