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[allocator.adaptor]
# 20 Memory management library [[mem]](./#mem)
## 20.6 Class template scoped_allocator_adaptor [allocator.adaptor]
### [20.6.1](#syn) Header <scoped_allocator> synopsis [[allocator.adaptor.syn]](allocator.adaptor.syn)
[🔗](#header:%3cscoped_allocator%3e)
namespace std {// class template scoped_allocator_adaptortemplate<class OuterAlloc, class... InnerAlloc>class scoped_allocator_adaptor; // [[scoped.adaptor.operators]](#scoped.adaptor.operators "20.6.5 Operators"), scoped allocator operatorstemplate<class OuterA1, class OuterA2, class... InnerAllocs>bool operator==(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a, const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b) noexcept;}
[1](#syn-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8608)
The class template scoped_allocator_adaptor is an allocator template that
specifies an allocator resource (the outer allocator) to be used by a container (as any
other allocator does) and also specifies an inner allocator resource to be passed to the
constructor of every element within the container[.](#syn-1.sentence-1)
This adaptor is instantiated with one
outer and zero or more inner allocator types[.](#syn-1.sentence-2)
If instantiated with only one allocator
type, the inner allocator becomes the scoped_allocator_adaptor itself, thus
using the same allocator resource for the container and every element within the
container and, if the elements themselves are containers, each of their elements
recursively[.](#syn-1.sentence-3)
If instantiated with more than one allocator, the first allocator is the
outer allocator for use by the container, the second allocator is passed to the
constructors of the container's elements, and, if the elements themselves are
containers, the third allocator is passed to the elements' elements, and so on[.](#syn-1.sentence-4)
If
containers are nested to a depth greater than the number of allocators, the last
allocator is used repeatedly, as in the single-allocator case, for any remaining
recursions[.](#syn-1.sentence-5)
[*Note [1](#syn-note-1)*:
The scoped_allocator_adaptor is derived from the outer
allocator type so it can be substituted for the outer allocator type in most
expressions[.](#syn-1.sentence-6)
— *end note*]
[🔗](#lib:scoped_allocator_adaptor)
namespace std {template<class OuterAlloc, class... InnerAllocs>class scoped_allocator_adaptor : public OuterAlloc {private:using OuterTraits = allocator_traits<OuterAlloc>; // *exposition only* scoped_allocator_adaptor<InnerAllocs...> inner; // *exposition only*public:using outer_allocator_type = OuterAlloc; using inner_allocator_type = *see below*; using value_type = typename OuterTraits::value_type; using size_type = typename OuterTraits::size_type; using difference_type = typename OuterTraits::difference_type; using pointer = typename OuterTraits::pointer; using const_pointer = typename OuterTraits::const_pointer; using void_pointer = typename OuterTraits::void_pointer; using const_void_pointer = typename OuterTraits::const_void_pointer; using propagate_on_container_copy_assignment = *see below*; using propagate_on_container_move_assignment = *see below*; using propagate_on_container_swap = *see below*; using is_always_equal = *see below*; template<class Tp> struct rebind {using other = scoped_allocator_adaptor< OuterTraits::template rebind_alloc<Tp>, InnerAllocs...>; };
scoped_allocator_adaptor(); template<class OuterA2> scoped_allocator_adaptor(OuterA2&& outerAlloc, const InnerAllocs&... innerAllocs) noexcept;
scoped_allocator_adaptor(const scoped_allocator_adaptor& other) noexcept;
scoped_allocator_adaptor(scoped_allocator_adaptor&& other) noexcept; template<class OuterA2> scoped_allocator_adaptor(const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& other) noexcept; template<class OuterA2> scoped_allocator_adaptor( scoped_allocator_adaptor<OuterA2, InnerAllocs...>&& other) noexcept;
scoped_allocator_adaptor& operator=(const scoped_allocator_adaptor&) = default;
scoped_allocator_adaptor& operator=(scoped_allocator_adaptor&&) = default; ~scoped_allocator_adaptor();
inner_allocator_type& inner_allocator() noexcept; const inner_allocator_type& inner_allocator() const noexcept;
outer_allocator_type& outer_allocator() noexcept; const outer_allocator_type& outer_allocator() const noexcept;
pointer allocate(size_type n);
pointer allocate(size_type n, const_void_pointer hint); void deallocate(pointer p, size_type n);
size_type max_size() const; template<class T, class... Args>void construct(T* p, Args&&... args); template<class T>void destroy(T* p);
