Subscript operator overload for partial template specialisation - c++

Question

I'm trying to learn about templates and template specialisation. I'm writing a template class for arrays, using template specialisation to avoid code bloat. Thus I have a fully specialised template MyArray and then I inherit from this one like class MyArray<T*> : private MyArray<void*>. I'm having trouble overloading the subscript operators (one for non-const refs, one for const refs). Here's a piece of code (far from complete, but contains my problem).
#include <iostream>
using std::cout;
using std::cin;
using std::endl;
/*** template class MyArray **/
template <class T>
class MyArray {};
/*** Full template specialization for MyArray holding pointers ***/
template <>
class MyArray<void*> {
public:
explicit MyArray(unsigned s = 100) : sz(s) {
data = new void*[s];
}
virtual ~MyArray() { delete[] data; }
/** subscript operator overload for non-const refs **/
void*& operator[](unsigned i) {
return data[i];
}
/** subscript operator overload for const refs **/
const void*& operator[](unsigned i) const {
return data[i]; // line 26
}
unsigned size() const { return sz; }
private:
void** data;
unsigned sz;
};
/** Partial specialization: create the template class by inheriting from the one above **/
template <class T>
class MyArray<T*> : private MyArray<void*> {
public:
explicit MyArray(unsigned s = 100) : MyArray<void*>::MyArray(s) {
data = new T*[s];
}
virtual ~MyArray() { delete[] data; }
/** subscript operator overload for non-const refs **/
T*& operator[](unsigned i) {
return reinterpret_cast<T*&>( // line 47
MyArray<void*>::operator[](i)
);
}
/** subscript operator overload for const refs **/
const T*& operator[](unsigned i) const {
return reinterpret_cast<const T*&>(
MyArray<void*>::operator[](i)
);
}
unsigned size() const { return MyArray<void*>::size(); }
private:
T** data;
};
/** input function for filling MyArray's **/
template <class T>
void InputMyArray(MyArray<T*>& my_array) {
unsigned size = 0;
T tmp;
while (cin >> tmp) {
T* i = new T;
*i = tmp;
my_array[size++] = i;
}
}
/** output function for printing elements of MyArray's **/
template <class T>
void OutputArray(const MyArray<T*>& my_array) {
for (unsigned i = 0; i < my_array.size(); i++) {
cout << *my_array[i] << " ";
}
cout << endl;
}
int main() {
/** Initialize array, fill it, print contents **/
MyArray<int*> p;
InputMyArray(p);
cout << "MyArray of pointer to ints holds int values: " << endl;
OutputArray(p);
return 0;
}
The compiler (clang) is complaining (error) about line 26
non-const lvalue reference to type 'const void *' cannot bind to a value of unrelated type 'void *'
I suppose I don't want the compiler to interpret this as a non-const reference - what I want is for it to be const. How can I properly overload this operator in this context? The corresponding piece of code works fine for a template class without specialisation, like MyArray<T>.
The compiler further complains (warnings) about the reinterpret_casts which apparently contain undefined behaviour
reinterpret_cast from 'void *' to 'int *&' has undefined behavior
(line 47). The reinterpret_casts are essentially copy-pasted from my instructions, so I would think they are to be used like this. I don't know why the reference to void* isn't picked up. What am I doing wrong here?

Answer 1

Your template is a container for void*, not const void*:
/** subscript operator overload for const refs **/
void* const& operator[](unsigned i) const {
return data[i]; // line 26
}
Same for T*:
/** subscript operator overload for const refs **/
T* const& operator[](unsigned i) const {
return reinterpret_cast<T*const&>(
MyArray<void*>::operator[](i)
);
}