STL vector清空元素的三种方法

目录

• 1. clear()方法
• 2. erase方法
• 3. swap方法

1. clear()方法

作用:
清空元素，但不回收空间。
vector有两个参数： 1.size，表示当前vector容器内存储的元素个数; 2. capacity，表示当前vector在内存中申请的这片区域所能容纳的元素个数。

通常capacity会比size大，如果往vector中push_back数据，这样就不用重新申请内存和拷贝元素到新内存区域了，便于节省时间。
所以vector.clear()的真正作用是：把size设置成0，capacity不变。代码示例：

/*------------------------------------------代码--------------------------------------------------*/
#include <iostream>
#include <fstream>
#include <memory>
#include <vector>
#include <string.h>

using namespace std;

int main()
{
    std::vector<double> vecnum;
    cout << "init: size = " << vecnum.size() << "; capacity = " << vecnum.capacity() << endl;
    vecnum.push_back(1.111);
    vecnum.push_back(2.222);
    vecnum.push_back(3.333);
    vecnum.push_back(4.333);
    cout << "after push_back: size = " << vecnum.size() << "; capacity = " << vecnum.capacity() << endl;
    vecnum.clear();
    cout << "after clear: size = " << vecnum.size() << "; capacity = " << vecnum.capacity() << endl;

    return 0;
}
// 结果
init: size = 0; capacity = 0
after push_back: size = 4; capacity = 4
after clear: size = 0; capacity = 4
/*------------------------------------------代码--------------------------------------------------*/

2. erase方法

作用:清空元素，但不回收空间

/*------------------------------------------代码--------------------------------------------------*/
#include <iostream>
#include <fstream>
#include <memory>
#include <vector>
#include <string.h>

using namespace std;

int main()
{
    std::vector<double> vecnum;
    cout << "init: size = " << vecnum.size() << "; capacity = " << vecnum.capacity() << endl;
    vecnum.push_back(1.111);
    vecnum.push_back(2.222);
    vecnum.push_back(3.333);
    vecnum.push_back(4.333);
    cout << "after push_back: size = " << vecnum.size() << "; capacity = " << vecnum.capacity() << endl;
    vector<double>::iterator it;
    for (it = vecnum.begin(); it != vecnum.end(); )
    {
       //注意使用erase()方法时，迭代不要随意的++
       it = vecnum.erase(it);
    }
    cout << "after erase: size = " << vecnum.size() << "; capacity = " << vecnum.capacity() << endl;
    return 0;
}
// 结果
init: size = 0; capacity = 0
after push_back: size = 4; capacity = 4
after clear: size = 0; capacity = 4
/*------------------------------------------代码--------------------------------------------------*/

注意：erase在每次操作时，迭代器指针会整体前移1，就是每次都会移动全部数据，所以vector不适合做频繁删除的容器

3. swap方法

作用：清除元素并回收内存
/*------------------------------------------代码--------------------------------------------------*/
#include <iostream>
#include <fstream>
#include <memory>
#include <vector>
#include <string.h>

using namespace std;

int main()
{
    std::vector<double> vecnum;
    cout << "init: size = " << vecnum.size() << "; capacity = " << vecnum.capacity() << endl;
    vecnum.push_back(1.111);
    vecnum.push_back(2.222);
    vecnum.push_back(3.333);
    vecnum.push_back(4.333);
    cout << "after push_back: size = " << vecnum.size() << "; capacity = " << vecnum.capacity() << endl;
    vector<double>().swap(vecnum);
    cout << "after swap: size = " << vecnum.size() << "; capacity = " << vecnum.capacity() << endl;
    return 0;
}
// 结果
init: size = 0; capacity = 0
after push_back: size = 4; capacity = 4
after swap: size = 0; capacity = 0
/*------------------------------------------代码--------------------------------------------------*/