C++中的allocator是如何实现的？

Question

我试图std::allocator在C++中更好地理解,我遇到了这个问题,在我看来实际上只有一个分配器类通常被容器使用(比如std::vector)我的问题是这个分配器是如何实现的？它是否像定期重新分配的堆栈一样？如果不是如何实际实施？

Answer 1

默认分配器是std::allocator,并且只::operator new在需要时使用,所以没什么特别的.它new与delete所需的每个对象或多或少相同.您可以[default.allocator]在标准中阅读更多相关信息.

分配器"接口"(实际上只是一组需求,在模板实例化期间强制执行)是这个过程的包装器,允许使用替代的内存供应方法.

例如,您可能提供的替代分配器可以实现内存池或其他特定于您需求的内容,从而减少诚实到良好的动态分配.

标准容器以及它们的元素类型具有分配器类型作为模板参数(您通常不会注意到这一点!)这就是您选择与该容器一起使用的替代实现的方式.

在这些情况下,你通常会预先分配一大块内存,然后在什么时候抛出一小块内存.从这个意义上讲,这样的实现可以被认为是一种"堆内堆",但实际上你根本没有必要给它堆语义.它只需要遵守概念分配器的要求.

Josuttis先生在http://www.josuttis.com/cppcode/allocator.html上提出了一个(无聊的)例子; 我在这里重现它:

/* The following code example is taken from the book
 * "The C++ Standard Library - A Tutorial and Reference"
 * by Nicolai M. Josuttis, Addison-Wesley, 1999
 *
 * (C) Copyright Nicolai M. Josuttis 1999.
 * Permission to copy, use, modify, sell and distribute this software
 * is granted provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied
 * warranty, and with no claim as to its suitability for any purpose.
 */
#include <limits>
#include <iostream>

namespace MyLib {
   template <class T>
   class MyAlloc {
     public:
       // type definitions
       typedef T        value_type;
       typedef T*       pointer;
       typedef const T* const_pointer;
       typedef T&       reference;
       typedef const T& const_reference;
       typedef std::size_t    size_type;
       typedef std::ptrdiff_t difference_type;

       // rebind allocator to type U
       template <class U>
       struct rebind {
           typedef MyAlloc<U> other;
       };

       // return address of values
       pointer address (reference value) const {
           return &value;
       }
       const_pointer address (const_reference value) const {
           return &value;
       }

       /* constructors and destructor
        * - nothing to do because the allocator has no state
        */
       MyAlloc() throw() {
       }
       MyAlloc(const MyAlloc&) throw() {
       }
       template <class U>
         MyAlloc (const MyAlloc<U>&) throw() {
       }
       ~MyAlloc() throw() {
       }

       // return maximum number of elements that can be allocated
       size_type max_size () const throw() {
           return std::numeric_limits<std::size_t>::max() / sizeof(T);
       }

       // allocate but don't initialize num elements of type T
       pointer allocate (size_type num, const void* = 0) {
           // print message and allocate memory with global new
           std::cerr << "allocate " << num << " element(s)"
                     << " of size " << sizeof(T) << std::endl;
           pointer ret = (pointer)(::operator new(num*sizeof(T)));
           std::cerr << " allocated at: " << (void*)ret << std::endl;
           return ret;
       }

       // initialize elements of allocated storage p with value value
       void construct (pointer p, const T& value) {
           // initialize memory with placement new
           new((void*)p)T(value);
       }

       // destroy elements of initialized storage p
       void destroy (pointer p) {
           // destroy objects by calling their destructor
           p->~T();
       }

       // deallocate storage p of deleted elements
       void deallocate (pointer p, size_type num) {
           // print message and deallocate memory with global delete
           std::cerr << "deallocate " << num << " element(s)"
                     << " of size " << sizeof(T)
                     << " at: " << (void*)p << std::endl;
           ::operator delete((void*)p);
       }
   };

   // return that all specializations of this allocator are interchangeable
   template <class T1, class T2>
   bool operator== (const MyAlloc<T1>&,
                    const MyAlloc<T2>&) throw() {
       return true;
   }
   template <class T1, class T2>
   bool operator!= (const MyAlloc<T1>&,
                    const MyAlloc<T2>&) throw() {
       return false;
   }
}

用法:

#include <vector>
#include "myalloc.hpp"

int main()
{
    // create a vector, using MyAlloc<> as allocator
    std::vector<int,MyLib::MyAlloc<int> > v;

    // insert elements
    // - causes reallocations
    v.push_back(42);
    v.push_back(56);
    v.push_back(11);
    v.push_back(22);
    v.push_back(33);
    v.push_back(44);
}