Recommended way to do RAII cleanly

[Jonathan M Davis]

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