Pair trivial move constructor

[Christian Rorvik]

Hi Pauolo,

Did you compile with -std=c++11?

On Thu, Nov 29, 2012 at 11:56 AM, Paolo Carlini
<paolo.carlini@oracle.com> wrote:
> Hi,
>
> just  a quick reply, for sure other have more technical points to make.
> But...
>
>
> On 11/29/2012 12:26 PM, Christian Rorvik wrote:
>>
>> It's my understanding that in GCC 4.7.2 ABI fixes were put in place to
>> ensure compatibility between code compiled with/without -std=c++11.
>>
>> As part of that std::pair's move constructor was made trivial, with
>> its current definition:
>> constexpr pair(pair&&) = default;
>>
>> The implication of this appears to be that all types contained in
>> std::pair must be either movable or trivially copyable, otherwise the
>> compiler will implicitly delete pair's move constructor and then
>> complain about its usage. Example:
>>
>> #include <utility>
>>
>> struct Foo
>> {
>>      Foo();
>>      Foo(const Foo&);
>> };
>>
>> typedef std::pair<Foo, Foo> FooPair;
>>
>> FooPair create();
>>
>> void test()
>> {
>>      const FooPair x = create();
>> }
>>
>>
>> Produces:
>>
>> a.cpp:15:30: error: use of deleted function 'constexpr std::pair<_T1,
>> _T2>::pair(std::pair<_T1, _T2>&&) [with _T1 = Foo; _T2 = Foo;
>> std::pair<_T1, _T2> = std::pair<Foo, Foo>]'
>> In file included from
>>
>> /usr/local/gcc-4.7.2/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/utility:72:0,
>>                   from a.cpp:1:
>>
>> /usr/local/gcc-4.7.2/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/stl_pair.h:120:17:
>> note: 'constexpr std::pair<_T1, _T2>::pair(std::pair<_T1, _T2>&&)
>> [with _T1 = Foo; _T2 = Foo; std::pair<_T1, _T2> = std::pair<Foo,
>> Foo>]' is implicitly deleted because the default definition would be
>> ill-formed:
>>
>> /usr/local/gcc-4.7.2/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/stl_pair.h:120:17:
>> error: non-static data member 'std::pair<Foo, Foo>::first' does not
>> have a move constructor or trivial copy constructor
>>
>> /usr/local/gcc-4.7.2/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/stl_pair.h:120:17:
>> error: non-static data member 'std::pair<Foo, Foo>::second' does not
>> have a move constructor or trivial copy constructor
>
> I can't reproduce this, neither with stock 4.7.2, neither with current
> 4_7-branch, neither in mainline. Maybe you have a pasto in the code above?
>
>> I'm not sure how this sits with the standard requirement for
>> std::pair,
>
> For sure C++11 wants a defaulted move constructor.
>
> Paolo.