Debugger won't evaluate stringlist contents or count
lazarus / 12-Debugger

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keithfre · 2011-09-21 18:20

I have a var tablist (a derivative of TStringlist) populated with
strings from a file. If I assign tablist[0] or InttoStr(tablist.count)
to a TEdit.Text the result is fine. But I can't get the debugger to
evaluate tablist[0] or tablist.count (using mouseover, Ctrl-F7 or a
watch).

0.9.30 (from the Downloads page), Windows XP

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Martin_fr · 2011-09-21 19:07

Unfortunately the answer is, it doesn't work (yet).
It is one of the big known issues.  And one that will require a lot of
time and work to fix.

The basic behind it is, that "Count" and "Items[x]" are properties
with a getter. So in order to get what you want to see, the getter (a
function like GetCount(): Integer) must actually be executed.
This is simply not yet implemented.

If you google, or even wait for further replies, you will find
idealogical arguments, if it should be done at all. Ignore them. It
should, and in time it will  (but it may be long time).

That much about the current situation. Feel free to ask, if you want
to know about internal details.

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Looking at your particular situation, it is possible to get the info,
but in some cases requires a lot of experience.

In case of "Count", it is very easy.

You can just try (using ctrl-F7 for evaluate, or open the watches
window from "View" menu ("Debug windows") ctrl-alt-w):
 Code: [Select]   FCountwill return the count.

Often properties map to a field of the same name, with an "F" in the
front. In some cases the debugger can actually resolve it for you. But
for TStringList it cant.
To confirm that FCount is indeed Count, in the base class TStrings:
Code: [Select]    property Count: Integer read GetCount;And in
TStringList:
Code: [Select]Function TStringList.GetCount: Integer;
begin
  Result:=FCount;
end;


2) in case of Items, it is harder:
Code: [Select]Function TStringList.Get(Index: Integer): string;
begin
  If (Index<0) or (INdex>=Fcount)  then
    Error (SListIndexError,Index);
  Result:=Flist^[Index].FString;
end;

You will have to follow the declaration of "FString"
Code: [Select]    FList: PStringItemList;

I will spare the details, if you want to see the items, use this:
Code: [Select]^TStringItem(FLazarusDirsHistory.Flist)[0]

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keithfre · 2011-09-21 19:21

Thanks, I get the picture.

So it looks like I can type in  "^TStringItem(tablist.Flist)[0]" to
evalute my tablist[0]. Even that's better than nothing at all ;-}

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Martin_fr · 2011-09-21 19:35

Yes, it should.

I should say, I tested with the latest snapshot, and there are changes
=> but I expect this one to work on 0.9.30 too

As a general rule:

you can always start (in ctrl-f7 / evaluate), by looking at the entire
object:
Code: [Select]tablist
It will give you a list, of all the fields in the object.

It should show the presence of an "FList", including type, and since
it is a pointer, it will show an address as value.

Normally you can just dereference (^) pointers, or use an index [] on
them:
Code: [Select]tablist.FList^
tablist.FList[0]

unfortunately, not in this case. gdb doesn't seem to like the special
way that FList is declared.

That's why in this case the typecast is needed. As a general rule, if
gdb has issues with arrays or pointers, try typecasting.

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gdb is the debugger used by Lazarus and fpc