Overwrite index operation

Question

I know that it's possible to overwritew index operation [] in C++ and work with class internal data like with an array. Is it possible to overwrite [][] and to work like with two demension array ?

sasha.sochka · Answer 1 · 2013-06-24T15:54:55.663

2

Yes, it's possible (and std::vector is an example of such a class).

You should overload operator[] in your class or structure.

In case you use positive integers to index data possible signature will be:

RETURN_TYPE operator[] (size_type n);

UPD: if you want two-dimensional data structure with non-negative integers as keys you can use vector of vectors:

std::vector<std::vector<YOUR_TYPE>> vector_name;

edited Jun 24 '13 at 15:54

answered Jun 24 '13 at 15:42

sasha.sochka

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Do you have an example how to do it. But std::vector is not two dimension – user1913557 Jun 24 '13 at 15:44

score 1 · Answer 2 · answered Jun 24 '13 at 16:03

1

In order to achieve it you need to overload operator[] to return some object that have operator[] too. For example:

class Proxy {
     std::vector<int>::iterator it;
     //constructor;
     int operator[] (size_t pos) {
         return *(it + pos);
     }
}

class MyMatrix {
    std::vector<std::vector<int>> v;
    Proxy operator[] (size_t pos) {
        return Proxy(v[pos].begin());
    }

}

answered Jun 24 '13 at 16:03

RiaD

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Note that if you're not doing bounds checking, and you use a single dimension implementation, a pointer is an adequate `Proxy`: `int* operator[]( int i ) { return &v[ i * columns ]; }`. Similarly, if with your implementation, `std::vector&` is a suitable proxy. – James Kanze Jun 24 '13 at 16:39

score 0 · Answer 3 · answered Jun 24 '13 at 15:44

0

Yes indeed you can; conventionally, you would overload [] return a reference to a row-type object, and overload [] on that row-type object to yield a reference to the scalar.

You should also consider overloading pointer dereference as well in order to maintain the equivalence of [x] and *(... + x).

answered Jun 24 '13 at 15:44

Bathsheba

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Your second suggestion really depends on the kind of the class in question. I can't really imagine `std::vector v(100); *(v + 7) = 5;` being legal code. – Angew is no longer proud of SO Jun 24 '13 at 15:49
@Angew; indeed. Hence the need to consider. If you were writing a matrix class then I think you should. – Bathsheba Jun 24 '13 at 15:57

score 0 · Answer 4 · answered Jun 24 '13 at 15:44

0

Let's say you have an array arr whenever you want to acess arr[x][y] for exemple it has an equivalence of (arr[x])[y] so first of all arr[x] is evaluated (let's name it a) and then a[y] is evaluated so you have to overload just []operator.

answered Jun 24 '13 at 15:44

Alexandru Barbarosie

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But (arr[x]) - must return a one dimension array, and a[y] - a value, how overload one operator for two different return types ? – user1913557 Jun 24 '13 at 16:00
@user1913557 array? Nop. It returns a pointer. Your left hand side of the `[]` operator is always a pointer. – Alexandru Barbarosie Jun 24 '13 at 16:37

Overwrite index operation

4 Answers4