[开发工具]C++标准库中string类遍历方式的性能差距

没有废话，直接上测试代码

#include <chrono>
#include <string>
#include <iostream>
#include <functional>

using namespace std;
using namespace chrono;

static uint64_t size = 12550820;
static string test_str(size, 'S');

uint64_t test(function<void()>& target) {
    auto time_begin = system_clock::now();
    target();
    auto time_end = system_clock::now();
    return static_cast<uint64_t>(duration_cast<microseconds>(time_end - time_begin).count());
}

int main() {
    std::function<void()> for_f = []()->void{
        uint64_t test = 0;
        for(char mychar : test_str) {
            test++;
        }
    };
    
    std::function<void()> normal_iterator_f = []()->void{
        uint64_t test = 0;
        for(auto i = test_str.begin(); i != test_str.end(); i++) {
            test++;
        }
    };
    
    std::function<void()> reverse_iterator_f = []()->void{
        uint64_t test = 0;
        for(auto i = test_str.rbegin(); i != test_str.rend(); i++) {
            test++;
        }
    };
    
    std::function<void()> const_iterator_f = []()->void{
        uint64_t test = 0;
        for(string::const_iterator i = test_str.begin(); i != test_str.end(); i++) {
            test++;
        }
    };
    
    std::function<void()> normal_cstyle_f = []()->void{
        uint64_t test = 0;
        char* char_ptr = const_cast<char*>(test_str.c_str());
        for(uint64_t i = 0; i < test_str.size(); i++) {
            char_ptr++;
            test++;
        }
    };
    
    std::function<void()> reverse_cstyle_f = []()->void{
        uint64_t test = 0;
        char* char_ptr = const_cast<char*>(test_str.c_str());
        char_ptr += test_str.size();
        for(uint64_t i = 0; i < test_str.size(); i++) {
            char_ptr--;
            test++;
        }
    };
    
    std::function<void()> index_f = []()->void{
        uint64_t test = 0;
        char mychar;
        for(uint64_t i = 0; i < test_str.size(); i++) {
            test++;
            mychar = test_str[i];
        }
    };
    
    cout << "(单位:毫秒)" << endl;
    cout << "标准for遍历用时 \t" << test(for_f) << endl;
    cout << "标准iterator用时 \t" << test(normal_iterator_f) << endl;
    cout << "反向iterator用时 \t" << test(reverse_iterator_f) << endl;
    cout << "常量iterator用时 \t" << test(const_iterator_f) << endl;
    cout << "标准C风格遍历用时 \t" << test(normal_cstyle_f) << endl;
    cout << "反向C风格遍历用时 \t" << test(reverse_cstyle_f) << endl;
    cout << "下标访问用时 \t" << test(index_f) << endl;
    
    return 0;
}

运行结果

~/build $ g++ 
~/app/potato/worldbox/util/speedtest.cc                                                     
~/build $ ./a.out                                                                                         
(单位:毫秒)                                                                                               
标准for遍历用时         475926                                                                            
标准iterator用时        1530713                                                                           
反向iterator用时        1503410                                                                           
常量iterator用时        1529692                                                                           
标准C风格遍历用时       537057                                                                            
反向C风格遍历用时       536231                                                                            
下标访问用时    1058003                                                                                   
~/build $