flat_map/flat_set with small_vector as container requires explicit allocator-type

[tobias-loew]

The latest changes to the allocator handling in small_vector now requires that the allocator-type has to be specified explicitely when used with flat_map/set:

This line doesn't compile anymore:
boost::container::flat_map<int, double, std::less, boost::container::small_vector<std::pair<int, double>,17>> my_map;

But this one does:
boost::container::flat_map<int, double, std::less, boost::container::small_vector<std::pair<int, double>,17, boost::container::new_allocator<std::pair<int, double>>>> my_map;

The reason for this is that in container_rebind.hpp:60ff

//Needed for non-conforming compilers like GCC 4.3
template <template <class, std::size_t, class> class Cont, typename V, std::size_t N, typename A, class U>
struct container_rebind<Cont<V, N, A>, U>
{
typedef Cont<U, N, typename allocator_traits::template portable_rebind_alloc::type> type;
};

The template doesn't use the "real_allocator" template to figure out the allocator but uses the small_vector's allocator-default "void".

Tobias

[tobias-loew]

Furthermore I came over another problem with custom allocators:
I am using the aligned allocator from http://blogs.msdn.com/b/vcblog/archive/2008/08/28/the-aligned_allocator.aspx (from Stephan T. Lavavej) which worked fine with small_vector.
But with the step from Boost 1.68 to 1.70 I had to add a specialization

template<std::size_t Alignment>
class aligned_allocator<void, Alignment> {

public:
	typedef void value_type;
	typedef void* pointer;
	typedef const void* const_pointer;

	template<class U>
	struct rebind {
		typedef aligned_allocator<U, Alignment> other;
	};
};

to make the code compile again.
I'm using VS 2019, i.e. MSVC 19.20

[tobias-loew]

The container rebind issue seems to be already fix. Thanks

[tobias-loew]

The problem with rebind are due to the non-type template parameter for the alignment of aligned_allocator: the helper-template for doing the rebind only has specializations for class-type templates.
Neither gcc, clang or msvc is able to rebind such an allocator with their std::allocator_trait, though I couldn't find any restriction on the kind of template parameters for allocators in the standard. the standard enables automatic rebind only for allocators with only class-type template arguments.

I fixed my problem by replacing the size_t parameter by std::integral_type