Hi,
I have done some experiments with the rvalue reference feature on the
cxx0x-branch.
If I execute the following program:
-------------------------
#include <iostream>
struct A {
A () { std::cout << "def ctor" << std::endl; }
A (const A&) { std::cout << "normal copy" << std::endl; }
A (A&&) { std::cout << "rvalue ref copy" << std::endl; }
~A () { std::cout << "dtor" << std::endl; }
};
A f() { return A(); }
int main() { A c = f(); }
------------------------
I get :
-----
def ctor
rvalue ref copy
dtor
rvalue ref copy
dtor
dtor
------
Now if I comment out the "rvalue reference constructor", I get the optimal:
----
def ctor
dtor
----
I am wondering: is this behavior (2 extra copies) required by the
rvalue-reference
specifications, or would gcc be allowed to do better?
Even if the whole point of rvalue-reference copies is that they are supposed to
be
cheap (can we count on them to be fully optimized by the lower level optimizers,
in principle?), it would probably be better to still be allowed to skip the 2
extra
copies completely using the RVO, even for objects defining an rvalue-ref copy
constructor, right?
BTW, http://gcc.gnu.org/projects/cxx0x.html does not yet mention the rvalue
reference patch.
--
Sylvain Pion
INRIA Sophia-Antipolis
Geometrica Project
CGAL, http://cgal.org/
