C ++有状态分配器解除分配问题 [英] C++ stateful allocator de-allocate issues

问题描述

这个问题是我对标准如何使用自定义分配器的误解。我有一个有状态的分配器，它保持分配的块的向量。此向量在分配时被推入，并在解除分配期间进行搜索。

This issue is my misunderstanding of how the standard is using my custom allocator. I have a stateful allocator that keeps a vector of allocated blocks. This vector is pushed into when allocating and searched through during de-allocation.

从我的调试看来，对象的不同实例（这是不同的）在取消分配时被调用。一个示例可能是调用MyAllocator（this * = 1）分配20个字节，然后一段时间后调用MyAllocator（this * = 2）取消分配先前分配的20个字节。显然，MyAllocator（this * = 2）中的向量不包含其他分配器分配的20字节块，因此无法取消分配。我的理解是C ++ 11允许有状态分配器，这是怎么回事，以及如何解决此问题？

From my debugging it appears that different instances of my object (this*'s differ) are being called on de-allocation. An example may be that MyAllocator (this* = 1) is called to allocate 20 bytes, then some time later MyAllocator (this* = 2) is called to de-allocate the 20 bytes allocated earlier. Abviously the vector in MyAllocator (this* = 2) doesn't contain the 20 byte block allocated by the other allocator so it fails to de-allocate. My understanding was that C++11 allows stateful allocators, what's going on and how do i fix this?

我已经将运算符==设置为仅在以下情况下返回true这==& rhs

I already have my operator == set to only return true when this == &rhs

伪代码：

template<typename T>
class MyAllocator
{
    ptr allocate(int n)
    {
       ...make a block of size sizeof(T) * n
       blocks.push_back(block);
       return (ptr)block.start;
    }

    deallocate(ptr start, int n)
    {
        /*This fails because the the block array is not the
        same and so doesn't find the block it wants*/ 
        std::erase(std::remove_if(blocks.begin,blocks.end, []()
        {
            return block.start >= (uint64_t)ptr && block.end <= ((uint64_t)ptr + sizeof(T)*n);
        }), blocks.end);
    }

    bool operator==(const MyAllocator& rhs)
    {
        //my attempt to make sure internal states are same
        return this == &rhs;
    }
private:
  std::vector<MemoryBlocks> blocks;
}

我在gcc上将此分配器用于std :: vector。据我所知，没有发生任何奇怪的重新绑定的事情

Im using this allocator for an std::vector, on gcc. So as far as i know no weird rebind stuff is going on

推荐答案

正如@Igor所述，分配器必须是可复制的。重要的是，尽管它们必须在副本之间共享状态，即使它们是从副本中复制的也是如此。在这种情况下，修复很容易，我按照建议将块向量设为了shared_ptr，然后将所有对该向量的更新复制到同一向量上，因为它们都指向同一事物。

As @Igor mentioned, allocators must be copyable. Importantly though they must share their state between copies, even AFTER they have been copied from. In this case the fix was easy, i made the blocks vector a shared_ptr as suggested and then now on copy all the updates to that vector occur to the same vector, since they all point to the same thing.

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