c++自定义对象支持按下标遍历

#include<iostream>
#include<string>
using namespace std;
class Array
{
public:
Array(){length = 0; num = NULL;};
Array(int n);
int & operator[]( int );
const int & operator[]( int )const;
int getlength() const {return length;}
private:
int length;
int * num;
};
Array::Array(int n)
{
try
{
num = new int[n];
}
catch(bad_alloc)
{
cerr<<"allocate storage failure!"<<endl;
throw;
}
length = n;
}
int& Array::operator[](int i)
{
if(i < 0 || i >= length)
throw string("out of bounds");
return num[i];
}
const int & Array::operator[](int i) const
{
if(i < 0 || i >= length)
throw string("out of bounds");
return num[i];
}
int main()
{
Array A(5);
int i;
try
{
for(i = 0; i < A.getlength(); i++)
A[i] = i;
for(i = 0 ;i < 6; i++ )
cout<< A[i] <<endl;
}
catch(string s)
{
cerr<< s <<", i = "<< i <<endl;
}
return 0;
}