David Abrahams escribió:
Well, maybe benefits don't outweigh problems... I added that overload so that assignment moves rvalues instead of copying them,
In the first version of the library we had two overloads ("const T &" and "rv<T> &") for copy/move constructor and assignments. But if T& operator=(const T&) is defined then: T produce(); T t; //... //Copy assignment called instead of move-assignment! t = produce(); This is suboptimal for assignments, because a move is replaced with a copy. In theory this also happens for copy-constructors but fortunately RVO is a much better approach and in practice this problem only happens with assignments: //In theory old approach would also call the copy constructor //but nearly all modern compilers use RVO. T t(produce()); That was the state of the library when I first proposed it. Then Klaus Trindle proposed an alternative overload set to catch non-const rvalues using "const rv<T> &" (the additional ovreloads "were rv<T> &" and "T&" --> note that is non-const) and that solves the assignment problem: http://lists.boost.org/Archives/boost/2009/06/153266.php However, this approach requires defining a non-const assignment operator, so we have a new problem: compiler-generated copy assignment takes the argument as non-const T &. So right know I have two options: the old approach that produces suboptimal assignments from non-const rvalues and the newer one that is optimal but hurts the compiler generated copy assignment. We need a third alternative: a new overload set that maintains "const T &" and properly catches non-const rvalues. Best, Ion