"Peter Dimov" <pdimov@mmltd.net> writes:
David Abrahams wrote:
Howard Hinnant <hinnant@twcny.rr.com> writes:
On Feb 9, 2004, at 11:33 AM, Peter Dimov wrote:
However (apologies for the follow up to self) the rest of Howard's analysis still seems to apply, i.e. the CA temporary is created with A::A(A&&), and the A const & parameter of foo is bound directly to that temporary (if not elided).
And thanks for the corrected analysis.
I don't know. If that analysis were correct, then Comeau would give the same error for this snippet in my example.
typedef X const XC; sink2(XC(X()));
but instead I see:
------ test 20, direct initialize a const A with an A ------- X() #24 MOVE #25 <== #24 in const lvalue sink2 destroy #25 destroy #24 (EMPTY)
That's because your "move" constructor is not a move constructor from language point of view (i.e. it has no special status). As I understand it, the compiler's thought process goes somewhat like: need to create a temporary, but it can be elided, so we'll just access check the copy constructor. The compiler doesn't want to backtrack and try not to elide the temporary, which would've succeeded.
OK, so 8.5.3/5 says: A temporary of type ``cv1 T2'' [sic] is created, and a constructor is called to copy the entire rvalue object into the temporary. The reference is bound to the temporary or to a sub-object within the temporary.* [Footnote: Clearly, if the reference initialization being processed is one for the first argument of a copy constructor call, an implementation must eventually choose the first alternative (binding without copying) to avoid infinite recursion. --- end foonote] The constructor that would be used to make the copy shall be callable whether or not the copy is actually done. The problem here is, IIUC, that the implementors read that as saying that, because "a constructor is called to copy..." that the constructor used must be a copy constructor? I don't understand why the converting constructor fails to be considered when the templated SFINAE'd constructor *will* be used to copy const lvalues. Templated constructors are never "copy ctors" either, IIUC. -- Dave Abrahams Boost Consulting www.boost-consulting.com