为什么我的重载转换运算符无法访问私有成员？

Question

I'm attempting to implement an overloaded casting operator on my templated array2d class using type T . So I'm to cast from array2d<T> to a new array2d<E> .

I'm able to perform the casting itself but problems arise when I try to set the casted data to the new instance of array2d<E> . The compiler tells me that the casting operator doesn't have access to the private members of array2d

Here's where I am so far (edited out unrelated code for brevity)

array2d.h

template<typename T>
class array2d {
private:
    // Member Variables
    T** data;
    size_t width, height;
public:
    // constructors, methods, etc...

    // Cast Operator
    template<typename E>
    operator array2d<E>() const;
};

// Other overloaded operators...

// Overloaded Casting Operator
template<typename T>
template<typename E>
array2d<T>::operator array2d<E>() const{
    // Create new instance
    array2d<E> castedArr(width, height);
    // Allocate memory for the casted data, then cast each element
    E** newData = new E*[castedArr.get_height()];

    for (size_t i = 0; i < castedArr.get_height(); i++){
        newData[i] = new E[castedArr.get_width()];
        for (size_t j = 0; j < castedArr.get_width(); j++){
            newData[i][j] = (E)data[i][j];
        }
    }
    // issue here, can't set data because it's private.
    castedArr.data = newData;

    delete [] newData;
    newData = nullptr;

    return castedArr;
}

main.cpp

#include "array2d.h"

int main(int argc, char *argv[]) {
// Cast Operator
    // Create an array2d<T> of
    // width = 5
    // height = 5
    // fill all elements with 42.1
    array2d<double> x(5, 5, 42.1);

    // Create a new array exactly the same as
    // x, where x is casted to int
    array2d<int> y = (array2d<int>) x;

    return 0;
}

This confused me as I have many other overloaded operators that can access the private members just fine using practically the exact same logic.

Why does this happen and what can I do to rectify it?

Answer 1

When writing a template, you don't nail down the actual type, you create a blue print for different types. array2d<double> and array2d<int> are different types, and by default, two instances of two different classes can't access their private members.

You can fix that by declaring every instantiation of array2d a friend class of the template array2d :

template<typename T>
class array2d {
    /* ... */

    template<class E> friend class array2d;

    /* ... */
};

As a side note, I'm not quite sure wheter

delete [] newData;

is a good idea. You're destroying parts of the resources that the new array2d instance is supposed to manage. If you delete[] that again in array2d::~array2d() , you'll have undefined behavior.