operator[][] C++

Question

I'd like to overload operator[][] to give internal access to a 2D array of char in C++.

Right now I'm only overloading operator[], which goes something like

class Object
{
    char ** charMap ;
    char* operator[]( int row )
    {
        return charMap[row] ;
    }
} ;

It works ok.. Is it possible to override operator[][] though?

Answer 1

Don’t try to do that – as others have said, overloading operator [] the way you do actually provides the [][] syntax for free. But that’s not a good thing.

On the contrary – it destroys the encapsulation and information hiding of your class by turning an implementation detail – the char* pointer – to the outside. In general, this is not advisable.

A better method would be to implement an operator [,] which takes more than one argument, or indeed an operator [][]. But neither exists in C++.

So the usual way of doing this is to ditch operator [] altogether for more than one dimension. The clean alternative is to use operator () instead because that operator can have more than one argument:

class Object
{
    char ** charMap ;
    char& operator ()(int row, int column)
    {
        return charMap[row][column];
    }
};

For more information, see the article in the C++ FAQ Lite.

Answer 2

There is no operator [][]: that's two [] operations in a row. You could:

• Have Object::operator[] return an object of a second class representing a row, which has its own operator[] method that takes a column number;
• Write a get(int row, int column) method and use that instead of operator overloading. I'd recommend this unless your object absolutely has to behave like an array.