Recommended way to do RAII cleanly

[Jacob Carlborg]

On 2010-07-12 08.25, Jonathan M Davis wrote:
> Okay. There are cases where you want a constructor to do something when the
> class/struct is created, and you want the destructor to do something when the
> class/struct goes out of scope. A classic example would be an autolock for a
> mutex. Another would be the hourglass in MFC - it's displayed when the object is
> created and disappears when the object is destroyed (so all you have to do is
> declare the object it at the beggining of the function and it automatically is
> displayed and then disappears). This is classic RAII.
>
> Obviously, because classes are reference types with infinite lifetime while
> structs are value types with their lifetime restricted to their scope, structs
> would be the better choice for RAII. I have noticed a bit of a snag however:
> structs can't have default constructors.
>
> After reading TDPL, I completely understand that structs can't have default
> constructors due to how the init property works. However, the classic case where
> you want to simply declare an object and have it do what it does through RAII
> then falls apart. Ideally, you'd have something like this
>
> struct S
> {
>      this()
>      {
>          /* do something */
>      }
>
>      ~this()
>      {
>         /* undo what you did before or do whatever clean up is required for it */
>      }
> }
>
> void main()
> {
>      auto s = S();
>     /* whatever the rest of main() does */
> }
>
>
> Thanks to the lack of default constructor, you can't do that. Therefore, I see 2
> options:
>
> 1.  Create a nonsensical constructor that takes an argument of _some_ kind which
> is totally ignored.
>
> 2. Create a factory function to create the struct, and it does whatever would
> have been in the default constructor.
>
>
> Out of those two options, the second seems the best, but it seems to me that
> there have got to be more options than that. So, the question is what would be
> the best option (be it one of those or another that I haven't though of) to do
> RAII in the general case? What would be "best practice" for D when dealing with
> structs intended for RAII without any arguments to their constructor when you
> can't have a default constructor?
>
> - Jonathan M Davis

You can use a static opCall method (if that hasn't been deprecated as 
well), like this:

struct S
{
     static S opCall ()
     {
         S s;
         // do something with s you would otherwise do in the constructor
         return s;
     }
}

auto s = S();

-- 
Jacob Carlborg