overload () operator for multidimensional data access

Question

I want to have access to multidimensional data inside a class, I found:

To provide multidimensional array access semantics, e.g. to implement a 3D array access a[i][j][k] = x;, operator[] has to return a reference to a 2D plane, which has to have its own operator[] which returns a reference to a 1D row, which has to have operator[] which returns a reference to the element. To avoid this complexity, some libraries opt for overloading operator() instead, so that 3D access expressions have the Fortran-like syntax a(i, j, k) = x;

on http://en.cppreference.com/w/cpp/language/operators

and I'd like to use suggested syntax, but I have trouble implementing that. How can I write overloaded assignment operator to work that way?

The `Eigen` library might be doing just that. Also: what troubles are you having? — Andras Deak -- Слава Україні, Aug 16 '17 at 11:06
Yes, I used Eigen, but funny thing, this time gcc 4.6 is my limit... :( But good idea, I'll try to search how they have done it. — corwin, Aug 16 '17 at 11:08
You don't need to overload assignment operator. You just have to make `()` return a reference. — HolyBlackCat, Aug 16 '17 at 11:08
I'm confused how to get the syntax right, can you post some example? — corwin, Aug 16 '17 at 11:12
What have you tried? How exactly do you have trouble implementing it? — YSC, Aug 16 '17 at 11:12
I have: T operator()(unsigned width, unsigned height) { return my_data[width + height * data_width]; } for reading the data like: my_class(7, 8), now I want to write to it in a simmilar way — corwin, Aug 16 '17 at 11:15
Just return a reference... Look at [that answer](https://stackoverflow.com/a/42628050/5470596) I posted on a similar topic. — YSC, Aug 16 '17 at 11:16

score 3 · Accepted Answer · answered Aug 16 '17 at 11:16

For example, it would be

template <typename T, std::size_t S1, std::size_t S2, S3>
struct Matrix3D
{
    // ...

     const T& operator()(std::size_t i, std::size_t j, std::size_t k) const {
          return data[i][j][k];
     }
     T& operator()(std::size_t i, std::size_t j, std::size_t k) {
          return data[i][j][k];
     }
private:
    T data[S1][S2][S3];
};

score 0 · Answer 2 · answered Aug 16 '17 at 11:20

0

Thanks to your help I have a solution:

T& operator()(unsigned width, unsigned height) {
    return my_data[width + height * data_width];
}

works both ways:

my_class(1,2) = value;
value = my_class(1,2);

answered Aug 16 '17 at 11:20

corwin

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overload () operator for multidimensional data access

2 Answers2