Does initializing a vector with one element require copy constructor? - c++

Question

The following example complies in clang but fails in GCC.
Original link
Code:
#include<vector>
struct Foo
{
Foo() = default;
Foo(Foo&&) = default;
};
Foo baz(std::vector<Foo> foos);
Foo bar1(Foo&& foo_)
{
std::vector<Foo> foos{std::move(foo_)};
return baz(std::move(foos));
}
Foo bar2(Foo&& foo_)
{
std::vector<Foo> foos;
foos.push_back(std::move(foo_));
return baz(std::move(foos));
}
template<typename... Foos>
Foo bar3(Foo&& foo_, Foos&&... foos_)
{
std::vector<Foo> foos{std::move(foo_), std::forward<Foos>(foos_)...};
return baz(std::move(foos));
}
void dummy()
{
Foo f1, f2;
bar3(std::move(f1), std::move(f2));
}
Errors:
In file included from /opt/gcc-4.9.0/include/c++/4.9.0/vector:62:0,
from /tmp/gcc-explorer-compiler11512-1-c5m2m1/example.cpp:1:
/opt/gcc-4.9.0/include/c++/4.9.0/bits/stl_construct.h: In instantiation of 'void std::_Construct(_T1*, _Args&& ...) [with _T1 = Foo; _Args = {const Foo&}]':
/opt/gcc-4.9.0/include/c++/4.9.0/bits/stl_uninitialized.h:75:53: required from 'static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const Foo*; _ForwardIterator = Foo*; bool _TrivialValueTypes = false]'
/opt/gcc-4.9.0/include/c++/4.9.0/bits/stl_uninitialized.h:125:41: required from '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const Foo*; _ForwardIterator = Foo*]'
/opt/gcc-4.9.0/include/c++/4.9.0/bits/stl_uninitialized.h:278:63: required from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = const Foo*; _ForwardIterator = Foo*; _Tp = Foo]'
/opt/gcc-4.9.0/include/c++/4.9.0/bits/stl_vector.h:1286:27: required from 'void std::vector<_Tp, _Alloc>::_M_range_initialize(_ForwardIterator, _ForwardIterator, std::forward_iterator_tag) [with _ForwardIterator = const Foo*; _Tp = Foo; _Alloc = std::allocator<Foo>]'
/opt/gcc-4.9.0/include/c++/4.9.0/bits/stl_vector.h:378:36: required from 'std::vector<_Tp, _Alloc>::vector(std::initializer_list<_Tp>, const allocator_type&) [with _Tp = Foo; _Alloc = std::allocator<Foo>; std::vector<_Tp, _Alloc>::allocator_type = std::allocator<Foo>]'
13 : required from here
/opt/gcc-4.9.0/include/c++/4.9.0/bits/stl_construct.h:75:7: error: use of deleted function 'constexpr Foo::Foo(const Foo&)'
{ ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
^
3 : note: 'constexpr Foo::Foo(const Foo&)' is implicitly declared as deleted because 'Foo' declares a move constructor or move assignment operator
struct Foo
^
Compilation failed
If it is a bug in GCC, can someone suggest an easy workaround for bar3 function?

Answer 1

It's not possible to move out of an initializer_list, so the corresponding constructor of std::vector has to copy. That's why you're getting the errors. bar2 should compile without errors.
The normal template pack expansion trick to repeatedly call push_back should work on the variadic version.