std::scoped_allocator_adaptor

 

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Dynamic memory management

Uninitialized memory algorithms uninitialized_copy uninitialized_fill uninitialized_move (C++17) uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17) destroy_at (C++17) voidify (C++20) uninitialized_copy_n (C++11) uninitialized_fill_n uninitialized_move_n (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17) construct_at (C++20) Constrained uninitialized memory algorithms ranges::uninitialized_copy (C++20) ranges::uninitialized_fill (C++20) ranges::uninitialized_move (C++20) ranges::construct_at (C++20) ranges::destroy (C++20) ranges::destroy_n (C++20) ranges::destroy_at (C++20) ranges::uninitialized_copy_n (C++20) ranges::uninitialized_fill_n (C++20) ranges::uninitialized_move_n (C++20) ranges::uninitialized_default_construct (C++20) ranges::uninitialized_value_construct (C++20) ranges::uninitialized_default_construct_n (C++20)   ranges::uninitialized_value_construct_n (C++20) Allocators allocator allocator_traits (C++11) allocation_result (C++23) allocator_arg (C++11) uses_allocator (C++11) uses_allocator_construction_args (C++20) make_obj_using_allocator (C++20) uninitialized_construct_using_allocator (C++20) scoped_allocator_adaptor (C++11) pmr::polymorphic_allocator (C++17) Memory resources pmr::memory_resource (C++17) pmr::get_default_resource (C++17) pmr::set_default_resource (C++17) pmr::new_delete_resource (C++17) pmr::null_memory_resource (C++17) pmr::synchronized_pool_resource (C++17) pmr::unsynchronized_pool_resource (C++17)   pmr::monotonic_buffer_resource (C++17) pmr::pool_options (C++17) Garbage collection support declare_reachable (C++11)(until C++23) undeclare_reachable (C++11)(until C++23) declare_no_pointers (C++11)(until C++23) undeclare_no_pointers (C++11)(until C++23) pointer_safety (C++11)(until C++23) get_pointer_safety (C++11)(until C++23) Uninitialized storage raw_storage_iterator (until C++20*) get_temporary_buffer (until C++20*) return_temporary_buffer (until C++20*) Explicit lifetime management start_lifetime_asstart_lifetime_as_array (C++23)(C++23)

Smart pointers unique_ptr (C++11) shared_ptr (C++11) weak_ptr (C++11) auto_ptr (until C++17*) owner_less (C++11) owner_less<void> (C++17) owner_hash (C++26) owner_equal (C++26) enable_shared_from_this (C++11) bad_weak_ptr (C++11) default_delete (C++11) out_ptr_t (C++23) inout_ptr_t (C++23) Low level memory management operator new operator new[] operator delete operator delete[] get_new_handler (C++11) set_new_handler launder (C++17) bad_alloc bad_array_new_length (C++11) nothrow_t align_val_t (C++17) destroying_delete_t (C++20) new_handler nothrow Miscellaneous pointer_traits (C++11) to_address (C++20) addressof (C++11) align (C++11) assume_aligned (C++20) C Library malloc calloc realloc    aligned_alloc (C++17) free

 

std::scoped_allocator_adaptor

Member functions

scoped_allocator_adaptor::scoped_allocator_adaptor

scoped_allocator_adaptor::~scoped_allocator_adaptor

scoped_allocator_adaptor::operator=

scoped_allocator_adaptor::inner_allocator

scoped_allocator_adaptor::outer_allocator

scoped_allocator_adaptor::allocate

scoped_allocator_adaptor::deallocate

scoped_allocator_adaptor::max_size

scoped_allocator_adaptor::construct

scoped_allocator_adaptor::destroy

scoped_allocator_adaptor::select_on_container_copy_construction

scoped_allocator_adaptor::outermost scoped_allocator_adaptor::outermost-construct scoped_allocator_adaptor::outermost-destroy

Non-member functions

operator==operator!= (until C++20)

Deduction guides(C++17)

 

Defined in header <scoped_allocator>
template< class OuterAlloc, class... InnerAllocs > 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.

Note that std::pmr::polymorphic_allocators propagate to nested containers following uses-allocator construction and do not need (and do not work with) std::scoped_allocator_adaptor.

Nested types

Type	Definition
outer_allocator_type	OuterAlloc
inner_allocator_type	scoped_allocator_adaptor<InnerAllocs...> if sizeof...(InnerAllocs) is zero scoped_allocator_adaptor<OuterAlloc> otherwise
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

Given the set of OuterAlloc and InnerAlloc... as Allocs:

Type	Definition
propagate_on_container_copy_assignment	std::true_type if there exists a type A in Allocs such that std::allocator_traits<A>::     propagate_on_container_copy_assignment::value is true std::false_type otherwise
propagate_on_container_move_assignment	std::true_type if there exists a type A in Allocs such that std::allocator_traits<A>::     propagate_on_container_move_assignment::value is true std::false_type otherwise
propagate_on_container_swap	std::true_type if there exists a type A in Allocs such that std::allocator_traits<A>::     propagate_on_container_swap::value is true std::false_type otherwise
is_always_equal	std::true_type if there exists a type A in Allocs such that std::allocator_traits<A>::     is_always_equal::value is true std::false_type otherwise

Member functions

(constructor)	creates a new scoped_allocator_adaptor object (public member function)
(destructor)	destructs a scoped_allocator_adaptor object (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)
Exposition-only function templates
outermost	obtains the outermost allocator (exposition-only member function*)
outermost-construct	constructs an object using the outermost allocator (exposition-only member function*)
outermost-destroy	destroys an object using the outermost allocator (exposition-only member function*)

Non-member functions

operator==operator!= (removed in C++20)

compares two scoped_allocator_adaptor objects (function template)

Nested classes

Class	Definition
rebind	template< class T > struct rebind {     using other = scoped_allocator_adaptor                       <std::allocator_traits<OuterAlloc>::template rebind_alloc<T>,                        InnerAllocs...>; };

Example

Run this code

#include <boost/interprocess/allocators/adaptive_pool.hpp>
#include <boost/interprocess/managed_shared_memory.hpp>
#include <scoped_allocator>
#include <vector>
 
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");
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2108	C++11	there was no way to show if scoped_allocator_adaptor is stateless	provided is_always_equal

See also

allocator_traits (C++11)	provides information about allocator types (class template)
uses_allocator (C++11)	checks if the specified type supports uses-allocator construction (class template)
allocator	the default allocator (class template)