标准是如何说明如何调整向量的清晰度改变容量？ [英] What does the standard say about how calling clear on a vector changes the capacity?

问题描述

此网站意味着清除向量可以更改容量：

This website implies that clearing a vector MAY change the capacity:

http://en.cppreference.com/w/cpp/container/vector/clear

许多实现不会在调用
之后释放分配的内存给clear（），有效地使向量
的capacity（）保持不变。

Many implementations will not release allocated memory after a call to clear(), effectively leaving the capacity() of the vector unchanged.

但是根据@JamesKanze，这是错误的，标准的任务明确将不会改变容量。

But according to @JamesKanze this is wrong and the standard mandates that clear will not change capacity.

标准说什么？ p>

What does the standard say?

推荐答案

取决于您正在查看的标准版本，
清除定义为 erase（begin（），end（））或（在C ++ 11中）的等价物：

破坏a中的所有元素。使所有
引用，指针和迭代器无效，引用
a的元素，并可能使
过去的迭代器无效。

Depending on the version of the standard you are looking at, clear is defined as the equivalent of erase(begin(), end()), or (in C++11):
"Destroys all elements in a. Invalidates all references, pointers, and iterators referring to the elements of a and may invalidate the past-the-end iterator."

在这两种情况下都不允许修改
的容量;以下代码由
标准保证安全：

In neither case is it allowed to modify the capacity; the following code is guaranteed safe by the standard:

std::vector<int> v;
for (int i = 0; i != 5; ++ i) {
    v.push_back(i);
}
assert(v.capacity() >= 5);
v.clear();
assert(v.capacity() >= 5);
v.push_back(10);
v.push_back(11);
std::vector<int>::iterator i = v.begin() + 1;
v.push_back(12);
v.push_back(13);
*i = 42;        //  i must still be valid, because none of 
                //  the push_back would have required an
                //  increase of capacity

（C ++ 11中的措辞改变的原因：委员会
不想要 MoveAssignable clear ，如果按 erase 定义，
就是这种情况。

(The reason for the change in wording in C++11: the committee didn't want to require MoveAssignable for clear, which would have been the case if it were defined in terms of erase.)

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