NAME¶

std::scoped_allocator_adaptor - std::scoped_allocator_adaptor

Synopsis¶

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

class scoped_allocator_adaptor (since C++11)

: public OuterAlloc;

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.

Member types¶

Type Definition
outer_allocator_type OuterAlloc
scoped_allocator_adaptor<InnerAllocs...> or, if
inner_allocator_type 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

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¶

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

Non-member functions¶

operator== compares two scoped_allocator_adaptor instances
operator!= (function template)
(removed in C++20)

Deduction guides(since C++17)

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 is_always_equal
scoped_allocator_adaptor is stateless provided

See also¶

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