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* [Bug c++/53234] New: [c++0x] unfriendly error message for missing move constructor
@ 2012-05-04 18:27 luto at mit dot edu
2012-05-04 19:07 ` [Bug c++/53234] " redi at gcc dot gnu.org
2020-03-20 4:25 ` luto at kernel dot org
0 siblings, 2 replies; 3+ messages in thread
From: luto at mit dot edu @ 2012-05-04 18:27 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53234
Bug #: 53234
Summary: [c++0x] unfriendly error message for missing move
constructor
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c++
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: luto@mit.edu
This code is correctly rejected by gcc 4.7 and up (4.6 incorrectly accepted
it). The error message confused me for a while, though.
struct move_only
{
move_only() = default;
move_only(move_only&&) = default;
move_only &operator = (move_only&&) = default;
};
struct is_it_moveable
{
//is_it_moveable() = default;
//is_it_moveable(is_it_moveable &&) = default;
~is_it_moveable();
move_only mo;
};
int main()
{
is_it_moveable j;
is_it_moveable k = (is_it_moveable&&)j;
}
A recent trunk build says:
move.cc: In function ‘int main()’:
move.cc:20:40: error: use of deleted function
‘is_it_moveable::is_it_moveable(const is_it_moveable&)’
is_it_moveable k = (is_it_moveable&&)j;
^
move.cc:8:8: note: ‘is_it_moveable::is_it_moveable(const is_it_moveable&)’ is
implicitly deleted because the default definition would be ill-formed:
struct is_it_moveable
^
move.cc:8:8: error: use of deleted function ‘constexpr
move_only::move_only(const move_only&)’
struct is_it_moveable
^
move.cc:1:8: note: ‘constexpr move_only::move_only(const move_only&)’ is
implicitly declared as deleted because ‘move_only’ declares a move constructor
or move assignment operator
struct move_only
^
This is, according to the standard, exactly correct. [class.copy] paragraph 9
says "If the definition of a class X does not explicitly declare a move
constructor, one will be implicitly declared as defaulted if and only if
[condition that does not apply here]." The note says "When the move
constructor is not implicitly declared or explicitly supplied, expressions that
otherwise would have invoked the move constructor may instead invoke a copy
constructor."
I think the error message could be improved to help C++11 newbies like myself,
though. An extra line like:
note: is_it_moveable has no move constructor because it has a user-declared
destructor
would be quite friendly. I spent a while staring at the error, thinking "of
course the copy constructor would be ill-formed. That's way I called the
*move* constructor, you dummy!"
(FWIW, this change is missing from the 4.7 release notes. I think it, or
something related, breaks boost 1.47's shared_ptr quite thoroughly.)
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug c++/53234] [c++0x] unfriendly error message for missing move constructor
2012-05-04 18:27 [Bug c++/53234] New: [c++0x] unfriendly error message for missing move constructor luto at mit dot edu
@ 2012-05-04 19:07 ` redi at gcc dot gnu.org
2020-03-20 4:25 ` luto at kernel dot org
1 sibling, 0 replies; 3+ messages in thread
From: redi at gcc dot gnu.org @ 2012-05-04 19:07 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53234
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2012-05-04
Ever Confirmed|0 |1
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-05-04 19:06:51 UTC ---
Yes, your suggested note would be helpful.
(In reply to comment #0)
> (FWIW, this change is missing from the 4.7 release notes. I think it, or
> something related, breaks boost 1.47's shared_ptr quite thoroughly.)
Older versions of Boost rarely work with newer versions of G++, especially in
C++11 mode, because noone tests Boost svn against GCC svn any more. Even if
the GCC 4.7 release notes had mentioned the change it wouldn't have made any
difference, I think Boost 1.47 had already been released when the change was
made and it would be too late to fix it.
Maybe we could add something to http://gcc.gnu.org/gcc-4.7/porting_to.html
about the change.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug c++/53234] [c++0x] unfriendly error message for missing move constructor
2012-05-04 18:27 [Bug c++/53234] New: [c++0x] unfriendly error message for missing move constructor luto at mit dot edu
2012-05-04 19:07 ` [Bug c++/53234] " redi at gcc dot gnu.org
@ 2020-03-20 4:25 ` luto at kernel dot org
1 sibling, 0 replies; 3+ messages in thread
From: luto at kernel dot org @ 2020-03-20 4:25 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53234
--- Comment #2 from Andy Lutomirski <luto at kernel dot org> ---
*** Bug 57998 has been marked as a duplicate of this bug. ***
^ permalink raw reply [flat|nested] 3+ messages in thread
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2012-05-04 18:27 [Bug c++/53234] New: [c++0x] unfriendly error message for missing move constructor luto at mit dot edu
2012-05-04 19:07 ` [Bug c++/53234] " redi at gcc dot gnu.org
2020-03-20 4:25 ` luto at kernel dot org
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