c++ - 容器使用的内部类型的内存分配

Question

C++11 标准在一般容器要求中有以下几行。

(23.2.1 - 3)

For the components affected by this subclause that declare an allocator_type, objects stored in these components shall be constructed using the allocator_traits::construct function and destroyed using the allocator_traits::destroy function (20.6.8.2). These functions are called only for the container’s element type, not for internal types used by the container

(23.2.1 - 7)

Unless otherwise specified, all containers defined in this clause obtain memory using an allocator

容器使用的所有内存都是由指定的分配器分配的，对不对？因为标准说内部类型不是用 allocator_traits::construct 构造的，所以应该有某种对 operator new 的调用。但是标准也说，这个条款中定义的所有容器都使用分配器获取内存，这在我看来意味着它不能是普通的 new 操作符，它必须是 placement new 操作符。我说得对吗？

让我举个例子，说明为什么这很重要。

假设我们有一个类，它拥有一些分配的内存:

#include <unordered_map>
#include <iostream>
#include <cstdint>
#include <limits>
#include <memory>
#include <new>

class Arena
{
public:
        Arena(std::size_t size)
        {
                size_     = size;
                location_ = 0;

                data_ = nullptr;
                if(size_ > 0)
                        data_ = new(std::nothrow) uint8_t[size_];
        }
        Arena(const Arena& other) = delete;
        ~Arena()
        {
                if(data_ != nullptr)
                        delete[] data_;
        }
        Arena& operator =(const Arena& arena) = delete;

        uint8_t* allocate(std::size_t size)
        {
                if(data_ == nullptr)
                        throw std::bad_alloc();

                if((location_ + size) >= size_)
                        throw std::bad_alloc();

                uint8_t* result = &data_[location_];
                location_ += size;
                return result;
        }

        void clear()
        {
                location_ = 0;
        }

        std::size_t getNumBytesUsed() const
        {
                return location_;
        }

private:
        uint8_t* data_;
        std::size_t location_, size_;

};

我们还有自定义分配器:

template <class T> class FastAllocator
{
public:
        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;

        template <class U> class rebind
        {
        public:
                typedef FastAllocator<U> other;

        };

        Arena* arena;

        FastAllocator(Arena& arena_): arena(&arena_) {}
        FastAllocator(const FastAllocator& other): arena(other.arena) {}
        template <class U> FastAllocator(const FastAllocator<U>& other): arena(other.arena) {}

        //------------------------------------------------------------------------------------
        pointer allocate(size_type n, std::allocator<void>::const_pointer)
        {
                return allocate(n);
        }
        pointer allocate(size_type n)
        {
                return reinterpret_cast<pointer>(arena->allocate(n * sizeof(T)));
        }

        //------------------------------------------------------------------------------------
        void deallocate(pointer, size_type) {}

        //------------------------------------------------------------------------------------
        size_type max_size() const
        {
                return std::numeric_limits<size_type>::max();
        }

        //------------------------------------------------------------------------------------
        void construct(pointer p, const_reference val)
        {
                ::new(static_cast<void*>(p)) T(val);
        }
        template <class U> void destroy(U* p)
        {
                p->~U();
        }

};

我们是这样使用它的:

typedef std::unordered_map<uint32_t, uint32_t, std::hash<uint32_t>, std::equal_to<uint32_t>,
                           FastAllocator<std::pair<uint32_t, uint32_t>>> FastUnorderedMap;

int main()
{
        // Allocate memory in arena
        Arena arena(1024 * 1024 * 50);
        FastAllocator<uint32_t> allocator(arena);
        FastAllocator<std::pair<uint32_t, uint32_t>> pairAllocator(arena);
        FastAllocator<FastUnorderedMap> unorderedMapAllocator(arena);

        FastUnorderedMap* fastUnorderedMap = nullptr;

        try
        {
                // allocate memory for unordered map
                fastUnorderedMap = unorderedMapAllocator.allocate(1);

                // construct unordered map
                fastUnorderedMap =
                        new(reinterpret_cast<void*>(fastUnorderedMap)) FastUnorderedMap
                        (
                                0,
                                std::hash<uint32_t>(),
                                std::equal_to<uint32_t>(),
                                pairAllocator
                        );

                // insert something
                for(uint32_t i = 0; i < 1000000; ++i)
                        fastUnorderedMap->insert(std::make_pair(i, i));
        }
        catch(std::bad_alloc badAlloc)
        {
                std::cout << "--- BAD ALLOC HAPPENED DURING FAST UNORDERED MAP INSERTION ---" << std::endl;
        }

        // no destructor of unordered map is called!!!!
        return 0;
}

如您所见，从未调用过 unordered_map 的析构函数，但在销毁 arena 对象期间释放了内存。会不会有内存泄漏，为什么？

如果能就此主题提供任何帮助，我将不胜感激。

Answer 1

分配器应该提供 4 个函数(这里很有趣):

• 2用于内存管理:allocate/deallocate
• 2用于对象生命周期管理:construct/destroy

您引用中的这些函数仅适用于construct 和destroy(在上一句中提到)，不适用于allocate/deallocate，因此不矛盾。

现在，关于内存泄漏，要让 arena 分配器工作，不仅容器中的对象应该使用 arena 分配器(容器保证)构建，而且那些对象分配的所有内存都应该也可以从此分配器获得；不幸的是，这可能会变得稍微复杂一些。