23 General utilities library [utilities]

23.12 Memory resources [mem.res]

23.12.3 Class template polymorphic_­allocator [mem.poly.allocator.class]

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A specialization of class template pmr​::​polymorphic_­allocator conforms to the Allocator requirements ([allocator.requirements]).
Constructed with different memory resources, different instances of the same specialization of pmr​::​polymorphic_­allocator can exhibit entirely different allocation behavior.
This runtime polymorphism allows objects that use polymorphic_­allocator to behave as if they used different allocator types at run time even though they use the same static allocator type.
template <class Tp>
class polymorphic_allocator {
  memory_resource* memory_rsrc; // exposition only

public:
  using value_type = Tp;

  // [mem.poly.allocator.ctor], constructors
  polymorphic_allocator() noexcept;
  polymorphic_allocator(memory_resource* r);

  polymorphic_allocator(const polymorphic_allocator& other) = default;

  template <class U>
    polymorphic_allocator(const polymorphic_allocator<U>& other) noexcept;

  polymorphic_allocator&
    operator=(const polymorphic_allocator& rhs) = delete;

  // [mem.poly.allocator.mem], member functions
  Tp* allocate(size_t n);
  void deallocate(Tp* p, size_t n);

  template <class T, class... Args>
  void construct(T* p, Args&&... args);

  template <class T1, class T2, class... Args1, class... Args2>
    void construct(pair<T1,T2>* p, piecewise_construct_t,
                   tuple<Args1...> x, tuple<Args2...> y);
  template <class T1, class T2>
    void construct(pair<T1,T2>* p);
  template <class T1, class T2, class U, class V>
    void construct(pair<T1,T2>* p, U&& x, V&& y);
  template <class T1, class T2, class U, class V>
    void construct(pair<T1,T2>* p, const pair<U, V>& pr);
  template <class T1, class T2, class U, class V>
    void construct(pair<T1,T2>* p, pair<U, V>&& pr);

  template <class T>
    void destroy(T* p);

  polymorphic_allocator select_on_container_copy_construction() const;

  memory_resource* resource() const;
};

23.12.3.1 polymorphic_­allocator constructors [mem.poly.allocator.ctor]

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polymorphic_allocator() noexcept;
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Effects: Sets memory_­rsrc to get_­default_­resource().
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polymorphic_allocator(memory_resource* r);
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Requires: r is non-null.
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Effects: Sets memory_­rsrc to r.
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Throws: Nothing.
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[ Note
:
This constructor provides an implicit conversion from memory_­resource*.
— end note
 ]
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template <class U> polymorphic_allocator(const polymorphic_allocator<U>& other) noexcept;
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#
Effects: Sets memory_­rsrc to other.resource().

23.12.3.2 polymorphic_­allocator member functions [mem.poly.allocator.mem]

