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内存分配器 Allocators

1. 概述 Summary

内存分配器Allocator把内存分配和释放封装起来。它们提供了一个底层的接口，允许有效的分配很多小对象。不同的内存分配器类型表示不同的内存管理策略。

Allocators encapsulate allocation and deallocation of memory. They provide a low-level interface that permits efficient allocation of many small objects; different allocator types represent different schemes for memory management.

注意，内存分配其只是简单的分配和释放内存，而不是创建和释放对象。STL中还包括一组底层的算法，用来操作为初始化的内存。

Note that allocators simply allocate and deallocate memory, as opposed to creating and destroying objects. The STL also includes several low-level algorithms for manipulating uninitialized memory.

注意，内存分配器也不会试图包装多种内存模型。C++之定义了一种内存模型（比如说，两个指针的差总是ptrdiff_t类型），这个内存模型是内存分配器唯一支持的内存模型。这是现在的STL与一开始的STL在内存分配器定义上的一个重大区别。[1]

Note also that allocators do not attempt to encapsulate multiple memory models. The C++ language only defines a single memory model (the difference of two pointers, for example, is always ptrdiff_t), and this memory model is the only one that allocators support. This is a major change from the definition of allocators in the original STL. [1]

2. 详述 Description

The details of the allocator interface are still subject to change, and we do not guarantee that specific member functions will remain in future versions. You should think of an allocator as a "black box". That is, you may select a container's memory allocation strategy by instantiating the container template with a particular allocator [2], but you should not make any assumptions about how the container actually uses the allocator.

The available allocators are as follows. In most cases you shouldn't have to worry about the distinction: the default allocator, alloc, is usually the best choice. alloc The default allocator. It is thread-safe, and usually has the best performance characteristics. pthread_alloc A thread-safe allocator that uses a different memory pool for each thread; you can only use pthread_alloc if your operating system provides pthreads. Pthread_alloc is usually faster than alloc, especially on multiprocessor systems. It can, however, cause resource fragmentation: memory deallocated in one thread is not available for use by other threads. single_client_alloc A fast but thread-unsafe allocator. In programs that only have one thread, this allocator might be faster than alloc. malloc_alloc An allocator that simply uses the standard library function malloc. It is thread-safe but slow; the main reason why you might sometimes want to use it is to get more useful information from bounds-checking or leak-detection tools while you are debugging.

3. 例子 Examples

   1 vector<double> V(100, 5.0);     // Uses the default allocator.
   2 vector<double, single_client_alloc> local(V.begin(), V.end());

4. 概念 Concepts

• Allocator

5. 类型 Types

• alloc
• pthread_alloc
• single_client_alloc
• malloc_alloc
• raw_storage_iterator

6. 函数 Functions

• construct
• destroy
• uninitialized_copy
• uninitialized_fill
• uninitialized_fill_n
• get_temporary_buffer
• return_temporary_buffer

7. 备注 Notes

[1] 做这个改变的原因是，新的接口可以减少内存碎片，并且能够实现出高效的并且线程安全的内存分配器。

[2] Different containers may use different allocators. You might, for example, have some containers that use the default allocator alloc and others that use pthread_alloc. Note, however, that vector<int> and vector<int, pthread_alloc> are distinct types.