用 vector::iterator 或 at() 迭代 STL 向量有什么更快的方法?

2022-01-10 00:00:00 performance iterator vector c++ stl

在性能方面，什么会更快?有区别吗?它依赖于平台吗?

In terms of performance, what would work faster? Is there a difference? Is it platform dependent? 

//1. Using vector<string>::iterator:
vector<string> vs = GetVector();

for(vector<string>::iterator it = vs.begin(); it != vs.end(); ++it)
{
   *it = "Am I faster?";
}

//2. Using size_t index:
for(size_t i = 0; i < vs.size(); ++i)
{
   //One option:
   vs.at(i) = "Am I faster?";
   //Another option:
   vs[i] = "Am I faster?";
}

推荐答案

为什么不写个测试看看呢?

Why not write a test and find out?

我的错 - 我以为我正在计时优化版本，但不是.在我的机器上，使用 g++ -O2 编译，迭代器版本比 operator[] 版本略慢，但可能不会明显.

My bad - I thought I was timing the optimised version but wasn't. On my machine, compiled with g++ -O2, the iterator version is slightly slower than the operator[] version, but probably not significantly so.

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

int main() {
    const int BIG = 20000000;
    vector <int> v;
    for ( int i = 0; i < BIG; i++ ) {
        v.push_back( i );
    }

    int now = time(0);
    cout << "start" << endl;
    int n = 0;
    for(vector<int>::iterator it = v.begin(); it != v.end(); ++it) {
        n += *it;
    }

    cout << time(0) - now << endl;
    now = time(0);
    for(size_t i = 0; i < v.size(); ++i) {
        n += v[i];
    }
    cout << time(0) - now << endl;

    return n != 0;
}

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