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C++ named requirements: Allocator - cppreference.com

Encapsulates strategies for access/addressing, allocation/deallocation and construction/destruction of objects.

Every standard library component that may need to allocate or release storage, from std::string, std::vector, and every container, except std::array(since C++11) and std::inplace_vector(since C++26), to std::shared_ptr and std::function(until C++17), does so through an Allocator: an object of a class type that satisfies the following requirements.

The implementation of many allocator requirements is optional because all AllocatorAwareContainer access allocators indirectly through std::allocator_traits, and std::allocator_traits supplies the default implementation of those requirements.

Given

• T, a non-const, non-reference type(until C++11)non-const object type(since C++11)(until C++17)cv-unqualified object type(since C++17),
• A, an Allocator type for type T,
• a, an object of type A,
• B, the corresponding Allocator type for some cv-unqualified object type U (as obtained by rebinding A),
• b, an object of type B,
• p, a value of type std::allocator_traits<A>::pointer, obtained by calling std::allocator_traits<A>::allocate(),
• cp, a value of type std::allocator_traits<A>::const_pointer, obtained by conversion from p,
• vp, a value of type std::allocator_traits<A>::void_pointer, obtained by conversion from p,
• cvp, a value of type std::allocator_traits<A>::const_void_pointer, obtained by conversion from cp or from vp,
• xp, a dereferenceable pointer to some cv-unqualified object type X,
• r, an lvalue of type T obtained by the expression *p,
• n, a value of type std::allocator_traits<A>::size_type.

• Satisfies NullablePointer, LegacyRandomAccessIterator, and LegacyContiguousIterator.

• Satisfies NullablePointer, LegacyRandomAccessIterator, and LegacyContiguousIterator.
• A::pointer is convertible to A::const_pointer.

• Satisfies NullablePointer.
• A::pointer is convertible to A::void_pointer.
• B::void_pointer and A::void_pointer are the same type.

• Satisfies NullablePointer.
• A::pointer, A::const_pointer, and A::void_pointer are convertible to A::const_void_pointer.
• B::const_void_pointer and A::const_void_pointer are the same type.

• An unsigned integer type.
• Can represent the size of the largest object A can allocate.

• A signed integer type.
• Can represent the difference of any two pointers to the objects allocated by A.

• For any U, B::template rebind<T>::other is A.

• true only if the storage allocated by the allocator a1 can be deallocated through a2.
• Establishes reflexive, symmetric, and transitive relationship.
• Does not throw exceptions.

• Same as !(a1 == a2).

• Does not throw exceptions.

• Does not throw exceptions.

• Does not throw exceptions.
• The value of a is unchanged and a1 == a.

• Does not throw exceptions.

• true if any two allocators of type A always compare equal.
• (If not provided, std::allocator_traits defaults this to std::is_empty<A>::type.)

• Provides an instance of A to be used by the container that is copy-constructed from the one that uses a currently.
• (Usually returns either a copy of a or a default-constructed A.)

• std::true_type or derived from it if the allocator of type A needs to be copied when the container that uses it is copy-assigned.
• If this member is std::true_type or derived from it, then A must satisfy CopyAssignable and the copy operation must not throw exceptions.
• Note that if the allocators of the source and the target containers do not compare equal, copy assignment has to deallocate the target's memory using the old allocator and then allocate it using the new allocator before copying the elements (and the allocator).

• std::true_type or derived from it if the allocator of type A needs to be moved when the container that uses it is move-assigned.
• If this member is std::true_type or derived from it, then A must satisfy MoveAssignable and the move operation must not throw exceptions.
• If this member is not provided or derived from std::false_type and the allocators of the source and the target containers do not compare equal, move assignment cannot take ownership of the source memory and must move-assign or move-construct the elements individually, resizing its own memory as needed.

• std::true_type or derived from it if the allocators of type A need to be swapped when two containers that use them are swapped.
• If this member is std::true_type or derived from it, type A must satisfy Swappable and the swap operation must not throw exceptions.
• If this member is not provided or derived from std::false_type and the allocators of the two containers do not compare equal, the behavior of container swap is undefined.

Notes:

1. ↑ See also fancy pointers below.
2. ↑ rebind is only optional (provided by std::allocator_traits) if this allocator is a template of the form SomeAllocator<T, Args>, where Args is zero or more additional template type parameters.

Given

• x1 and x2, objects of (possibly different) types X::void_pointer, X::const_void_pointer, X::pointer, or X::const_pointer

Then, x1 and x2 are equivalently-valued pointer values, if and only if both x1 and x2 can be explicitly converted to the two corresponding objects px1 and px2 of type X::const_pointer, using a sequence of static_casts using only these four types, and the expression px1 == px2 evaluates to true.

