[fpc-devel] (not) initialized managed variable / regarding ansistring

[Martin Frb]

On 23/02/2025 20:42, Martin Frb wrote:
> function Foo: ansistring;
> var s: ansistring;
> begin
>   SetLenght(s, 10000); fillchar(@s[1], 10000, ' ');
>   SetLenght(Result, 10000); fillchar(@Result[1], 10000, ' ');
> end;
>
...
> So in the original example above, if either string is having a large 
> content (pointing to a length = 1MB chars area), then SetLengh would 
> copy "10000" chars of that into the new unique string.
> Such a copy would be an unnecessary cost.
>
> I know this will happen (with smaller length) when using -gt.
> But will it ever happen in any other case?
>

So yes I found an example for result. The length of result will have the 
previous value.

program Project1; {$Mode objfpc}

function bar: AnsiString;
begin
   writeln(Length(Result));
   SetLength(Result, random(20));
end;

procedure foo;
var   a: String;
begin
   repeat     a := bar;   until a = '';
end;

begin
   foo;  readln;
end.


Leaves the question: What about local vars?




>
> -------------------
> As a side note
>
> program Project1;
> procedure foo;
> var
>   s: ansistring;
> begin
>   SetLength(s, 100);
> end;
>
> begin
>   foo;
> end.
>
> Compile with -O3 and somehow the compiler realizes that s is 
> initialized. The warning is gone.
>
> Interestingly -O3 will also do that for "result" in "function foo: 
> ansistring;" => Doesn't that depend on the caller, or is there some 
> magic?

Any idea why -O3 does not print the warning?

In my new example "result" will be having the previous value, even with 
O3. So it should be giving that warning?