C++ named requirements: Allocator

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, 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 allocator-aware classes, including standard library containers, access allocators indirectly through std::allocator_traits, and std::allocator_traits supplies the default implementation of those requirements.

Requirements

Given.

• T, a non-const, non-reference type (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

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

Given.

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

Given.

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

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

Stateful and stateless allocators

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.

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.

Fancy pointers

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).

Standard library

The following standard library components satisfy the Allocator requirements:

Examples

#include <cstdlib>
#include <new>
#include <limits>
#include <iostream>
#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);
}

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

Defect reports

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.