std::scoped_allocator_adaptor

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std::scoped_allocator_adaptor
 
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.

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
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 instance
(public member function)
destructs a scoped_allocator_adaptor instance
(public member function)
assigns a scoped_allocator_adaptor
(public member function)
obtains an inner_allocator reference
(public member function)
obtains an outer_allocator reference
(public member function)
allocates uninitialized storage using the outer allocator
(public member function)
deallocates storage using the outer allocator
(public member function)
returns the largest allocation size supported by the outer allocator
(public member function)
constructs an object in allocated storage, passing the inner allocator to its constructor if appropriate
(public member function)
destructs an object in allocated storage
(public member function)
copies the state of scoped_allocator_adaptor and all its allocators
(public member function)

Non-member functions

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

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 scoped_allocator_adaptor is stateless is_always_equal provided

See also

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