object states

[Henning Pohl]

On Monday, 8 October 2012 at 08:53:39 UTC, simendsjo wrote:
> 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);
> }

There is a difference between invariants and what I call states. 
States clear things up when you actually need to do a runtime 
check and when not. Invariants are always checked, because you 
don't care about performance in debug mode. A Number<positive> is 
positive already and will stay positive while you can access it. 
De facto you cannot change a Number<positive> to a negative 
number.