std::scoped...ator_adaptor(3) C++ Standard Libary std::scoped...ator_adaptor(3) 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 allo- cator, etc. If there are more levels to the container than there are inner alloca- tors, the last inner allocator is reused for all further nested containers. The purpose of this adaptor is to correctly initialize stateful alloca- tors 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 alloca- tor'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 contain- ers 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>::differ- ence_type pointer std::allocator_traits<OuterAlloc>::pointer const_pointer std::allocator_traits<OuterAl- loc>::const_pointer void_pointer std::allocator_traits<OuterAl- loc>::void_pointer const_void_pointer std::allocator_traits<OuterAl- loc>::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_con- tainer_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_alloca- tor_adaptor constructor instance (public member function) destructor destructs a scoped_allocator_adap- tor instance (public member function) operator= assigns a scoped_allocator_adaptor (public member function) inner_allocator obtains an inner_allocator refer- ence (public member function) outer_allocator obtains an outer_allocator refer- ence (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_alloca- tor_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::seg- ment_manager>; using ipc_row = std::vector<int, alloc<int>>; using ipc_matrix = std::vector<ipc_row, std::scoped_allocator_adaptor<al- loc<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 retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior LWG 2108 C++11 there was no way to show if is_always_equal scoped_allocator_adaptor is stateless pro- vided 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) http://cppreference.com 2024.06.10 std::scoped...ator_adaptor(3)
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