From cppreference.com
template<classOuterAlloc,class...InnerAllocs>classscoped_allocator_adaptor:publicOuterAlloc;(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,
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
and do not need (and do not work with) std::scoped_allocator_adaptor.
Nested types
Type Definition outer_allocator_typeOuterAllocinner_allocator_typescoped_allocator_adaptor<OuterAlloc> if sizeof...(InnerAllocs) is zero
scoped_allocator_adaptor<InnerAllocs...> otherwise
value_typestd::allocator_traits<OuterAlloc>::value_typesize_typestd::allocator_traits<OuterAlloc>::size_typedifference_typestd::allocator_traits<OuterAlloc>::difference_typepointerstd::allocator_traits<OuterAlloc>::pointerconst_pointerstd::allocator_traits<OuterAlloc>::const_pointervoid_pointerstd::allocator_traits<OuterAlloc>::void_pointerconst_void_pointerstd::allocator_traits<OuterAlloc>::const_void_pointer
Given the set of OuterAlloc and InnerAlloc... as Allocs:
Type Definition propagate_on_container_copy_assignment
if there exists a type A in Allocs such that std::allocator_traits<A>::
propagate_on_container_copy_assignment::value is true
otherwise
propagate_on_container_move_assignment
if there exists a type A in Allocs such that std::allocator_traits<A>::
propagate_on_container_move_assignment::value is true
otherwise
propagate_on_container_swap
if there exists a type A in Allocs such that std::allocator_traits<A>::
propagate_on_container_swap::value is true
otherwise
is_always_equal
if there exists a type A in Allocs such that std::allocator_traits<A>::
is_always_equal::value is true
otherwise
Member functions
creates a new scoped_allocator_adaptor object
(public member function)
destructs a scoped_allocator_adaptor object
(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)
select_on_container_copy_construction
copies the state of scoped_allocator_adaptor and all its allocators
(public member function)
Exposition-only function templates
obtains the outermost allocator
(exposition-only member function*)
constructs an object using the outermost allocator
(exposition-only member function*)
destroys an object using the outermost allocator
(exposition-only member function*)Non-member functions
(since C++17)
Nested classes
Class Definition rebindtemplate<classT>structrebind{usingother=scoped_allocator_adaptor<std::allocator_traits<OuterAlloc>::templaterebind_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>namespacebi=boost::interprocess;template<classT>usingalloc=bi::adaptive_pool<T,bi::managed_shared_memory::segment_manager>;usingipc_row=std::vector<int,alloc<int>>;usingipc_matrix=std::vector<ipc_row,std::scoped_allocator_adaptor<alloc<ipc_row>>>;intmain(){bi::managed_shared_memorys(bi::create_only,"Demo",65536);// create vector of vectors in shared memoryipc_matrixv(s.get_segment_manager());// for all these additions, the inner vectors obtain their allocator arguments// from the outer vector's scoped_allocator_adaptorv.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
C++11 there was no way to show if scoped_allocator_adaptor is stateless provided is_always_equalSee also