c – 我们应该为函数的返回值使用临时变量吗？

ClassA a = function1();
ClassB b = function2(a);
function3(b);

function3(function2(function1()));

#include <iostream>
#include <ctime>
#include <math.h>
using namespace std;

int main()
{
   clock_t start = clock();
   clock_t ends = clock();

   // Case 1.
   start = clock();
   for (int i=0; i<10000000; i++)
   {
      double a = cos(1);
      double b = pow(a,2);
      sqrt(b);
   }
   ends = clock();
   cout << (doublE) (ends - start) / CLOCKS_PER_SEC << endl;

   // Case 2.
   start = clock();
   for (int i=0; i<10000000; i++)
      sqrt(pow(cos(1),2));
   ends = clock();
   cout << (doublE) (ends - start) / CLOCKS_PER_SEC << endl;
   return 0;
}

如果您想要计算紧缩和数字变得更加一致,请打破这个全面的优化.确保获得适当值的代码实际上是运行而不是完全抛出的,我已将两个测试中的结果分配给一个易变的本地(这不是完全正确的使用volatile,而是确保只有价值创造是显着的三角洲).

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

int main()
{
    clock_t start;
    volatile double val;

    for (int j=1;j<=10;j++)
    {
        // Case 1.
        start = clock();
        for (int i=0; i<2000000; i++)
        {
            double a = cos(1);
            double b = pow(a,2);
            val = sqrt(b);
        }
        cout << j << ':' << (doublE) (clock() - start) / CLOCKS_PER_SEC << endl;

        // Case 2.
        start = clock();
        for (int i=0; i<2000000; i++)
            val = sqrt(pow(cos(1),2));
        cout << j << ':' << (doublE) (clock() - start) / CLOCKS_PER_SEC << endl << endl;
    }
    return 0;
}

在我的MACBook Air上生成以下版本编译输出(这是没有任何延伸的速度恶魔)：

1:0.001465
1:0.001305

2:0.001292
2:0.001424

3:0.001297
3:0.001351

4:0.001366
4:0.001342

5:0.001196
5:0.001376

6:0.001341
6:0.001303

7:0.001396
7:0.001422

8:0.001429
8:0.001427

9:0.001408
9:0.001398

10:0.001317
10:0.001353