Re: Rvalue-references, binding and conversions

SG <>
Fri, 5 Dec 2008 20:34:25 CST
On 5 Dez., 02:18, SG <> wrote:

I appreciate the existence of rvalue references but IMHO their use
should be limited to exactly what they were designed for: move
semantics and perfect forwarding. While testing with concepts and
rvalue references I came across this problem:


Here's an update. The problem is actually more subtle:

    #include <iostream>

    struct S {};

    concept C<typename T> {}
    concept_map C<S> {}

    auto concept A<typename T> {}

    template<C T> void foo(const T & x) { // #1
       std::cout << "foo(const T &)\n";

    template<C T> void foo(T && x) { // #2
       std::cout << "foo(T &&)\n";

    template<A T> void bar(const T & x) { // #3
       std::cout << "bar(const T &)\n";

    template<A T> void bar(T && x) { // #4
       std::cout << "bar(T &&)\n";

    int main() {
       S lvl;
       foo(lvl); // #1 with T=S where C<S>, !C<S&>
       foo(S()); // #2 with T=S where C<S>
       bar(lvl); // #4 with T=S& where A<S&>
       bar(S()); // #4 with T=S where A<S>

Tested wit conceptg++ svn 727. So, function #4 attracts all non-const
objects (rvalue AND lvalue) which is NOT what was intended. The lesson
to be learned: Don't mix rvalue references with template parameter
deduction unless you want the type parameter deduced to be a reference
in case of lvalues.

Is there a way to somehow "disable" this deduction rule so #3 is
called on lvalues?


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