C++中获得vector中最后一个元素，使用迭代器.end()，和使用size()取得长度，哪个效率更高？

Question

如
vector v;
int l=v.size();
v[l-1];
和
vector v;
auto p＝v.end();
p--;

Answer 1

size()取得长度的效率更高

#include <iostream>
#include <vector>
#include<ctime>
using namespace std;

int main()
{
    vector <int> vec;
    int i = 0;
    for (i = 0; i < 100000; ++i)
        vec.push_back(i);

    clock_t startTime, endTime;
    startTime = clock();//计时开始
    for(int i = 0; i < 10000; ++i)
        int l = vec.size();
    endTime = clock();//计时结束
    cout << "vec.size运行10000次的时间为: " << (double)(endTime - startTime) / CLOCKS_PER_SEC << "s" << endl;

    startTime = clock();//计时开始
    for (int i = 0; i < 10000; ++i)
        vector<int> ::iterator p = vec.end();
    endTime = clock();//计时结束
    cout << "vec.end运行10000次的时间为: " << (double)(endTime - startTime) / CLOCKS_PER_SEC << "s" << endl;

    system("pause");
    return 0;
}