[fpc-devel] Wrong docs: not initialized global variables

[Ondrej Pokorny]

On 03.07.2018 20:06, Martin wrote:
> On 03/07/2018 19:27, Ondrej Pokorny wrote:
>>> On 7/3/2018 9:40 AM, Ondrej Pokorny wrote:
>>>> program Project1;
>>>> var
>>>>    S: string;
>>>> begin
>>>>    SetLength(S, 10); // <<< warning here --->> WHY ????
>>>>    Writeln(S[2]);    // <<< no warning here --->> WHY ????
>>>> end.
>>>
>>
>> The compiler tells me that the line "SetLength(S, 10);" is 
>> potentially dangerous, which it is not.
> "writeln(a)" where a is "integer" is not >dangerous< either. Yet it is 
> an uninitialized value that you write, and therefore likely not what 
> you want (or you would have used random).
> "potential error" <> "dangerous"
>
> In any case,
> - "S" is the array (the container, the length, and internally the 
> pointer to the memory).
> - SetLength is a function taking a "var param"
>
> S has no specific value yet. So SetLenght gets a random value as 
> param. Therefore this warning is correct.
> To initialize "S":
>    S := nil; // no warning
>
>> The line "Writeln(S[2]);" is potentially dangerous, but the compiler 
>> does not warn me about it.
> It is a "potential error", dangerous or not.
>
> I agree in a perfect world the compiler would keep track of each 
> individual element in an array, and know which ones are initialized. 
> In reality this doesn't work.
>
> In theory there are 3 states that the compiler can have:
> - it knows a variable is not initialized => It warns (even if the case 
> it harmless)
> - it knows a variable is initialized => it doesn't warn
> - it doesn't know if a variable is initialized => it doesn't warn (and 
> that is IMHO correct)
>
> SetLength(S, 10); // The compiler knows S is not initialized
> Writeln(S[2]);    // The compiler does not know (this is about S[2], 
> not about S)
>
> In the "s[2]" case, it could theoretically be implemented as a very 
> special case. But the majority of array element access is not 
> practical to implement.
> And even in this case the cost/value is not balancing out.

You three (Ralf, Stefan and you) completely missed my point.

The warning and my point is not about array elements at all. Yes, I used 
an array element as an example but I could easily use the whole 
array/string in a different way, too.


>> => The compiler warns me about the wrong line.
> No, but it only gives you one out of 2 warnings.

No, the compiler warns me about the wrong line. SetLength/FillChar 
should not emit a warning at all because there is nothing to warn about. 
See my reply to Florian if my first email with the points was not clear 
enough.

Ondrej