C++ running error in my class

C++ running error in my class

I have tried to make my own list for C++. I have this Class:
template <class T>
class List
{
private:
T *value, *valueHelper;
int valueSize;
public:
int size;
List()
{
valueSize = 2;
value = (T*)malloc(sizeof(T) * valueSize);
size = 0;
}
void Add(T val)
{
size++;
if (size > valueSize)
{
valueSize *= 2;
valueHelper = (T*)malloc(sizeof(T) * valueSize);
memcpy(valueHelper, value, sizeof(T) * (valueSize / 2));
free(value);
value = valueHelper;;
}
value[size - 1] = val;
}
T operator[](int P)
{
return value[P];
}
};
When I try to use it in the main it works fine to Int. Buy to Struct it is
doing problems:
struct Material
{
string materialName;
int faceNum;
int meshNum;
Material(): materialName(""), faceNum(0), meshNum(0){};
};
void main()
{
List <Material> myList = List<Material>();
myList.Add(Material());
}
I got running error in the class on the line:
value[size - 1] = val;
Why?