C++ STL编程 vector的插入、删除、扩容机制、随机访问和内存交换

vector有两种插入方式，见以下代码

#include <iostream>
#include <vector>
using namespace std;
/*
插入方式
1 push_back
2 insert
*/
int main() {vector<int> v;for (int i = 0; i < 10; ++i) {v.push_back(i);}v.insert(v.begin(), 888);return 0;
}

vector有三种删除方式，代码见下

#include <iostream>
#include <vector>
using namespace std;
/*
删除方式
1 pop_back
2 erase
3 clear
*/
int main() {vector<int> v = { 9, 8, 5, 221 };v.pop_back();vector<int>::iterator it = v.erase(v.begin() + 1);cout << *it << endl;v.clear();return 0;
}

vector扩容机制，代码及详细见下：

#include <iostream>
#include <vector>
using namespace std;void printVector(vector<int>& v) {for (vector<int>::iterator iter = v.begin(); iter != v.end(); iter++) {cout << *iter << " ";}cout << endl;
}/*
容量 capacity()
大小 size()
*/int main() {vector<int> v = { 8, 8, 8, 9};printVector(v);cout << v.size() << endl;// 如果是size等于capacity的话，下一次push_back一个元素后，capacity就会扩容到原先的1.5倍。cout << v.capacity() << endl;v.push_back(7);cout << "size: " << v.size() << endl;cout << "capacity" << v.capacity() << endl;v.push_back(3);cout << "size: " << v.size() << endl;cout << "capacity" << v.capacity() << endl;v.push_back(1);cout << "size: " << v.size() << endl;cout << "capacity" << v.capacity() << endl;v.push_back(4);cout << "size: " << v.size() << endl;cout << "capacity" << v.capacity() << endl;//当capacity为9时 9*1.5=13.5 他会下取整，取capacity为13for (int i = 0; i < 3; ++i) {v.push_back(i);}cout << "size: " << v.size() << endl;cout << "capacity" << v.capacity() << endl;v.resize(18);cout << "size: " << v.size() << endl;cout << "capacity" << v.capacity() << endl;v.resize(20, 6);cout << "size: " << v.size() << endl;cout << "capacity" << v.capacity() << endl;v.resize(100);// 如果发现size远远小于传入的，就直接变成那个对应sizecout << "size: " << v.size() << endl;cout << "capacity" << v.capacity() << endl;v.resize(5);// 如果是缩容的情况，其实capacity不会缩回去的。cout << "size: " << v.size() << endl;cout << "capacity" << v.capacity() << endl;return 0;
}

运行结果截图见下：

原理可见以下截图

vector随机访问，代码见下：

#include <iostream>
#include <vector>
using namespace std;void printVector(vector<int>& v) {for (vector<int>::iterator iter = v.begin(); iter != v.end(); iter++) {cout << *iter << " ";}cout << endl;
}/*
容量 capacity()
大小 size()
*/int main() {vector<int> v = { 1, 2, 3, 9};cout << v[2] << endl; //用中括号不进行下标检测，下标越界不会报错，这个访问效率高cout << v.at(2) << endl;// 这个越界会报错，但是访问效率低cout << "front: " << v.front() << endl;cout << "back: " << v.back() << endl;return 0;
}

vector的内存交换，代码及详细见下：

#include <iostream>
#include <vector>
using namespace std;void printVector(vector<int>& v) {for (vector<int>::iterator iter = v.begin(); iter != v.end(); iter++) {cout << *iter << " ";}cout << endl;
}int main() {vector<int> v1 = { 1, 2, 3, 4, 5 };vector<int> v2 = { 9, 8, 7, 6, 5 };// 1 内存交换cout << "v1: ";printVector(v1);cout << "v2: ";printVector(v2);v1.swap(v2);cout << "v1: ";printVector(v1);cout << "v2: ";printVector(v2);// 2 缩容v1.resize(1000);v1.resize(5);cout << "v1.capacity(): " << v1.capacity() << endl;vector<int>(v1).swap(v1); // vector<int>(v1) 这个是一个匿名对象，直接进行拷贝构造，然后将v1数组拷贝到对应位置cout << "v1.capacity(): " << v1.capacity() << endl;vector<int> x(v1);//如果是拷贝构造过程中，capacity设置的值过大没有意义，它是直接看size的cout << "x.capacity(): " << x.capacity() << endl;// 3 内存清理v2.resize(10002);v2.clear();// 不会清理内存，只是清理size内容cout << "v2.capacity(): " << v2.capacity() << endl;vector<int>({}).swap(v2);// 这里才是清理内存cout << "v2.capacity(): " << v2.capacity() << endl;return 0;
}