std::scoped_allocator_adaptor
| Defined in | scoped_allocator |
template< class OuterAlloc, class... InnerAllocs >
class scoped_allocator_adaptor : 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.
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
| pub | Type | Definition |
| pub | outer_allocator_type | OuterAlloc |
| pub | inner_allocator_type | scoped_allocator_adaptor<InnerAllocs...> or, if sizeof...(InnerAllocs) == 0, scoped_allocator_adaptor<OuterAlloc> |
| pub | value_type | |
| pub | size_type | |
| pub | pointer | |
| pub | const_pointer | |
| pub | const_void_pointer | |
| pub | 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... |
| pub | 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... |
| pub | 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... |
| pub | is_always_equal | 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... |
| pub | rebind | |
Member functions
| pub | (constructor) | creates a new scoped_allocator_adaptor instance |
| pub | (destructor) | destructs a scoped_allocator_adaptor instance |
| pub | operator= | assigns a scoped_allocator_adaptor |
| pub | inner_allocator | obtains an inner_allocator reference |
| pub | outer_allocator | obtains an outer_allocator reference |
| pub | allocate | allocates uninitialized storage using the outer allocator |
| pub | deallocate | deallocates storage using the outer allocator |
| pub | max_size | returns the largest allocation size supported by the outer allocator |
| pub | construct | constructs an object in allocated storage, passing the inner allocator to its constructor if appropriate |
| pub | destroy | destructs an object in allocated storage |
| pub | select_on_container_copy_construction | copies the state of scoped_allocator_adaptor and all its allocators |
Non-member functions
| pub | operator== operator!= (removed in C++20) | compares two scoped_allocator_adaptor instances |
Deduction guides (od C++17)
Defined in header <scoped_allocator>
template<class OuterAlloc, class... InnerAllocs>
scoped_allocator_adaptor(OuterAlloc, InnerAllocs...)
-> scoped_allocator_adaptor<OuterAlloc, InnerAllocs...>;
One deduction guide is provided for std::scoped_allocator_adaptor to make it possible to deduce its outer allocator.
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");
}
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 |