On 02.07.2019 23:34, Jonas Maebe wrote:
On 02/07/2019 22:31, Ondrej Pokorny wrote:
This is similar to the object-is operator that gets evaluated as well
even if the type of the left-side value is the type at right side:
var
Value: TPersistent;
begin
Value := TPersistent(TObject.Create);
IsValid := Value is TPersistent; // IsValid gains false
This is an invalid program. If you compile with -CR, the program will
abort with an error when the typecast is performed, because it will get
replaced by an "as" operation. In that sense, "integer as enum" would
indeed be somewhat similar, and -CR might even be extended to perform
the same replacement of explicit typecasts with "as" operators for these
types.
No, this is a perfectly valid program with a perfectly defined behavior!
It is a perfectly valid program even with -CR. When I use -CR I expect
an exception and I will handle it - in this case I don't need the is-check:
var
Value: TPersistent;
begin
try
Value := TPersistent(TObject.Create); // or: TObject.Create as
TPersistent (without -CR)
except
Writeln('Wrong value supplied! Exit.');
Exit;
end;
// do something
Readln;
end.
A good real-word example is the cast from a pointer:
program Project1;
uses Classes;
var
Value: TPersistent;
P: Pointer;
begin
P := TObject.Create;
Value := TPersistent(P);
if Value is TPersistent then
Writeln('Value is TPersistent')
else
Writeln('Value is not TPersistent');
Readln;
end.
As an example of an operation on the resulting "Value" that is already
undefined: if you would call a TPersistent virtual method on it, and
whole-program optimization devirtualised that call, it may call the
"correct" method of TPersistent instead of using the VMT of whatever
other class instance type Value points to.
Yes, I agree that calling an (unavailable) method on an object casted to
a wrong class is an invalid operation. But this is not what I did in my
example. Preventing calls of unavailable methods or access of
unavailable fields is exactly what I the IS-operator is for.
Invalid data means undefined behaviour, always. "is" is not a special
case that is immune to this.
We are again at the very fundamental question "what invalid data is".
Storing a TObject in TPersistent is not an invalid operation IMO. Both
are class objects and both can be checked for inheritance with the
IS-operator. Calling methods, accessing fields etc. on the TObject-value
in TPersistent-variable is invalid, indeed. But not a simple IS-operator.
What I understand as invalid for the IS-operator is e.g. trying to call
it for an interface in TPersistent variable:
var
Value: TPersistent;
P: Pointer;
I: IUnknown;
begin
I := TInterfacedObject.Create;
P := I;
Value := TPersistent(P);
if Value is TPersistent then
And e.g. in the context of generics,
simplifying/removing such checks where possible would probably be quite
desirable.
Not really. The IS-operator returns false for NIL that is a valid value,
so you cannot remove such checks:
var
Value: TPersistent;
IsValid: Boolean;
begin
Value := nil;
IsValid := Value is TPersistent; // IsValid gains false - you cannot
simply replace this check with true
Writeln(IsValid);
Readln;
end.
Ondrej
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