How to control value assigned by operator [] - c++

Question

I know how to overload operator[] as follows :
T& operator [](int idx) {
return TheArray[idx];
}
T operator [](int idx) const {
return TheArray[idx];
}
But what I want is to control values assigned by arr[i] = value.
I want to control value to be between 0 and 9.
Is there any syntax to do so?

Answer 1

You would have to write a template class that holds a reference to the element in the array (of type T), in this template you implement the assignment operator, and there you can implement your check. Then you return an object of this template class from your [] operator.
Something like this:
template< typename T> class RangeCheck
{
public:
RangeCheck( T& dest): mDestVar( dest) { }
RangeCheck& operator =( const T& new_value) {
if ((0 <= new_value) && (new_value < 9)) { // <= ??
mDestVar = new_value;
} else {
... // error handling
}
return *this;
}
private:
T& mDestVar;
};

Answer 2

Rene has provided a good answer. In addition to this, here is a full example. Note that I added a "user-defined conversion", i.e., operator T, in the proxy_T class.
#include <iostream>
#include <array>
#include <stdexcept>
template <class T>
class myClass
{
std::array<T, 5> TheArray; // Some array...
class proxy_T
{
T& value; // Reference to the element to be modified
public:
proxy_T(T& v) : value(v) {}
proxy_T& operator=(T const& i)
{
if (i >= 0 and i <= 9)
{
value = i;
}
else
{
throw std::range_error(std::to_string(i));
}
return *this;
}
operator T() // This is required for getting a T value from a proxy_T, which make the cout-lines work
{
return value;
}
};
public:
proxy_T operator [](int const idx)
{
return TheArray.at(idx);
}
T operator [](int const idx) const
{
return TheArray[idx];
}
};
int main() {
myClass<int> A;
std::cout << A[0] << std::endl;
A[0] = 2;
std::cout << A[0] << std::endl;
A[1] = 20;
}