Hi,

I have a class Ground which defines some playground constants (at least constant to the class):

    class Ground
    {
        immutable WALL = -2;
        immutable playgroundWidth = 77;
        immutable playgroundHeight = 22;

...
}

Now, in another class "Player", I would like to use those playground constants:

    import ground;
    class Player
    {
        this()
        {
            x = Ground::playgroundWidth/2;
            y = Ground::playgroundHeight/2;
        }
    ...
    }

I used the "::" notation as a reference to C++, but obviously here it does not compile:
|Error: need this for playgroundWidth of type immutable(int)|

A class must have a "new" initialization in dlang, alright. But then I replaced immutable by enum in the playgroundWidth declaration, and it compiles fine with:

        enumplaygroundWidth = 77;
...
        x = Ground.playgroundWidth/2;

My question is : Is enum declaration the right way to go? Or did I just find a trick around the problem? Could not find anything in the documentation. Why enum works and immutable does not?

thanks

On Tuesday, 16 November 2021 at 18:12:34 UTC, chopchop wrote:

    class Ground
    {
        immutable WALL = -2;
        immutable playgroundWidth = 77;
        immutable playgroundHeight = 22;

...
}

    import ground;
    class Player
    {
        this()
        {
            x = Ground::playgroundWidth/2;
            y = Ground::playgroundHeight/2;
        }
    ...
    }

The reason you get this error is that you have declared playgroundWidth and playgroundHeight as instance variables of Ground. Of course, in order to access an instance variable, you need to have an instance. When the compiler says "need this", it means you need an instance of the class.

The correct fix is to declare the variables as static:

class Ground
{
    static immutable WALL = -2;
    static immutable playgroundWidth = 77;
    static immutable playgroundHeight = 22;
}

As far as I know, this works the same way in D as it does in C++. Here's an example of C++ code that has the same error:

class C
{
public:
    int x = 42;
};

int main()
{
    // error: invalid use of non-static data member 'C::x'
    int y = C::x;
    return 0;
}

Interactive version: https://godbolt.org/z/a5xvzooev

The version with the enum declarations works because enum declarations are always considered static.

Thanks guys, it is very clear now.