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Tp* allocate(size_t n);
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Returns: Equivalent to 
return static_cast<Tp*>(memory_rsrc->allocate(n * sizeof(Tp), alignof(Tp)));
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void deallocate(Tp* p, size_t n);
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Requires: p was allocated from a memory resource x, equal to *memory_­rsrc, using x.allocate(n * sizeof(Tp), alignof(Tp)).
3
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Effects: Equivalent to memory_­rsrc->deallocate(p, n * sizeof(Tp), alignof(Tp)).
4
#
Throws: Nothing.
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template <class T, class... Args> void construct(T* p, Args&&... args);
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Requires: Uses-allocator construction of T with allocator resource() (see [allocator.uses.construction]) and constructor arguments std​::​forward<Args>(args)... is well-formed.
[ Note
:
Uses-allocator construction is always well formed for types that do not use allocators.
— end note
 ]
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#
Effects: Construct a T object in the storage whose address is represented by p by uses-allocator construction with allocator resource() and constructor arguments std​::​forward<Args>(args)....
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Throws: Nothing unless the constructor for T throws.
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template <class T1, class T2, class... Args1, class... Args2> void construct(pair<T1,T2>* p, piecewise_construct_t, tuple<Args1...> x, tuple<Args2...> y);
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[ Note
:
This method and the construct methods that follow are overloads for piecewise construction of pairs ([pairs.pair]).
— end note
 ]
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Effects: Let xprime be a tuple constructed from x according to the appropriate rule from the following list.
[ Note
:
The following description can be summarized as constructing a pair<T1, T2> object in the storage whose address is represented by p, as if by separate uses-allocator construction with allocator resource() ([allocator.uses.construction]) of p->first using the elements of x and p->second using the elements of y.
— end note
 ]
  • (9.1)
    If uses_­allocator_­v<T1,memory_­resource*> is false
    and is_­constructible_­v<T1,Args1...> is true,
    then xprime is x.
  • (9.2)
    Otherwise, if uses_­allocator_­v<T1,memory_­resource*> is true
    and is_­constructible_­v<T1,allocator_­arg_­t,memory_­resource*,Args1...> is true,
    then xprime is tuple_­cat(make_­tuple(allocator_­arg, resource()), std​::​move(x)).
  • (9.3)
    Otherwise, if uses_­allocator_­v<T1,memory_­resource*> is true
    and is_­constructible_­v<T1,Args1...,memory_­resource*> is true,
    then xprime is tuple_­cat(std​::​move(x), make_­tuple(resource())).
  • (9.4)
    Otherwise the program is ill formed.
Let yprime be a tuple constructed from y according to the appropriate rule from the following list:
  • (9.5)
    If uses_­allocator_­v<T2,memory_­resource*> is false
    and is_­constructible_­v<T2,Args2...> is true,
    then yprime is y.
  • (9.6)
    Otherwise, if uses_­allocator_­v<T2,memory_­resource*> is true
    and is_­constructible_­v<T2,allocator_­arg_­t,memory_­resource*,Args2...> is true,
    then yprime is tuple_­cat(make_­tuple(allocator_­arg, resource()), std​::​move(y)).
  • (9.7)
    Otherwise, if uses_­allocator_­v<T2,memory_­resource*> is true
    and is_­constructible_­v<T2,Args2...,memory_­resource*> is true,
    then yprime is tuple_­cat(std​::​move(y), make_­tuple(resource())).
  • (9.8)
    Otherwise the program is ill formed.
Then, using piecewise_­construct, xprime, and yprime as the constructor arguments, this function constructs a pair<T1, T2> object in the storage whose address is represented by p.
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template <class T1, class T2> void construct(pair<T1,T2>* p);
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Effects: Equivalent to: 
construct(p, piecewise_construct, tuple<>(), tuple<>());
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template <class T1, class T2, class U, class V> void construct(pair<T1,T2>* p, U&& x, V&& y);
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Effects: Equivalent to: 
construct(p, piecewise_construct,
          forward_as_tuple(std::forward<U>(x)),
          forward_as_tuple(std::forward<V>(y)));
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template <class T1, class T2, class U, class V> void construct(pair<T1,T2>* p, const pair<U, V>& pr);
12
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Effects: Equivalent to: 
construct(p, piecewise_construct,
          forward_as_tuple(pr.first),
          forward_as_tuple(pr.second));
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template <class T1, class T2, class U, class V> void construct(pair<T1,T2>* p, pair<U, V>&& pr);
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Effects: Equivalent to: 
construct(p, piecewise_construct,
          forward_as_tuple(std::forward<U>(pr.first)),
          forward_as_tuple(std::forward<V>(pr.second)));
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template <class T> void destroy(T* p);
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Effects: As if by p->~T().
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polymorphic_allocator select_on_container_copy_construction() const;
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Returns: polymorphic_­allocator().
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[ Note
:
The memory resource is not propagated.
— end note
 ]
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memory_resource* resource() const;
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Returns: memory_­rsrc.

23.12.3.3 polymorphic_­allocator equality [mem.poly.allocator.eq]

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template <class T1, class T2> bool operator==(const polymorphic_allocator<T1>& a, const polymorphic_allocator<T2>& b) noexcept;
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Returns: *a.resource() == *b.resource().
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template <class T1, class T2> bool operator!=(const polymorphic_allocator<T1>& a, const polymorphic_allocator<T2>& b) noexcept;
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Returns: !(a == b).