|Category: allocators||Component type: overview|
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.
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. 
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.|
vector<double> V(100, 5.0); // Uses the default allocator. vector<double, single_client_alloc> local(V.begin(), V.end());
 The reason for this change is that the new interface reduces memory fragmentation, and that it allows an implementation that is both efficient and thread-safe.
 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.
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