scoped_allocator_adaptor select_on_container_copy_construction() const; }; template<class OuterAlloc, class... InnerAllocs> scoped_allocator_adaptor(OuterAlloc, InnerAllocs...)-> scoped_allocator_adaptor<OuterAlloc, InnerAllocs...>;}
### [20.6.2](#types) Member types [[allocator.adaptor.types]](allocator.adaptor.types)
[🔗](#lib:inner_allocator_type,scoped_allocator_adaptor)
`using inner_allocator_type = see below;
`
[1](#types-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8722)
*Type*: scoped_allocator_adaptor<OuterAlloc> if sizeof...(InnerAllocs) is
zero; otherwise,
scoped_allocator_adaptor<InnerAllocs...>[.](#types-1.sentence-2)
[🔗](#lib:propagate_on_container_copy_assignment,scoped_allocator_adaptor)
`using propagate_on_container_copy_assignment = see below;
`
[2](#types-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8733)
*Type*: true_type ifallocator_traits<A>​::​propagate_on_container_copy_assignment​::​value istrue for any A in the set of OuterAlloc andInnerAllocs...; otherwise, false_type[.](#types-2.sentence-1)
[🔗](#lib:propagate_on_container_move_assignment,scoped_allocator_adaptor)
`using propagate_on_container_move_assignment = see below;
`
[3](#types-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8746)
*Type*: true_type ifallocator_traits<A>​::​propagate_on_container_move_assignment​::​value istrue for any A in the set of OuterAlloc andInnerAllocs...; otherwise, false_type[.](#types-3.sentence-1)
[🔗](#lib:propagate_on_container_swap,scoped_allocator_adaptor)
`using propagate_on_container_swap = see below;
`
[4](#types-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8759)
*Type*: true_type ifallocator_traits<A>​::​propagate_on_container_swap​::​value istrue for any A in the set of OuterAlloc andInnerAllocs...; otherwise, false_type[.](#types-4.sentence-1)
[🔗](#lib:is_always_equal,scoped_allocator_adaptor)
`using is_always_equal = see below;
`
[5](#types-5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8772)
*Type*: true_type ifallocator_traits<A>​::​is_always_equal​::​value istrue for every A in the set of OuterAlloc andInnerAllocs...; otherwise, false_type[.](#types-5.sentence-1)
### [20.6.3](#cnstr) Constructors [[allocator.adaptor.cnstr]](allocator.adaptor.cnstr)
[🔗](#lib:scoped_allocator_adaptor,constructor)
`scoped_allocator_adaptor();
`
[1](#cnstr-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8787)
*Effects*: Value-initializes the OuterAlloc base class and the inner allocator
object[.](#cnstr-1.sentence-1)
[🔗](#lib:scoped_allocator_adaptor,constructor_)
`template<class OuterA2>
scoped_allocator_adaptor(OuterA2&& outerAlloc, const InnerAllocs&... innerAllocs) noexcept;
`
[2](#cnstr-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8800)
*Constraints*: is_constructible_v<OuterAlloc, OuterA2> is true[.](#cnstr-2.sentence-1)
[3](#cnstr-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8804)
*Effects*: Initializes the OuterAlloc base class withstd​::​forward<OuterA2>(outerAlloc) and inner with innerAllocs... (hence recursively initializing each allocator within the adaptor with the corresponding
allocator from the argument list)[.](#cnstr-3.sentence-1)
[🔗](#lib:scoped_allocator_adaptor,constructor__)
`scoped_allocator_adaptor(const scoped_allocator_adaptor& other) noexcept;
`
[4](#cnstr-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8818)
*Effects*: Initializes each allocator within the adaptor with the corresponding allocator
from other[.](#cnstr-4.sentence-1)
[🔗](#lib:scoped_allocator_adaptor,constructor___)
`scoped_allocator_adaptor(scoped_allocator_adaptor&& other) noexcept;
`
[5](#cnstr-5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8830)
*Effects*: Move constructs each allocator within the adaptor with the corresponding allocator
from other[.](#cnstr-5.sentence-1)
[🔗](#lib:scoped_allocator_adaptor,constructor____)
`template<class OuterA2>
scoped_allocator_adaptor(
const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& other) noexcept;
`
[6](#cnstr-6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8844)
*Constraints*: is_constructible_v<OuterAlloc, const OuterA2&> is true[.](#cnstr-6.sentence-1)
[7](#cnstr-7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8848)
*Effects*: Initializes each allocator within the adaptor with the corresponding allocator
from other[.](#cnstr-7.sentence-1)
[🔗](#lib:scoped_allocator_adaptor,constructor_____)
`template<class OuterA2>
scoped_allocator_adaptor(scoped_allocator_adaptor<OuterA2, InnerAllocs...>&& other) noexcept;
`
[8](#cnstr-8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8861)
*Constraints*: is_constructible_v<OuterAlloc, OuterA2> is true[.](#cnstr-8.sentence-1)
[9](#cnstr-9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8865)
