On Mon, 12 Jul 2010 08:25:32 +0200, Jonathan M Davis
<jmdavisp...@gmail.com> 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
Do you know about the scope storage class, or about scope classes?
{
scope tmp = new A();
// use tmp;
tmp destructor is called.
}
scope classes are similar:
http://www.digitalmars.com/d/2.0/class.html
