是否允许vector :: shrink_to_fit重新分配？ [英] Is vector::shrink_to_fit allowed to reallocate?

问题描述

此成员函数在标准中没有定义的作用（仅备注），如果不允许重新分配，则其使用将受到限制。但是我在标准中发现的唯一适用于23.2.1 / 11的段落：

This member function, that has no defined effect in the standard (only remarks), would have limited use if not allowed to reallocate. But the only paragraph I found in the standard that seems to apply would be 23.2.1/11:

或通过在其他函数方面定义一个函数），调用容器成员函数或将容器作为参数传递给库函数，都不应使对该容器内对象的迭代器或更改其值无效。

"Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container."

由于重新分配使迭代器无效，这是否意味着shrink_to_fit无法正式重新分配？如果是这样，则该函数仅对减少容量不会重新分配的实现有效（如果有）...

Since reallocations invalidate iterators, would this imply that shrink_to_fit cannot formally reallocate? If so, the function could only have effect on implementations where reducing capacity does not reallocate, if any...

推荐答案

我同意，其目的显然是 shrink_to_fit（）可以重新分配，因此应该允许它使迭代器无效。

I agree, the intention is obviously that shrink_to_fit() can reallocate, so it should be allowed to invalidate iterators.

它仅在实现为分配器提供类似于 realloc（）的功能的情况下才可能兑现请求，该功能可确保在缩小块时不移动内存，这不会使内存无效迭代器。但是，我不认为这是我们的意图，正如最近添加的要求 T 是向量中的 MoveInsertable ，因为该要求仅在元素移至新元素时才有意义

It would be possible to only honour the request if the implementation provides a realloc()-like feature for allocators that guaranteed not to move the memory when shrinking the block, which would not invalidate iterators. But I don't believe that's what's intended, as implied by the recently-added requirement that T is MoveInsertable into the vector, because that requirement is only relevant if the elements are moved to new locations, which would invalidate iterators.

我认为您应该提交问题以澄清问题， shrink_to_fit（）会使迭代器无效。该问题将适用于 basic_string ，双端队列和 vector 。

I think you should submit an issue to get it clarified that shrink_to_fit() can invalidate iterators. The issue would apply to basic_string, deque and vector.

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