Difference between revisions of "cpp/named req/Allocator"

Revision as of 20:53, 27 August 2020

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, 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 cv-unqualified object type
• 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 allocator_traits<A>::pointer, obtained by calling allocator_traits<A>::allocate()
• cp, a value of type allocator_traits<A>::const_pointer, obtained by conversion from p
• vp, a value of type allocator_traits<A>::void_pointer, obtained by conversion from p
• cvp, a value of type allocator_traits<A>::const_void_pointer, obtained by conversion from cp or from vp
• xp, a dereferencable pointer to some cv-unqualified object type X
• r, an lvalue of type T obtained by the expression *p
• n, a value of type 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

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.

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_alloc();
 
    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.