A template construct like using()

[user1234]

On Tuesday, 26 April 2022 at 21:33:43 UTC, Chris Katko wrote:
> I swear I asked something like this before years ago but it 
> doesn't show up in my previous forum posts.
>
> I'm looking for a construct that mimics using(var)/with(var)
>
> ```D
> bitmap* b;
>
> draw_with(b)
>   {
>   draw_pixel(red, 16, 16); //draw red pixel to bitmap b (b is 
> implied above)
>   }
> ```
>
> But the code ends up being:
> ```D
> bitmap* b;
>
> set_target_bitmap(b); //entry code
> draw_pixel(red, 16, 16); // body
> set_target_bitmap(original_target); // exit code
> ```
>
> The essence is wrapping something the code up in a kind of 
> RAII-like entry and exit code that references a given target 
> variable.
>
> Perhaps a mixin is what I'm looking for?

assuming `set_target_bitmap` returns the previous target :

```d
struct Bitmap
{
}

struct PushPopBitmap
{
     Bitmap* bitmap;
     Bitmap* old;
     alias bitmap this;

     this(Bitmap* bmp) {
         old = set_target_bitmap(bitmap = bmp);
     }

     ~this() {
         set_target_bitmap(old);
     }
}

Bitmap* set_target_bitmap(Bitmap* bmp);
void draw_pixel();

void main()
{
     Bitmap* bmp;
     with (PushPopBitmap(bmp)) draw_pixel();
}
```

At the end of the WithStatement block that restores the previous 
context.