New `finally` keyword in GCC 15.1

General
1

Hi all,

I failed to document (or maybe it was added a bit later than when I finished the changelog for GCC 15) that there is a new GNAT extension available in GNAT/GCC 15, it is the finally keyword. You can read about it in the GNAT Reference Manual. There is not much documentation about it though nor examples. However, I came across this SPARK commit which provided some examples. I tested the first one with a quick gnatmake -gnatX0 XXX.adb and it compiled without much fuss.

Sooo… how could this be used? I am asking as a complete newbee, not as a critique :slight_smile: It is also nice to see that SPARK now supports it too!

Best regards,
Fer

1 Like
2

The first thought that comes to mind is something like files that you want to close if you successfully open them but later encounter an exception while parsing them. You can use the finally section to close the file regardless of whether the full process completed or exceptioned out.

3 Likes
3

It is a feature complement to object finalization. Finalization is an OO approach to the general problem of safe finalization. Finally is a procedural counterpart.

For example, let you have a mutex you seize to protect a critical code section. To ensure that the mutex gets released you can create a holder object that seizes the mutex on initialization and releases it on finalization. Then you place the holder in a scope like this:

declare
   Lock : Holder (Mutex’Access);
begin
   ... -- Critical section
end;

Now the procedural alternative would be:

Mutex.Seize;
begin
   ... -- Critical section
finally
   Mutex.Release;
end;

You can use it in smallish designs when procedural decomposition is sufficient.

Note also. You need to be very careful with the scope where you place finalize. In the example above it would be a bug to place Mutex.Seize after the begin. The compiler cannot protect you here.

4 Likes