关于#C++程序填空#的问题，如何解决？

这个代码怎么完善成对的呀

#include <iostream>
using namespace std;
class IndexError{};
template 

class ARRAY
{
    size_t m_size;
    T *m_ptr;
public:
    ARRAY(size_t size) : m_size(size)
    {    
        m_ptr = new T[size];    
        memset(m_ptr, 0, size*sizeof(int));
    }
    ~ARRAY()
    {    
        delete[] m_ptr;    
    }
    T& at(int index);
};

template <typename T>
::at(int index)  
{
    if(index<0||
)  
    {
        
 IndexError();  
    }
    return m_ptr[index];
}

int main()
{
    ARRAY<int> a(50);
    int i;
    cin >> i;
    
  
    {
        for(int j=0;j<i;j++)
            a.at(i) = j;
    }
    catch(IndexError e)
    {
        return 0;
    }
    return 0;
}

#include <iostream>
#include <cstring>
using namespace std;
class IndexError : public exception
{
public:
    const char *what()
    {
        return "下标越界异常";
    }
};
template <typename T>
class ARRAY
{
    size_t m_size;
    T *m_ptr;

public:
    ARRAY(size_t size) : m_size(size)
    {
        m_ptr = new T[size];
        memset(m_ptr, 0, size * sizeof(int));
    }
    ~ARRAY()
    {
        delete[] m_ptr;
    }
    T &at(int index);
};

template <typename T>
T &ARRAY<T>::at(int index)
{
    if (index < 0 || index >= m_size)
    {

        throw IndexError();
    }
    return m_ptr[index];
}

int main()
{
    ARRAY<int> a(50);
    int i;
    cin >> i;
    try
    {
        for (int j = 0; j < i; j++)
            a.at(i) = j;
    }
    catch (IndexError &e)
    {
        cout << e.what() << endl;
        return 0;
    }
    return 0;
}