Pass by value or reference, to a C++ constructor that needs to store a copy?

Question

Should a C++ (implicit or explicit) value constructor accept its parameter(s) by value or reference-to-const, when it needs to store a copy of the argument(s) in its object

Answer 1

In C++98 and C++03, you should pass const& bar and then copy. In C++0x, you should pass bar and then do a move (provided bar has a move constructor).

#include 

struct foo
{
    bar _b;

    foo(bar b) : _b(std::move(b)) {}
};

If you construct foo with an lvalue parameter, the copy constructor will be called to create a copy b, and that copy will be moved into _b. If you construct foo with an rvalue parameter, bar's move constructor will be called to move into b, and then it will be moved again into _b.