Re: On the path to move constructors: compile error.

"Alf P. Steinbach" <>
Mon, 30 Nov 2009 18:55:44 CST
* Andy Venikov:

Some time has passed since my previous post and I still don't know the

Can somebody explain why do I still need a "normal" copy-constructor
when passing an object by const-reference?

Here's my original post:

Andrew Venikov wrote:

On most compilers the following will not compile:

struct A
     A() {}
     A(A &) {} //Note: no const

void Func(A const &);
Func(A()); //Compile-time error here in gcc prior to 4.3 and Comeau

The error that gcc emits is:
error: no matching function for call to 'A::A(A)'
test.cpp:11: note: candidates are: A::A(A&)

The error that Comeau emits is:
"ComeauTest.c", line 20: error: "A::A(A &)", required for copy that
was eliminated,
           is not callable because reference parameter cannot be bound
to rvalue

But if I use gcc 4.3, or enable c++0x extensions in Comeau, then this
compiles without
any errors.

I've seen this behavior referenced in several places
(Alexandrescu's MOJO article:
(Dave Abraham's clarification:
Also, I've encountered it myself a couple of times (especially when
dealing with auto_ptr, since
it doesn't have a copy-constructor that takes a const reference).

My question is why? I couldn't find any plausible reference that
mandates this behavior.

In the current standard's ?8.5.3/5,

   -- If the initializer expression is an rvalue, with T2 a class type,
      and "cv1 T1" is reference-compatible with "cv2 T2", the reference
      is bound in one of the following ways (the choice is implementation-

      -- The reference is bound to the object represented by the rvalue
         (see 3.10) or to a sub-object within that object.

      -- 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.

      The constructor that would be used to make the copy shall be callable
      whether or not the copy is actually done.

This language is removed/changed in the C++0x draft, so that the binding is
direct when that's possible, in which case having an accessible copy constructor
is not required.

The C++0x change influences e.g. passing of a std::auto_ptr rvalue.

The intuition tells me that if a function takes something by a const
reference, then there
should be no copy-constructors invoked.

Dave Abrahams' article that I mentioned talks about a specific clause
in the standard: 8.5.3/5.
But reading that clause I couldn't infer anything that would suggest
the mentioned behavior.

See above.

Cheers & hth.,

- Alf

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