W3cubDocs

/C++

std::scoped_allocator_adaptor

Defined in header `<scoped_allocator>`
template< class OuterAlloc, class... InnerAlloc > class scoped_allocator_adaptor : public OuterAlloc;		(since C++11)

The std::scoped_allocator_adaptor class template is an allocator which can be used with multilevel containers (vector of sets of lists of tuples of maps, etc). It is instantiated with one outer allocator type OuterAlloc and zero or more inner allocator types InnerAlloc.... A container constructed directly with a scoped_allocator_adaptor uses OuterAlloc to allocate its elements, but if an element is itself a container, it uses the first inner allocator. The elements of that container, if they are themselves containers, use the second inner allocator, etc. If there are more levels to the container than there are inner allocators, the last inner allocator is reused for all further nested containers.

The purpose of this adaptor is to correctly initialize stateful allocators in nested containers, such as when all levels of a nested container must be placed in the same shared memory segment. The adaptor's constructor takes the arguments for all allocators in the list, and each nested container obtains its allocator's state from the adaptor as needed.

For the purpose of scoped_allocator_adaptor, if the next inner allocator is A, any class T for which std::uses_allocator<T,A>::value == true participates in the recursion as if it was a container. Additionally, std::pair is treated as such a container by specific overloads of scoped_allocator_adaptor::construct.

Typical implementation holds an instance of a std::scoped_allocator_adaptor<InnerAllocs...> as a member object.

Member types

Type	Definition
`outer_allocator_type`	`OuterAlloc`
`inner_allocator_type`	`scoped_allocator_adaptor<InnerAllocs...>` or, if `sizeof...(InnerAllocs) == 0`, `scoped_allocator_adaptor<OuterAlloc>`
`value_type`	`std::allocator_traits<OuterAlloc>::value_type`
`size_type`	`std::allocator_traits<OuterAlloc>::size_type`
`difference_type`	`std::allocator_traits<OuterAlloc>::difference_type`
`pointer`	`std::allocator_traits<OuterAlloc>::pointer`
`const_pointer`	`std::allocator_traits<OuterAlloc>::const_pointer`
`void_pointer`	`std::allocator_traits<OuterAlloc>::void_pointer`
`const_void_pointer`	`std::allocator_traits<OuterAlloc>::const_void_pointer`
`propagate_on_container_copy_assignment` `std::true_type` if `std::allocator_traits<A>::propagate_on_container_copy_assignment::value` is `true` for at least one allocator `A` among `OuterAlloc` and `InnerAlloc...`
`propagate_on_container_move_assignment` `std::true_type` if `std::allocator_traits<A>::propagate_on_container_move_assignment::value` is `true` for at least one allocator `A` among `OuterAlloc` and `InnerAlloc...`
`propagate_on_container_swap` `std::true_type` if `std::allocator_traits<A>::propagate_on_container_swap::value` is `true` for at least one allocator `A` among `OuterAlloc` and `InnerAlloc...`
`is_always_equal`(C++17) `std::true_type` if `std::allocator_traits<A>::is_always_equal::value` is `true` for every allocator `A` among `OuterAlloc` and `InnerAlloc...`
`rebind` template< class T > struct rebind { typedef scoped_allocator_adaptor< std::allocator_traits<OuterAlloc>::template rebind_alloc<T>, InnerAllocs... > other; };

Member functions

(constructor)	creates a new scoped_allocator_adaptor instance (public member function)
(destructor)	destructs a scoped_allocator_adaptor instance (public member function)
operator=	assigns a `scoped_allocator_adaptor` (public member function)
inner_allocator	obtains an `inner_allocator` reference (public member function)
outer_allocator	obtains an `outer_allocator` reference (public member function)
allocate	allocates uninitialized storage using the outer allocator (public member function)
deallocate	deallocates storage using the outer allocator (public member function)
max_size	returns the largest allocation size supported by the outer allocator (public member function)
construct	constructs an object in allocated storage, passing the inner allocator to its constructor if appropriate (public member function)
destroy	destructs an object in allocated storage (public member function)
select_on_container_copy_construction	copies the state of scoped_allocator_adaptor and all its allocators (public member function)

Non-member functions

operator==operator!=

compares two scoped_allocator_adaptor instances
(public member function)

Deduction guides(since C++17)

Example

#include <vector>
#include <scoped_allocator>
#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/interprocess/allocators/adaptive_pool.hpp>
namespace bi = boost::interprocess;
template<class T> using alloc = bi::adaptive_pool<T,
                                    bi::managed_shared_memory::segment_manager>;
using ipc_row = std::vector<int, alloc<int>>;
using ipc_matrix = std::vector<ipc_row, std::scoped_allocator_adaptor<alloc<ipc_row>>>;
int main ()
{
   bi::managed_shared_memory s(bi::create_only, "Demo", 65536);
 
   // create vector of vectors in shared memory
   ipc_matrix v(s.get_segment_manager());
 
   // for all these additions, the inner vectors obtain their allocator arguments
   // from the outer vector's scoped_allocator_adaptor
   v.resize(1); v[0].push_back(1);
   v.emplace_back(2);
   std::vector<int> local_row = {1,2,3};
   v.emplace_back(local_row.begin(), local_row.end());
 
   bi::shared_memory_object::remove("Demo");
}