Given

• w1 and w2, objects of type X::void_pointer

Then, for the expression w1 == w2 and w1 != w2 either or both objects may be replaced by an equivalently-valued object of type X::const_void_pointer with no change in semantics.

Given

• p1 and p2, objects of type X::pointer

Then, for the expressions p1 == p2, p1 != p2, p1 < p2, p1 <= p2, p1 >= p2, p1 > p2, p1 - p2 either or both objects may be replaced by an equivalently-valued object of type X::const_pointer with no change in semantics.

The above requirements make it possible to compare Container's iterators and const_iterators.

An allocator type X for type T additionally satisfies the allocator completeness requirements if both of the following are true regardless of whether T is a complete type:

• X is a complete type.
• Except for value_type, all the member types of std::allocator_traits<X> are complete types.

Every Allocator type is either stateful or stateless. Generally, a stateful allocator type can have unequal values which denote distinct memory resources, while a stateless allocator type denotes a single memory resource.

Although custom allocators are not required to be stateless, whether and how the use of stateful allocators in the standard library is implementation-defined. Use of unequal allocator values may result in implementation-defined runtime errors or undefined behavior if the implementation does not support such usage.

Custom allocators may contain state. Each container or another allocator-aware object stores an instance of the supplied allocator and controls allocator replacement through std::allocator_traits.

Instances of a stateless allocator type always compare equal. Stateless allocator types are typically implemented as empty classes and suitable for empty base class optimization.

The member type is_always_equal of std::allocator_traits is intendedly used for determining whether an allocator type is stateless.

When the member type pointer is not a raw pointer type, it is commonly referred to as a "fancy pointer". Such pointers were introduced to support segmented memory architectures and are used today to access objects allocated in address spaces that differ from the homogeneous virtual address space that is accessed by raw pointers. An example of a fancy pointer is the mapping address-independent pointer boost::interprocess::offset_ptr, which makes it possible to allocate node-based data structures such as std::set in shared memory and memory mapped files mapped in different addresses in every process. Fancy pointers can be used independently of the allocator that provided them, through the class template std::pointer_traits(since C++11). The function std::to_address can be used to obtain a raw pointer from a fancy pointer.(since C++20)

Use of fancy pointers and customized size/different type in the standard libary are conditionally supported. Implementations may require that member type pointer, const_pointer, size_type, and difference_type are value_type*, const value_type*, std::size_t, and std::ptrdiff_t, respectively.

For the definition of the query object std::get_allocator, the following exposition-only concept is defined.

The exposition-only concept /*simple-allocator*/ defines the minimal usability constraints of the Allocator requirement.

The following standard library components satisfy the Allocator requirements:

Demonstrates a C++11 allocator, except for [[nodiscard]] added to match C++20 style.

#include <cstdlib>
#include <iostream>
#include <limits>
#include <new>
#include <vector>
 
template<class T>
struct Mallocator
{
    typedef T value_type;
 
    Mallocator() = default;
 
    template<class U>
    constexpr Mallocator(const Mallocator <U>&) noexcept {}
 
    [[nodiscard]] T* allocate(std::size_t n)
    {
        if (n > std::numeric_limits<std::size_t>::max() / sizeof(T))
            throw std::bad_array_new_length();
 
        if (auto p = static_cast<T*>(std::malloc(n * sizeof(T))))
        {
            report(p, n);
            return p;
        }
 
        throw std::bad_alloc();
    }
 
    void deallocate(T* p, std::size_t n) noexcept
    {
        report(p, n, 0);
        std::free(p);
    }
private:
    void report(T* p, std::size_t n, bool alloc = true) const
    {
        std::cout << (alloc ? "Alloc: " : "Dealloc: ") << sizeof(T) * n
                  << " bytes at " << std::hex << std::showbase
                  << reinterpret_cast<void*>(p) << std::dec << '\n';
    }
};
 
template<class T, class U>
bool operator==(const Mallocator <T>&, const Mallocator <U>&) { return true; }
 
template<class T, class U>
bool operator!=(const Mallocator <T>&, const Mallocator <U>&) { return false; }
 
int main()
{
    std::vector<int, Mallocator<int>> v(8);
    v.push_back(42);
}

Possible output:

Alloc: 32 bytes at 0x2020c20
Alloc: 64 bytes at 0x2023c60
Dealloc: 32 bytes at 0x2020c20
Dealloc: 64 bytes at 0x2023c60

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

1. ↑ The member types reference and const_reference of std::allocator are defined as T& and const T& respectively.
   • If T is a reference type, reference and const_reference are ill-formed because reference to reference cannot be formed (reference collapsing was introduced in C++11).
   • If T is const-qualified, reference and const_reference are the same, and the overload set of address() is ill-formed.

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