With statement become like C#'s using?

[Jacob Carlborg]

On 2013-08-05 14:40, Bosak wrote:
> In C# there is this using construct:
>
> using(Bitmap image = this.OpenImage("filename.bmp")) {
>      image.Name = "foo";
>      //use image like image.sth
> }
>
> which is translated to:
>
> {
>      Bitmap image = this.OpenImage("filename.bmp");
>      try {
>          image.Name = "foo";
>          //use image like image.sth
>      }
>      finally {
>          IDisposable obj = image as IDisposable;
>          if(obj != null)
>              obj.Dispose();
>      }
> }
>
> I know that the with statement is different, but it can be improved so
> that you can declare things in it like an using statement:
>
> with(Bitmap image = open("filename.bmp")) {
>      name = "foo";
>      //no need to specify image.sth
> }
>
> or even a more implicit one:
> with(open("filename.bmp")) {
> //ditto
> }
>
> And both of the above to be translated to:
>
> {
>      Bitmap temp = expression;
>      //use bitmap
>      delete temp; // Call destructor/finallizer of the object
>      //I'm not sure if delete was the proper way to call a destructor in D
> }
>
> And I hope you got the point. Tell me what you think.

You can replicate the C# using statement with a library function:


module test;

import std.stdio;
alias writeln println;

template isDisposable (T)
{
     enum isDisposable = __traits(compiles, { T t; t.dispose(); });
}

void using (alias block, T)(T t) if (isDisposable!(T))
{
     block(t);

     scope (exit)
         t.dispose();
}

void using (string block, T)(T t) if (isDisposable!(T))
{
     with (t)
         mixin(block);

     scope (exit)
         t.dispose();
}

class Foo
{
     void bar ()
     {
         writeln("bar");
     }

     void dispose ()
     {
         writeln("dispose");
     }
}

void main ()
{
     using!(foo => foo.bar())(new Foo);

     using!q{ bar(); }(new Foo);
}

If D could have a better syntax for delegates/blocks, it could look like 
this:

using(new Foo ; foo) {
     foo.bar();
}


-- 
/Jacob Carlborg