Thestd::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 typeOuterAlloc and zero or more inner allocator typesInnerAlloc.... A container constructed directly with ascoped_allocator_adaptor usesOuterAlloc 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 ofscoped_allocator_adaptor, if the next inner allocator isA, any classT for whichstd::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 ofscoped_allocator_adaptor::construct.

Typical implementation holds an instance of astd::scoped_allocator_adaptor<InnerAllocs...> as a member object.

Note thatstd::pmr::polymorphic_allocators propagate to nested containers followinguses-allocator construction and do not need (and do not work with)std::scoped_allocator_adaptor.

[edit]Nested types

Given the set ofOuterAlloc andInnerAlloc... asAllocs:

[edit]Member functions

[edit]Non-member functions

[edit]Deduction guides(since C++17)

[edit]Nested classes

[edit]Example

#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");}

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

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