operator[][] C++

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佛祖请我去吃肉 2021-02-15 16:24

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

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  • 2021-02-15 17:02

    There is no [][] operator. What actually happens is that the second [] operates on the variable returned by the first []. Because there is already that functionality, it would create ambiguity were there to exist a [][] operator.


    For example: let's say you have a variable x of some type T.

    T x = new T();
    

    If we use the [] operator, let's say a variable of other type Q is returned:

    Q y = x[0];
    

    And then using the [] operator on a variable of type Q might return a variable of type R:

    R z = y[0];
    

    Therefore x[][] returns a variable of t ype R.

    Let's say we actually were able to overload [][] for type T such that it returned a type S:

    S a = x[0][0];
    

    The compiler would have no way of knowing if it should use the [][] operator on x to return a type S variable, or use the [] operator twice in a row to return a type R variable. This is the ambiguity I mentioned above.


    Your best bet if you're stuck on using square brackets is to have operator[] return a variable which also has [] overloaded (or perhaps a variable of the same type, with a flag set), and have that initially returned variable deal with the second [].

    But the best solution here (as mentioned already in another answer) is to use a different operator such as ().

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  • 2021-02-15 17:04

    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.

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  • 2021-02-15 17:10

    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.
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  • 2021-02-15 17:18

    There is no operator[][]. Evaluating a[x][y] first calls operator[] on a, and then operator[] again on the result of that.

    So the operator[] of your object has to return another object with its own operator[], which then will access the requested value.

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  • 2021-02-15 17:23

    As far as I know there is no such thing as operator[][]. What you can do is you could return from your operator[] method something that has overloaded operator[].

    Actually you are doing it now, because you return char* which can be indexed using [] again.

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