使用string vs char数组时有多大的性能差异？

Question

我有以下代码:

char fname[255] = {0}
snprintf(fname, 255, "%s_test_no.%d.txt", baseLocation, i);

VS

std::string fname = baseLocation + "_test_no." + std::to_string(i) + ".txt";

哪一个表现更好？第二个是否涉及临时创作？有没有更好的方法来做到这一点？

Answer 1

让我们运行数字:

代码(我使用PAPI计时器)

main.cpp中

#include <iostream>
#include <string>
#include <stdio.h>
#include "papi.h"
#include <vector>
#include <cmath>
#define TRIALS 10000000

class Clock
{
  public:
    typedef long_long time;
    time start;
    Clock() : start(now()){}
    void restart(){ start = now(); }
    time usec() const{ return now() - start; }
    time now() const{ return PAPI_get_real_usec(); }
};


int main()
{
  int eventSet = PAPI_NULL;
  PAPI_library_init(PAPI_VER_CURRENT);
  if(PAPI_create_eventset(&eventSet)!=PAPI_OK) 
  {
    std::cerr << "Failed to initialize PAPI event" << std::endl;
    return 1;
  }

  Clock clock;
  std::vector<long_long> usecs;

  const char* baseLocation = "baseLocation";
  //std::string baseLocation = "baseLocation";
  char fname[255] = {};
  for (int i=0;i<TRIALS;++i)
  {
    clock.restart();
    snprintf(fname, 255, "%s_test_no.%d.txt", baseLocation, i);
    //std::string fname = baseLocation + "_test_no." + std::to_string(i) + ".txt";
    usecs.push_back(clock.usec());
  }

  long_long sum = 0;
  for(auto vecIter = usecs.begin(); vecIter != usecs.end(); ++vecIter)
  {
    sum+= *vecIter;
  }

  double average = static_cast<double>(sum)/static_cast<double>(TRIALS);
  std::cout << "Average: " << average << " microseconds" << std::endl;

  //compute variance
  double variance = 0;
  for(auto vecIter = usecs.begin(); vecIter != usecs.end(); ++vecIter)
  {
    variance += (*vecIter - average) * (*vecIter - average);
  }

  variance /= static_cast<double>(TRIALS);
  std::cout << "Variance: " << variance << " microseconds" << std::endl;
  std::cout << "Std. deviation: " << sqrt(variance) << " microseconds" << std::endl;
  double CI = 1.96 * sqrt(variance)/sqrt(static_cast<double>(TRIALS));
  std::cout << "95% CI: " << average-CI << " usecs to " << average+CI << " usecs" << std::endl;  
}

玩评论以获得这种或那种方式.使用编译行在我的机器上进行两千万次迭代:

g ++ main.cpp -lpapi -DUSE_PAPI -std = c ++ 0x -O3

使用char数组:

Average: 0.240861 microseconds
Variance: 0.196387microseconds
Std. deviation: 0.443156 microseconds
95% CI: 0.240586 usecs to 0.241136 usecs

使用字符串方法:

Average: 0.365933 microseconds
Variance: 0.323581 microseconds
Std. deviation: 0.568842 microseconds
95% CI: 0.365581 usecs to 0.366286 usecs

所以至少在我的机器上使用MY代码和我的编译器设置时,我看到移动到字符串时减速约50%.使用以下公式,字符数组比字符串加速34%:

((字符串的时间) - (字符数组的时间))/(字符串的时间)

这给出了方法之间的时间差异,单独为字符串的时间百分比.我的原始百分比是正确的; 我使用字符数组方法作为参考点,当移动到字符串时显示52%的减速,但我发现它具有误导性.

我将采取任何和所有评论我是如何做错的:)

编辑:使用GCC 4.8.4编译:

串

Average: 0.338876 microseconds
Variance: 0.853823 microseconds
Std. deviation: 0.924026 microseconds
95% CI: 0.338303 usecs to 0.339449 usecs

字符数组

Average: 0.239083 microseconds
Variance: 0.193538 microseconds
Std. deviation: 0.439929 microseconds
95% CI: 0.238811 usecs to 0.239356 usecs

因此字符数组方法保持明显更快,尽管不那么快.在这些测试中,它的速度提高了约29%.