Is there a version of uninitialized_value_construct
that uses the an allocator to construct the element in place instead of placement new
?
This below is the prototypical implementation of uninitialized_value_construct
; however I am looking for one where the allocator is passed so I can use the line alloc.construct(std::addressof(*current))
instead of ::new (std::addressof(*current))
.
template<class ForwardIt>
void uninitialized_value_construct(ForwardIt first, ForwardIt last)
{
using Value = typename std::iterator_traits<ForwardIt>::value_type;
ForwardIt current = first;
try {
for (; current != last; ++current) {
::new (static_cast<void*>(std::addressof(*current))) Value();
}
} catch (...) {
std::destroy(first, current);
throw;
}
}
In C++20, there is uninitialized_construct_using_allocator but it is not clear what it is for or how to use it.
EDIT: After a conversation with @NicolBolas, I ended up implementing these pair of functions (that I wish were in the std::
). For my needs (and without loss of generality, I will do this for other uninitialized_X
functions.
template<class Alloc, class ForwardIt, class Size, class AT = typename std::allocator_traits<Alloc>>
ForwardIt uninitialized_value_construct_n(Alloc& a, ForwardIt first, Size n){
using T = typename std::iterator_traits<ForwardIt>::value_type;
ForwardIt current = first;
try{
for(; n > 0; (void) ++current, --n)
AT::construct(a, std::addressof(*current), T());
// ::new (static_cast<void*>(std::addressof(*current))) T();
return current;
}catch(...){destroy(a, first, current); throw;}
}
template<class Alloc, class ForwardIt, typename AT = typename std::allocator_traits<Alloc> >
void destroy(Alloc& a, ForwardIt first, ForwardIt last){
for(; first != last; ++first)
AT::destroy(a, std::addressof(*first));
// destroy_at(std::addressof(*first));
}
Sadly, there is no ranged version of uninitialized_construct
that takes an allocator.
As for what uninitialized_construct_using_allocator
is for, it construct a single object of the given type with the given arguments through the allocator. That might seem trivial to do (std::allocator_traits<Alloc>::construct(t, alloc, std::forward<Args>(args)...)
), but it does it in such a way that the allocator will be propagated correctly to scoped_allocator
s used by that T
. This requires a bunch of techniques that are not well known and are not easily understood, so there's a specific function to do it.