Re: Override member variables

[Ali Çehreli via Digitalmars-d]

On 05/19/2018 11:09 AM, Gheorghe Gabriel wrote:
> I've worked with a lot of programming languages and I've found something
> interesting in Kotlin. You can override member variables. Would you like
> to have this feature in D?
It's needed in C++ and I'm sure any object-oriented programming language 
as well. As you seem do indicate, the current solution is to provide 
member functions. Assuming that the values are constant:

class Rectangle {
    int width() const {
        return 0;
    }
    int height() const {
        return 0;
    }
}

class Table : Rectangle {
    override int width() const {
        return 10;
    }
    override int height() const {
        return 14;
    }
}

void main() {
    auto r = new Table();
    assert(r.width == 10);
    assert(r.height == 14);
}

The compiler can optimize virtual function calls away when it can prove 
the actual type.

Here is quick exercise with current language features:

mixin template overridable(string name, T...) {
    static assert(T.length == 1, "You must provide a single value for 
member '" ~ name ~"'");

    import std.string : format;
    mixin (format(q{
                %s %s() const { return %s; }
            }, typeof(T[0]).stringof, name, T[0]));
}

mixin template overrided(string name, T...) {
    static assert(T.length == 1, "You must provide a single value for 
member '" ~ name ~"'");

    import std.string : format;
    mixin (format(q{
                override %s %s() const { return %s; }
            }, typeof(T[0]).stringof
            , name, T[0]));
}

class Rectangle {
    mixin overridable!("width", 0);
    mixin overridable!("height", 0);
}

class Table : Rectangle {
    mixin overrided!("width", 10);
    mixin overrided!("height", 14);
}

void main() {
    auto r = new Table();
    assert(r.width == 10);
    assert(r.height == 14);
}

Ali