2015-第16周项目1-平方根中的异常

#include<iostream>
#include<cmath>
using namespace std;
double squareroot(double &a)
{
double n;
if(a>=0)
n=sqrt(a);
else
throw a;
return n;
}

int main()
{
double m;
try
{
cout<<"请输入一个需开方的数："<<endl;
cin>>m;
cout<<m<<" 的平方根为 "<<squareroot(m)<<endl<<endl;
}
catch(double)
{
cout<<"输入的数为负数，无法开方！"<<endl<<endl;
}
return 0;
}