wall_clock
用途：wall_clock函数用于计时
类型：普通函数
示例代码如下：
wall_clock timer;
mat A = randu<mat>(100,100);
mat B = randu<mat>(100,100);
mat C;
timer.tic();
for(uword i=0; i<10000; ++i)
{
C = A + B + A + B;
}
double n = timer.toc();
cout << "number of seconds: " << n << endl;运行结果：
number of seconds: 5.776
请按任意键继续. . .