Write code to detect whether or not a given number is prime

Write code to detect whether or not a given number is prime

bool foo(int num)
{
 if(num<=1) return false;
 if(num==2) return true;
 if(!num%2) return false;
 for(int i=3;i < sqrt(num);i++)
  {if(!num%i)return false;}
 return true;
}

Better solution, see wiki