Re: C++0x: unfortunate interaction of rvalue references and concepts

SG <>
Mon, 8 Dec 2008 16:49:34 CST
On 8 Dez., 04:12, "Joe Smith" <> wrote:

"SG" wrote:

[...] rvalue references are allowed to bind to lvalues. [...]

Really? I was under the impression that a T or a T& (with T not a reference
type) will never bind to a 'T&&'. As I understand it a non-template function
with an argument of type T&& should only be called on temporaries or if a T
has been converted to an rvalue by way of std::move (or some other

Yes. The way it is currently defined is that rvalue references also
bind to lvalues UNLESS there's a more attractive overload. Move
semantics relies on overload resolution to pick the right function.

The motivation behind it was probably to reduce the need for
overloading when:
(*) you don't intent to copy/move objects
(*) but still want to be able to refer/alter to temporary objects.


   class A {
     int data;
     A() : data(0) {}
     void setdata(int x) { data = x; }
     void backup(ostream && os) const { os << data; }

   int main() {
     A foo;

This is clearly a trade-off. If you made && only bind to rvalues you
could safe some function declarations a la

    void push_back(T const&) = delete; // T movable but not copyable.

but would require additional overloads for examples like 'A::backup'
to be able to accept both, lvalue and rvalue arguments.


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