Dynamic allocation with template - c++

Question

I create a class Vector that contain two variables that are template variable, I am trying to build such a dictionary that tell mark of a specific student.
The problem is I am struggling dynamic allocating memory with template. I have to do this without map or STL help. Can you explain, how to allocate them properly.
#define DEF_CAPACITY 20
template <class U, class T>
class Vector {
protected:
T* _data;
U* _keys;
int _size; //size in use
int _capacity; //available capacity
public:
//constructors
Vector(int capacity = DEF_CAPACITY);
~Vector();
int getSize() const { return size; }
void insert(U key, T data);
T operator[](U key);
};
template<class U, class T>
inline Vector<U, T>::Vector(int capacity)
{
this->_capacity = capacity;
this->_size = 0;
}
template<class U, class T>
Vector<U, T>::~Vector()
{
if (_data)
delete[] _data;
if (_keys)
delete[] _keys;
}
template<class U, class T>
void Vector<U, T>::insert(U key, T data)
{
_keys[_size] = new U;
//Maybe I have to do something like that, but all options I test doesn't work
//keys[_size] = new U[Some size];
_keys[_size] = key;
_data[_size] = new T;
_data[_size] = data;
_size++;
}
template<class U, class T>
T Vector<U, T>::operator[](U key)
{
int index = 0;
for (int i = 0; i < size; i++)
if (_keys[i] == key)
index = i;
return _data[index];
}
int main() {
Vector<string, int> grades;
grades.insert("john", 90);
grades.insert("marc", 100);
grades.insert("ron", 87);
grades.insert("lynda", 95);
//...
grades.insert("Tome", 93);
//...
cout << grades["marc"] << endl;
cout << grades["lynda"] << endl;
cout << grades["john"] << endl;
return 0;
}

Answer 1

To allocate n elements of type T you use:
T* array = new T[n];
so to allocate space for your keys and values:
// be aware: this will call the constructor for each element!!!
_keys = new U[n];
_data = new T[n];
If your "Vector" has a fixed capacity, this should be done in your constructor:
template<class U, class T>
inline Vector<U, T>::Vector(int capacity)
: _data(new T[capacity]);
, _keys(new U[capacity]);
, _size(0);
, _capacity(capacity);
{
}
after that, you can insert key/value pairs like this:
template<class U, class T>
void Vector<U, T>::insert(const U& key, const T& data)
{
if(_size == _capacity)
throw std::out_of_range("no space left!!!");
_keys[_size] = key; // copy assignment
_data[_size] = data; // copy assignment
_size++;
}
This design will unnecessarily call constructors for unused keys and values.
You could also do this with malloc and free to prevent this, though I wouldn't recommend going that way and instead use the standard library.