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

Revision as of 18:00, 2 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.

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