Error with `std::vector< std::unique_ptr< T > >` - c++

Question

I'm seeing some errors passing std::vector< std::unique_ptr< T > > around with std::move. The code that reproduces the problem is this:
#include <memory> // for std::unique_ptr
#include <utility> // for std::move
#include <vector> // for std::vector
struct bar {};
using vtype = std::vector<std::unique_ptr<bar>>;
struct foo
{
foo(vtype v) : _v(std::move(v)) { }
private:
vtype _v;
};
vtype getVector()
{
return { std::move( std::unique_ptr<bar>(new bar()) ) };
};
int main()
{
foo f(std::move(getVector()));
};
With clang 3.4, this code produces this error:
$ clang++ -std=c++11 test.cpp -o xtest
In file included from test.cpp:1:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/memory:64:
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_construct.h:75:38: error: call to deleted constructor of
'std::unique_ptr<bar, std::default_delete<bar> >'
{ ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_uninitialized.h:75:8: note: in instantiation of function template specialization
'std::_Construct<std::unique_ptr<bar, std::default_delete<bar> >, const std::unique_ptr<bar, std::default_delete<bar> > &>' requested here
std::_Construct(std::__addressof(*__cur), *__first);
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_uninitialized.h:117:2: note: in instantiation of function template specialization
'std::__uninitialized_copy<false>::__uninit_copy<const std::unique_ptr<bar, std::default_delete<bar> > *, std::unique_ptr<bar, std::default_delete<bar> > *>'
requested here
__uninit_copy(__first, __last, __result);
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_uninitialized.h:258:19: note: in instantiation of function template specialization
'std::uninitialized_copy<const std::unique_ptr<bar, std::default_delete<bar> > *, std::unique_ptr<bar, std::default_delete<bar> > *>' requested here
{ return std::uninitialized_copy(__first, __last, __result); }
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:1204:11: note: in instantiation of function template specialization
'std::__uninitialized_copy_a<const std::unique_ptr<bar, std::default_delete<bar> > *, std::unique_ptr<bar, std::default_delete<bar> > *, std::unique_ptr<bar,
std::default_delete<bar> > >' requested here
std::__uninitialized_copy_a(__first, __last,
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:368:2: note: in instantiation of function template specialization
'std::vector<std::unique_ptr<bar, std::default_delete<bar> >, std::allocator<std::unique_ptr<bar, std::default_delete<bar> > > >::_M_range_initialize<const
std::unique_ptr<bar, std::default_delete<bar> > *>' requested here
_M_range_initialize(__l.begin(), __l.end(),
^
test.cpp:17:12: note: in instantiation of member function 'std::vector<std::unique_ptr<bar, std::default_delete<bar> >, std::allocator<std::unique_ptr<bar,
std::default_delete<bar> > > >::vector' requested here
return { std::move( std::unique_ptr<bar>(new bar()) ) };
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/unique_ptr.h:273:7: note: function has been explicitly marked deleted here
unique_ptr(const unique_ptr&) = delete;
^
1 error generated.
The situations doesn't seem to be any better with g++ 4.8:
$ g++-4.8 -std=c++11 test.cpp -o xtest
In file included from /usr/include/c++/4.8/memory:64:0,
from test.cpp:1:
/usr/include/c++/4.8/bits/stl_construct.h: In instantiation of ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = std::unique_ptr<bar>; _Args = {const std::unique_ptr<bar, std::default_delete<bar> >&}]’:
/usr/include/c++/4.8/bits/stl_uninitialized.h:75:53: required from ‘static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const std::unique_ptr<bar>*; _ForwardIterator = std::unique_ptr<bar>*; bool _TrivialValueTypes = false]’
/usr/include/c++/4.8/bits/stl_uninitialized.h:117:41: required from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const std::unique_ptr<bar>*; _ForwardIterator = std::unique_ptr<bar>*]’
/usr/include/c++/4.8/bits/stl_uninitialized.h:258:63: required from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = const std::unique_ptr<bar>*; _ForwardIterator = std::unique_ptr<bar>*; _Tp = std::unique_ptr<bar>]’
/usr/include/c++/4.8/bits/stl_vector.h:1206:27: required from ‘void std::vector<_Tp, _Alloc>::_M_range_initialize(_ForwardIterator, _ForwardIterator, std::forward_iterator_tag) [with _ForwardIterator = const std::unique_ptr<bar>*; _Tp = std::unique_ptr<bar>; _Alloc = std::allocator<std::unique_ptr<bar> >]’
/usr/include/c++/4.8/bits/stl_vector.h:369:36: required from ‘std::vector<_Tp, _Alloc>::vector(std::initializer_list<_Tp>, const allocator_type&) [with _Tp = std::unique_ptr<bar>; _Alloc = std::allocator<std::unique_ptr<bar> >; std::vector<_Tp, _Alloc>::allocator_type = std::allocator<std::unique_ptr<bar> >]’
test.cpp:17:59: required from here
/usr/include/c++/4.8/bits/stl_construct.h:75:7: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = bar; _Dp = std::default_delete<bar>]’
{ ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
^
In file included from /usr/include/c++/4.8/memory:81:0,
from test.cpp:1:
/usr/include/c++/4.8/bits/unique_ptr.h:273:7: error: declared here
unique_ptr(const unique_ptr&) = delete;
^
According to this answer and the comments, this shouldn't be happening on these compilers, but I'm not doing exactly the same: I'm trying to initialize the vector with an initializer list.
Any idea what needs to happen in order for this code to build correctly?

Answer 1

The used of the braced-init-list in the return statement within getVector
return { std::move( std::unique_ptr<bar>(new bar()) ) };
results in a call to the std::vector<T> constructor that takes an initializer_list<T> argument. Even though you're moving the unique_ptr, an initializer_list only allows const access to its elements, due to which the vector will attempt to copy the unique_ptr, leading to the error you see.
You can fix the error by resorting to a more verbose manner of constructing the vector
vtype getVector()
{
vtype v;
v.push_back(std::unique_ptr<bar>(new bar()));
return v;
}
Live demo
For curiosity's sake, it is possible to construct a vector from an array of move-only objects, but you need to go through std::move_iterator to move the elements.
vtype getVector()
{
std::unique_ptr<bar> arr[] = {std::unique_ptr<bar>(new bar())};
return {std::make_move_iterator(std::begin(arr)),
std::make_move_iterator(std::end(arr))};
}

Answer 2

your vtype is a vector of unique pointers and you returning to it a unique pointer.
since you are doing the move in the constructor foo you dont need to return a move
take a look to this code:
#include <memory> // for std::unique_ptr
#include <utility> // for std::move
#include <vector> // for std::vector
struct bar {};
using vtype = /*std::vector<*/std::unique_ptr<bar>/*>*/;
struct foo
{
foo(vtype v) : _v(std::move(v)) { }
private:
vtype _v;
};
vtype getVector()
{
return /*{ std::move(*/ std::unique_ptr<bar>(new bar()) /*) }*/;
};
int main()
{
foo f(std::move(getVector()));
};