c++ - Overload resolution between object, rvalue reference, const reference -

given 3 functions, call ambiguous.

int f( int ); int f( int && ); int f( int const & );  int q = f( 3 ); 

removing f( int ) causes both clang , gcc prefer rvalue reference on lvalue reference. instead removing either reference overload results in ambiguity f( int ).

overload resolution done in terms of strict partial ordering, int seems equivalent 2 things not equivalent each other. rules here? seem recall defect report this.

is there chance int && may preferred on int in future standard? reference must bind initializer, whereas object type not constrained. overloading between t , t && mean "use existing object if i've been given ownership, otherwise make copy." (this similar pure pass-by-value, saves overhead of moving.) these compilers work, must done overloading t const & , t &&, , explicitly copying. i'm not sure strictly standard.

what rules here?

as there 1 parameter, rule 1 of 3 viable parameter initializations of parameter must a better match both other two. when 2 initializations compared, either 1 better other, or neither better (they indistinguishable).

without special rules direct reference binding, 3 initializations mentioned indistinguishable (in 3 comparisons).

the special rules direct reference binding make int&& better const int&, neither better or worse int. therefore there no best match:

s1    s2 int   int&&         indistinguishable int   const int&    indistinguishable int&& const int&    s1 better 

int&& better const int& because of 13.3.3.2:

s1 , s2 reference bindings (8.5.3) , neither refers implicit object parameter of non-static member function declared without ref-qualifier, , s1 binds rvalue reference rvalue , s2 binds lvalue reference.

but rule not apply when 1 of initializations not reference binding.

is there chance int && may preferred on int in future standard? reference must bind initializer, whereas object type not constrained. overloading between t , t && mean "use existing object if i've been given ownership, otherwise make copy."

you propose make reference binding better match non-reference binding. why not post idea isocpp future proposals. not best subjective discussion / opinion.