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);
}