Difference between revisions of "cpp/memory/scoped allocator adaptor"

Revision as of 13:38, 1 March 2017

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.

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

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

Member functions

Non-member functions

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, "MySharedMemory", 65536);
   alloc<int> a(s.get_segment_manager());
 
   // create vector of vectors in shared memory
   ipc_matrix v(a);   // note: rebinds alloc<ipc_row> from this alloc<int>
   v.emplace_back(3); // the inner vector obtains its allocator argument
                      // from scoped_allocator_adaptor
 
   bi::shared_memory_object::remove("MySharedMemory");
}

See also