I had some free time recently so I decided as a learning experience to fork the
compiler and implement the “auto var” idea that was mentioned a few weeks ago.
What I found is that it’s a pretty lightweight (in terms of impact on the
compiler) and unintrusive way to manage memory on a per-scope basis which
solves a minor but common pattern. This was highly debated but I know from my
personal experience I often know at the time of declaring a class that I will
allocate the class at the top of the function and free at the end. If this was
C++ I would declare the class on the stack but we don’t have that option in
Pascal so we’re forced into either a class or record paradigm. The better
option would probably be full blown ARC but I don’t know if that’s ever going
to be on the agenda of FPC.
I understand there’s lots of potential problems like passing an auto var out of
scope, classes with constructors that require parameters (rather big problem)
or perhaps the “auto” modifier (I did that because it was easy to parse) but is
there any merit to this idea if it was cleaned up and made safe? I did some
tests to see if you could prevent passing auto vars out of scope and it’s
possible to prevent most ways but not 100%.
========================================================
program auto_test_1;
type
TMyClass = class
data: TObject; auto;
end;
var
obj: TMyClass; auto;
begin
// obj is auto so TMyClass.Create is called along with subsequent auto
var members
writeln('obj:', obj.classname);
writeln('data:', obj.data.classname);
// when TMyClass.Free is called auto var members call Free also
end.
program auto_test_2;
uses
fgl;
var
list: specialize TFPGList<integer>; auto;
i: integer;
begin
// list is auto so TFPGList<integer>.Create is called
// TFPGList as an auto var is a compelling alternative to dynamic arrays
list.Add(1);
list.Add(2);
list.Add(3);
for i in list do
writeln(i);
end.
Regards,
Ryan Joseph
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