*Effects*: Initializes each allocator within the adaptor with the corresponding allocator rvalue
from other[.](#cnstr-9.sentence-1)
### [20.6.4](#members) Members [[allocator.adaptor.members]](allocator.adaptor.members)
[1](#members-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8873)
In the construct member functions,*OUTERMOST*(x) is*OUTERMOST*(x.outer_allocator()) if
the expression x.outer_allocator() is
valid ([[temp.deduct]](temp.deduct "13.10.3 Template argument deduction")) andx otherwise;*OUTERMOST_ALLOC_TRAITS*(x) isallocator_traits<remove_reference_t<decltype(*OUTERMOST*(x))>>[.](#members-1.sentence-1)
[*Note [1](#members-note-1)*:
*OUTERMOST*(x) and*OUTERMOST_ALLOC_TRAITS*(x) are recursive operations[.](#members-1.sentence-2)
It
is incumbent upon the definition of outer_allocator() to ensure that the
recursion terminates[.](#members-1.sentence-3)
It will terminate for all instantiations ofscoped_allocator_adaptor[.](#members-1.sentence-4)
— *end note*]
[🔗](#lib:inner_allocator,scoped_allocator_adaptor)
`inner_allocator_type& inner_allocator() noexcept;
const inner_allocator_type& inner_allocator() const noexcept;
`
[2](#members-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8897)
*Returns*: *this if sizeof...(InnerAllocs) is zero; otherwise,inner[.](#members-2.sentence-1)
[🔗](#lib:outer_allocator,scoped_allocator_adaptor)
`outer_allocator_type& outer_allocator() noexcept;
`
[3](#members-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8909)
*Returns*: static_cast<OuterAlloc&>(*this)[.](#members-3.sentence-1)
[🔗](#lib:outer_allocator,scoped_allocator_adaptor_)
`const outer_allocator_type& outer_allocator() const noexcept;
`
[4](#members-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8920)
*Returns*: static_cast<const OuterAlloc&>(*this)[.](#members-4.sentence-1)
[🔗](#lib:allocate,scoped_allocator_adaptor)
`pointer allocate(size_type n);
`
[5](#members-5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8931)
*Returns*: allocator_traits<OuterAlloc>​::​allocate(outer_allocator(), n)[.](#members-5.sentence-1)
[🔗](#lib:allocate,scoped_allocator_adaptor_)
`pointer allocate(size_type n, const_void_pointer hint);
`
[6](#members-6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8942)
*Returns*: allocator_traits<OuterAlloc>​::​allocate(outer_allocator(), n, hint)[.](#members-6.sentence-1)
[🔗](#lib:deallocate,scoped_allocator_adaptor)
`void deallocate(pointer p, size_type n) noexcept;
`
[7](#members-7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8953)
*Effects*: As if by:allocator_traits<OuterAlloc>​::​deallocate(outer_allocator(), p, n);
[🔗](#lib:max_size,scoped_allocator_adaptor)
`size_type max_size() const;
`
[8](#members-8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8965)
*Returns*: allocator_traits<OuterAlloc>​::​max_size(outer_allocator())[.](#members-8.sentence-1)
[🔗](#lib:construct,scoped_allocator_adaptor)
`template<class T, class... Args>
void construct(T* p, Args&&... args);
`
[9](#members-9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8977)
*Effects*: Equivalent to:apply([p, this](auto&&... newargs) {*OUTERMOST_ALLOC_TRAITS*(*this)::construct(*OUTERMOST*(*this), p,
std::forward<decltype(newargs)>(newargs)...); },
uses_allocator_construction_args<T>(inner_allocator(),
std::forward<Args>(args)...));
[🔗](#lib:destroy,scoped_allocator_adaptor)
`template<class T>
void destroy(T* p);
`
[10](#members-10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L8998)
*Effects*: Calls *OUTERMOST_ALLOC_TRAITS*(*this)​::​destroy(*OUTERMOST*(*this), p)[.](#members-10.sentence-1)
[🔗](#lib:select_on_container_copy_construction,scoped_allocator_adaptor)
`scoped_allocator_adaptor select_on_container_copy_construction() const;
`
[11](#members-11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L9009)
*Returns*: A new scoped_allocator_adaptor object
where each allocator a1 within the adaptor
is initialized withallocator_traits<A1>​::​select_on_container_copy_construction(a2),
where A1 is the type of a1 anda2 is the corresponding allocator in *this[.](#members-11.sentence-1)
### [20.6.5](#scoped.adaptor.operators) Operators [[scoped.adaptor.operators]](scoped.adaptor.operators)
[🔗](#lib:operator==,scoped_allocator_adaptor)
`template<class OuterA1, class OuterA2, class... InnerAllocs>
bool operator==(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a,
const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b) noexcept;
`
[1](#scoped.adaptor.operators-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L9029)
*Returns*: If sizeof...(InnerAllocs) is zero,a.outer_allocator() == b.outer_allocator() otherwisea.outer_allocator() == b.outer_allocator() && a.inner_allocator() == b.inner_allocator()