是否有一个使用 alloca 的分配器，否则是 C++符合STL标准吗？

发布于 2024-09-30 12:22:46 字数 427 浏览 0 评论 0原文

我有两个问题：

1）是否可以实现一个使用 alloca 在堆栈上分配内存并且符合 C++ STL 的分配器？

如果有代码，您只需将 URL 指向我即可让我高兴。 :-) 如果没有代码，也许您可以画出分配和释放函数的草图？

2）如果上述问题的答案是“是”，我想了解如何在堆栈上为类成员分配内存。作为一个例子，考虑一个

std::vector<int, AllocaAllocator<int> >

并假设对该向量的成员函数“resize”的调用首先调用分配器的“deallocate”，然后调用“allocate”。

调用 allocate 的范围是成员函数 resize 的范围。这是否意味着在该函数调用结束时分配的内存将从堆栈中删除？

亲切的问候，比约恩

原文

I have two questions:

1) Is it possible to implement an allocator that uses alloca to allocate memory on the stack and is otherwise C++ STL compliant?

If there is code out there, you can make me happy by simply pointing me to the URL. :-)
If there is no code out there, perhaps you can sketch the functions allocate and deallocate?

2) If the answer to the above question is 'yes', I'd like to understand how it is possible to allocate memory on the stack for class members. As an example, consider an

std::vector<int, AllocaAllocator<int> >

and suppose that a call of the member function 'resize' of this vector calls first 'deallocate' and then 'allocate' of the allocator.

The scope from which allocate is called is that of the member function resize. Doesn't this mean that the allocated memory is removed from the stack at the end of that function call?

Kind regards,
Bjoern

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过去的过去 2024-10-07 12:22:46

Bjoern，看来您从根本上误解了堆栈和分配的工作原理。阅读有关他们的信息。

您所要求的是不可能的，因为当您从分配它的函数返回时，alloca 分配的内存将被“释放”（与帕特里克所说不同，内联不能改变其行为）。我写“freed”是因为它实际上并没有被释放，它只是像任何其他堆栈变量一样超出了范围。因此之后使用它会导致未定义的行为。

假设您在 YourAllocator::allocate 中分配了一块内存，该内存是从 d.push_back() 调用的：

deque<int, AllocaAllocator> d;
d.push_back(42); // calls alloca
printf("Hello\n");
printf("%d\n", d[0]);

alloca 分配的内存可能会被以下堆栈帧覆盖push_back 和 printf，因此输出可能不是 42，它可能会崩溃，或者发生任何其他情况。

Bjoern, it looks like you fundamentally misunderstand how stack and alloca work. Read about them.

What you are asking is impossible because the memory allocated by alloca is "freed" when you return from the function that allocated it (and unlike Patrick said, inlining cannot change its behavior). I write "freed" because it's not actually freed, it just goes out of scope as any other stack variable. So using it afterwards causes undefined behavior.

Suppose you allocate a chunk of memory in YourAllocator::allocate which is called from d.push_back():

deque<int, AllocaAllocator> d;
d.push_back(42); // calls alloca
printf("Hello\n");
printf("%d\n", d[0]);

The memory allocated by alloca may be overwritten by the stack-frames of push_back and printf, so the output may not be 42, it may crash, or any other thing.

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