Re: Why we need scope(success) if I can write at the end?

[spir]

On 01/21/2011 09:56 PM, Jonathan M Davis wrote:
On Friday, January 21, 2011 05:18:15 tamir wrote:
or what's the differents between theese two:
void transactionalCreate(string filename) {
   string tempFilename = filename - ".fragment";
   scope(success) {
     std.file.rename(tempFilename, filename);
   }
   auto f = File(tempFilename, "w");
}
and:
void transactionalCreate(string filename) {
   string tempFilename = filename - ".fragment";
   auto f = File(tempFilename, "w");
   std.file.rename(tempFilename, filename);
}

scope(success) will be run _regardless_ of how you leave that scope (except for
if an exepction is thrown). So, you can have one line with scope(success) and
multiple return statements or breaks or continues or whatnot, and that
scope(success) statement will be run in all cases. If you didn't use
scope(success), then you'd have to duplicate that code at every point that the
scope could be exited successfully.

Now, if there's only one way to exit a particular scope successfully, then
scope(success) is less useful. However, even in cases like that it can improve
code maintanence and readability when it comes to stuff like resources. For
instance, if you were to open a file at the beginning of a function, having a
scope(success) statement immediately following it makes it clear that closing
the function has been taken care of, whereas without the scope statement, you'd
have to worry about putting it at the end of the function separate from where
the file was opened. Having them together makes the code clearer. Now, a file
isn't the best example, since you'd want success(exit) in that case, and the
File struct std.stdio deals with it anyway through its destructor, but the
example should give you the basic idea.

- Jonathan M Davis

I just realised scope(success) can avoid 2 kinds of bugs, one common, 
one naughty:
* at first coding time, simply forgetting releasing resources (one 
thinks at that when accessing it, but then, it's easy to forget), or any 
other exit task
* at maintenance time, opening a new exit point for a function (new 
return), and forgetting the exit task there

Denis
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