Template of Template c++ - c++

Question

#include <iostream>
using namespace std;
template <typename T>
class DynamicArray
{
T *array;
unsigned int elements;
unsigned int size;
public:
void expand_array(int extra_size)
{
T *new_array= new T[size + extra_size];
//copy integers from old array
memcpy(new_array,array,elements*sizeof(T));
//make 0 the integers from new part of the array
memset(new_array + size, 0, extra_size * sizeof(T));
delete [] array;
array = new_array;
size += extra_size;
}
//constructors
DynamicArray()
{
array = new T[8];
elements = 0;
size = 8;
memset(array,0,size*sizeof(T));
}
DynamicArray(const unsigned int size)
{
array= new T[size];
}
virtual ~DynamicArray()
{
delete [] array;
}
void add(T new_element)
{
if(elements>=size)
expand_array(8);
array[elements++] = new_element;
}
int get (const unsigned int index) const
{
if(index< elements)
return array[index];
return -1;
}
void add(const unsigned int index, T new_element)
{
if(index>size)
expand_array(index- size +1);
array[index] = new_element;
elements = index +1;
}
DynamicArray &operator=(DynamicArray &ab)
{
elements=ab.elements;
size=ab.size;
if (this == &ab)
return *this;
delete array;
if(ab.array)
{
array= new T[size];
memcpy(array,ab.array,elements*sizeof(T));
}
else
{
array=0;
}
return *this;
}
DynamicArray(const DynamicArray& source)
{
elements=ab.elements;
size=ab.size;
if(ab.array)
{
array= new T[size];
memcpy(array,ab.array,elements*sizeof(T));
}
else
{
array=0;
}
}
};
int main()
{
DynamicArray<int> da(2);
DynamicArray<int> db(2);
DynamicArray< DynamicArray<int> > array_of_arrays(2);
array_of_arrays[0] = da;/
//array_of_arrays[1] = db;
/*
da[0]=5;
da[1]=2;
db[0]=3;
db[1]=4;
cout<<array_of_arrays[0][0]<<endl;
*/
system("color 0C");
system("pause");
return 0;
}
I need help in understanding why this gives me an error:
error C2676: binary '[' : 'DynamicArray<T>' does not define this operator or a conversion to a type acceptable to the predefined operator
i added this
T& operator[](unsigned int index)//important to provide this operator
{
if (index > size)
{
expand_array(index-size);
}
return array[index];
}
but subequently i got invalid allocation size error during runtime.I try to try it and figure that the problem is somewhere about this area
if(ab.array)
{
array= new T[size];
memcpy(array,ab.array,elements*sizeof(T));
}
but i have no idea why is that so.
Any ideas?
Thanks I figured it out: One of the attributes are not initialise to a proper valuein one of the constructors
DynamicArray(const unsigned int size)
.
Thanks for all the help.

Answer 1

The error has nothing to do with templates. The problem is that you use the [] operator on array_of_arrays. However the DynamicArray class does not define operator[].
What it does define is a method named add, so maybe you meant to call that instead? I.e. array_of_arrays.add(0, da); instead of array_of_arrays[0] = da;.
However it would probably be more idiomatic to simply define operator[] on your class.

Answer 2

DynamicArray does not define a subscript operator.
This would typically look like:
T& operator[] (unsigned int index)
{
if (index > size)
{
// Handle error
}
return array[index];
}