四种方式
- 下标
- 迭代器
- auto迭代器
- for range
#include <iostream>
#include <vector>
#include <stdint.h>
#include <ctime>
int main()
{
const uint32_t loop = 10000000;
std::vector<int32_t> vec;
clock_t timeStart = 0;
for (uint32_t i = 0; i < loop; ++i)
{
vec.push_back(i);
}
// test time use
// 1.by index
timeStart = clock();
uint64_t sum1 = 0;
for (uint32_t i = 0; i < vec.size(); ++i)
{
sum1+=vec[i];
}
std::cout << sum1<<" "<< clock() - timeStart << "ms" << std::endl;
// 2.by iterator
timeStart = clock();
uint64_t sum2 = 0;
for (std::vector<int32_t>::const_iterator it = vec.begin(); it != vec.end(); ++it)
{
sum2 +=*it;
}
std::cout << sum2 << " " << clock() - timeStart << "ms" << std::endl;
// 3.by auto iterator
uint64_t sum3 = 0;
timeStart = clock();
for (auto it = vec.begin(); it != vec.end(); ++it)
{
sum3 += *it;
}
std::cout << sum3 << " " << clock() - timeStart << "ms" << std::endl;
// 4.by for range
uint64_t sum4 = 0;
timeStart = clock();
for (const auto & it :vec)
{
sum4 += it;
}
std::cout << sum4 << " " << clock() - timeStart << "ms" << std::endl;
return 0;
}
VS2019
debug
loop=1kw
49999995000000 354ms
49999995000000 3867ms
49999995000000 4089ms
49999995000000 18ms 为什么这种写法最快？
forrange最快。index比iterator快10倍，auto iterator略慢于iterator。
release
loop=1kw
49999995000000 12ms
49999995000000 13ms
49999995000000 14ms
49999995000000 4ms
forrange仍然最快，其他3个差不多，约为forrange的3倍
loop=10kw
4999999950000000 118ms
4999999950000000 130ms
4999999950000000 129ms
4999999950000000 33ms
同上
gcc 4.8.5
debug
loop=1kw
g++ -DDEBUG -std=c++11 -g main.cpp
./a.out
49999995000000 80ms 为什么这种写法最快？
49999995000000 240ms
49999995000000 240ms
49999995000000 180ms
index最快
loop=10kw
g++ -DDEBUG -std=c++11 -g main.cpp
./a.out
4999999950000000 740ms
4999999950000000 2350ms
4999999950000000 2450ms
4999999950000000 1800ms
同上
release
loop=10kw
g++ -std=c++11 -O2 main.cpp
4999999950000000 50ms
4999999950000000 40ms
4999999950000000 40ms
4999999950000000 40ms
4种写法都差不多
结论
优化后，vs for range最快，gcc 4种写法几乎没区别。
for range遍历vector是最快的一种写法。跨平台时，优先选for range。
尽量用新版本特性，语法简单，性能也好。