2016-07-28 13:30 GMT+02:00 Michael Van Canneyt <[email protected]>:
>
> Assume a is null before the call to test1:
>
> test1(a);
>
That won't compile.
>
> 1. What happens if this procedure is empty, i.e. no write is performed ?
> (or it is performed conditionally)
> On return, is a null or not ?
>
You are not able to call test1 in presented form (you will get error:
Incompatible types).
> 2. Assume your variable is null before the call to Test1.
> What is the value on entry into Test1 ? i.e. what happens if
>
> begin
> if X=0 then
> x:=1
> else
> X:=23;
> end
>
That won't compile too. The correct code:
=== code begin ===
if x.HasValue and (x^ = 0) then
x := 1
else
x := 23;
=== code end ===
or
=== code begin ===
if x.GetValueOrDefault = 0 then
x := 1
else
x := 23;
=== code end ===
3. How do you detect an actual write ?
>
> 4. What happens if Test1 is an external C procedure ?
It depend on context. If we have:
x := 1; // is detected in class operator Implicit(A: T): TNullable<T>;
when you use
x^ := 1; // you are operating directly on Instance field
in that case if x.HasValue is false you will get EAccessViolation
--
Best regards,
Maciej Izak
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