自定义分配器可以发挥作用的一个领域是游戏开发，特别是在游戏机上，因为它们只有少量内存，没有交换空间。在这样的系统上，您要确保对每个子系统都有严格的控制，这样一个不重要的系统就不能从一个重要的系统窃取内存。池分配器等其他功能可以帮助减少内存碎片。你可以在这里找到一篇关于这个主题的详细的长篇论文:

EASTL—Electronic Arts标准模板库

我正在研究一个mmap-分配器，它允许向量使用内存 内存映射文件。我们的目标是让向量使用这样的存储 直接在由mmap映射的虚拟内存中。我们的问题是 提高真正大的文件(>10GB)的读取到内存，而不复制 开销，因此我需要这个自定义分配器。

到目前为止，我已经有了一个自定义分配器的骨架 (它来源于std::allocator)，我认为这是一个很好的开始 指向写自己的分配器。请随意使用这段代码 以任何你想要的方式:

#include <memory>
#include <stdio.h>

namespace mmap_allocator_namespace
{
        // See StackOverflow replies to this answer for important commentary about inheriting from std::allocator before replicating this code.
        template <typename T>
        class mmap_allocator: public std::allocator<T>
        {
public:
                typedef size_t size_type;
                typedef T* pointer;
                typedef const T* const_pointer;

                template<typename _Tp1>
                struct rebind
                {
                        typedef mmap_allocator<_Tp1> other;
                };

                pointer allocate(size_type n, const void *hint=0)
                {
                        fprintf(stderr, "Alloc %d bytes.\n", n*sizeof(T));
                        return std::allocator<T>::allocate(n, hint);
                }

                void deallocate(pointer p, size_type n)
                {
                        fprintf(stderr, "Dealloc %d bytes (%p).\n", n*sizeof(T), p);
                        return std::allocator<T>::deallocate(p, n);
                }

                mmap_allocator() throw(): std::allocator<T>() { fprintf(stderr, "Hello allocator!\n"); }
                mmap_allocator(const mmap_allocator &a) throw(): std::allocator<T>(a) { }
                template <class U>                    
                mmap_allocator(const mmap_allocator<U> &a) throw(): std::allocator<T>(a) { }
                ~mmap_allocator() throw() { }
        };
}

为了使用它，像下面这样声明一个STL容器:

using namespace std;
using namespace mmap_allocator_namespace;

vector<int, mmap_allocator<int> > int_vec(1024, 0, mmap_allocator<int>());

例如，每当分配内存时，就可以使用它记录日志。什么是必要的 是重新绑定结构，否则向量容器使用超类分配/释放 方法。

更新:内存映射分配器现在可以在https://github.com/johannesthoma/mmap_allocator上获得，并且是LGPL。您可以在项目中使用它。

使用自定义分配器来使用内存池而不是堆可能会很有用。这只是众多例子中的一个。

对于大多数情况，这肯定是一个不成熟的优化。但它在某些情况下(嵌入式设备、游戏等)非常有用。

在图形模拟中，我看到自定义分配器用于

std::allocator不直接支持的对齐约束。 通过为短期分配(只是这个框架)和长期分配使用单独的池来最小化碎片。

One example of I time I have used these was working with very resource constrained embedded systems. Lets say you have 2k of ram free and your program has to use some of that memory. You need to store say 4-5 sequences somewhere that's not on the stack and additionally you need to have very precise access over where these things get stored, this is a situation where you might want to write your own allocator. The default implementations can fragment the memory, this might be unacceptable if you don't have enough memory and cannot restart your program.

One project I was working on was using AVR-GCC on some low powered chips. We had to store 8 sequences of variable length but with a known maximum. The standard library implementation of the memory management is a thin wrapper around malloc/free which keeps track of where to place items with by prepending every allocated block of memory with a pointer to just past the end of that allocated piece of memory. When allocating a new piece of memory the standard allocator has to walk over each of the pieces of memory to find the next block that is available where the requested size of memory will fit. On a desktop platform this would be very fast for this few items but you have to keep in mind that some of these microcontrollers are very slow and primitive in comparison. Additionally the memory fragmentation issue was a massive problem that meant we really had no choice but to take a different approach.

So what we did was to implement our own memory pool. Each block of memory was big enough to fit the largest sequence we would need in it. This allocated fixed sized blocks of memory ahead of time and marked which blocks of memory were currently in use. We did this by keeping one 8 bit integer where each bit represented if a certain block was used. We traded off memory usage here for attempting to make the whole process faster, which in our case was justified as we were pushing this microcontroller chip close to it's maximum processing capacity.

在嵌入式系统上下文中，我还可以看到编写自己的自定义分配器的其他情况，例如，如果序列的内存不在主ram中，而在这些平台上可能经常出现这种情况。

对于共享内存来说，不仅容器头存储在共享内存中，而且容器头包含的数据也存储在共享内存中，这一点至关重要。

Boost::Interprocess的分配器就是一个很好的例子。然而，正如你在这里读到的，这个allone是不够的，要使所有STL容器共享内存兼容(由于不同进程中的映射偏移量不同，指针可能会“中断”)。