调用 std::vector::clear() 会将 std::vector::capacity() 设置

问题描述

如果我在向量上使用 .reserve(items)，向量将分配足够的内存来让我猜测我需要的项目数量.

If I use .reserve(items) on a vector, the vector will allocate enough memory for my guess of the number of items that I'll need.

如果我以后使用 .clear()，这只是清除向量还是保存我之前定义的储备?

If I later on use .clear(), will that just clear the vector or save my earlier defined reserve?

谢谢.

推荐答案

指定 std::vector::clear() 影响大小.它可能不会影响容量.要重置容量，请使用交换技巧:

It is specified that std::vector<T>::clear() affects the size. It might not affect the capacity. For resetting the capacity, use the swap trick:

    std::vector<int> v1;

    // somehow increase capacity

    std::vector<int>().swap(v1);

注意:由于这个旧答案仍然得到赞成(因此人们阅读它)，我觉得有必要添加C++11 添加了 std::vector<...>::shrink_to_fit()，它请求向量删除未使用的容量.

Note: Since this old answer is still getting upvotes (thus people read it), I feel the need to add that C++11 has added std::vector<...>::shrink_to_fit(), which requests the vector to remove unused capacity.

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