Re: object states

[simendsjo]

On Sunday, 7 October 2012 at 20:46:09 UTC, Henning Pohl wrote:

On Sunday, 7 October 2012 at 20:18:15 UTC, Ali Çehreli wrote:

Sounds good.

Thanks.

You haven't mentioned the invariant keyword, so perhaps you are not aware of that feature?

 http://dlang.org/class.html#Invariant

 http://dlang.org/dbc.html

Although the docs seem to favor classes, invariant is available for structs as well.

Ali

How would you resolve the issue I wrote in my first post using invariants? You cannot add/remove checks from invariants while using the object. That's what I made different. But there are still some limits given by the language. Look at point 7 for instance.

The problem about contract programming in general is you cannot use it in public library code. Distinction between logic and user input is a very important thing, we really need to improve this. You may have noticed that I did not make use of either assert or enforce. I didn't want to specify this.

You can create a wrapper struct that includes an invariant:

import std.stdio;

struct Image
{
    int width;

    void doSomething()
    {
    }

    void modifiesWidth()
    {
        --width;
    }
}

void func(Image img)
{
    struct ImgWrapper
    {
        Image* img;
        this(ref Image img)
        {
            this.img = &img;
        }

        invariant()
        {
            assert(img.width == 512);
        }

        void opDispatch(string s)()
        {
            mixin("img."~s~"();");
        }
    }

    auto wrapper = ImgWrapper(img);
    wrapper.doSomething();
    wrapper.modifiesWidth(); // assertion failure
}

void main()
{
    Image img = { 512 };
    func(img);
}