std::vector shrink_to_fit() 方法

自 C++20 起

// Non const version only
constexpr void shrink_to_fit();

自 C++11 起

// Non const version only
void shrink_to_fit();

请求移除未使用的容量。这是一个非强制性请求，旨在将capacity()减少到size()。请求是否被满足取决于实现。

失效

如果发生重新分配，所有迭代器（包括past-the-end 迭代器）以及所有对元素的引用都将失效。

如果未发生重新分配，则没有迭代器或引用失效。

参数

(无)

类型要求

• T （容器的元素类型）必须满足 MoveInsertable 的要求。

返回值

(无)

复杂度

最多与容器大小线性相关 - O(size())。

异常

如果抛出的异常不是由 T 的移动构造函数引起的，则没有副作用（强异常保证）。

示例

Main.cpp

#include <iostream>
#include <vector>
 
int main()
{
    std::vector<int> v;
    std::cout << "Default-constructed capacity is " << v.capacity() << '\n';
    v.resize(100);
    std::cout << "Capacity of a 100-element vector is " << v.capacity() << '\n';
    v.resize(50);
    std::cout << "Capacity after resize(50) is " << v.capacity() << '\n';
    v.shrink_to_fit();
    std::cout << "Capacity after shrink_to_fit() is " << v.capacity() << '\n';
    v.clear();
    std::cout << "Capacity after clear() is " << v.capacity() << '\n';
    v.shrink_to_fit();
    std::cout << "Capacity after shrink_to_fit() is " << v.capacity() << '\n';
    for (int i = 1000; i < 1300; ++i)
        v.push_back(i);
    std::cout << "Capacity after adding 300 elements is " << v.capacity() << '\n';
    v.shrink_to_fit();
    std::cout << "Capacity after shrink_to_fit() is " << v.capacity() << '\n';
}

可能输出

Default-constructed capacity is 0
Capacity of a 100-element vector is 100
Capacity after resize(50) is 100
Capacity after shrink_to_fit() is 50
Capacity after clear() is 50
Capacity after shrink_to_fit() is 0
Capacity after adding 300 elements is 512
Capacity after shrink_to_fit() is 300