Comparing Exceptions and Errors

[Steven Schveighoffer]

On 6/12/22 4:11 PM, kdevel wrote:
> On Tuesday, 7 June 2022 at 18:37:13 UTC, Steven Schveighoffer wrote:
> [...]
>> My very common use of `scope(failure)` for my DB code:
>>
>> ```d
>> conn.exec("START TRANSACTION");
>> scope(success) conn.exec("COMMIT");
>> scope(failure) conn.exec("ROLLBACK");
>> ```
> 
> Are there multiple (successful) returns in your code or why are you 
> executing the COMMIT under a scope exit clause?

No, usually just one (the implicit final return).

> If there is only one 
> successful return wouldn't it enhance code readability when an explicit 
> COMMIT precedes the return?

It just introduces another place where I can mess up the transaction. I 
can write these 3 lines, and be done, never having to worry about the 
transaction code again.

This is one of the major features of scope guards.

> Is the ROLLBACK really necessary?

Yes. If you don't execute the rollback and start executing more DB 
calls, they all get included in the transaction (and might be expected 
to be).

> Isn't the transaction not committed 
> (i.e. rolled back) when the db handle is closed?

Possibly, but I don't close the handle. It goes back to a pool to get 
reused.

> Why not have a 
> Transaction class (struct)?

A transaction class/struct cannot hook normal return and throwing 
differently (the destructor has no way of knowing whether an exception 
is in flight).

-Steve