[fpc-devel] Hom to initialize an object (old style object, not TObject)

[Tomas Hajny]

On Fri, November 1, 2013 16:18, Martin wrote:
> On 01/11/2013 15:10, Tomas Hajny wrote:
>> Sorry if I overlooked something, but where is the variable of that type
>> declared in your code? These objects may be either declared statically
>> ("var B: TBar") or dynamically ("Result := New (TBar.Init)" - but then
>> the
>> return value must be of type PBar, not TBar).
> Objects are  like (advanced) records.
>
> So Result is a TBar, and result already has the memory it needs. "New"
> is not needed.

Result is stored in a temporarily allocated memory. You are right that New
is not absolutely needed (as witnessed by the compiler accepting your code
- unlike TP/BP which wouldn't accept TBar as return value, because it
doesn't support performing assignment on objects or even records), but
using the temporarily allocated memory implies that you initialize the
memory in one place first (Init) and then you need to copy the data to
another memory location afterwards. Depending on the object size, this may
or may not be an issue, but it doesn't look like the most efficient
solution to me in any case.


>> If I understand it
>> correctly, you initialize the object on the stack, but it would
>> disappear
>> from there (gets overwritten) immediately after the function return (the
>> following function call would overwrite it).
>
> It gets lost only, because i do not use the result of the function call.
> But even if D did "SomVarBar := Foo;", I still get the warning.

Yes. Still, if the (formally incorrect) warning helps in pointing out that
the chosen solution may not be the most efficient one, it isn't so bad
from my point of view. ;-)

Tomas