I had another idea yesterday while working with Swift and I was curious to hear
what anyone thought. In Pascal we have “const”, “constref" “var”, “out” params
for functions but no way to document or enforce optional params (which are
pointers/classes).
It’s a very common pattern that a function takes a pointer as a parameter and
we don’t know if we’re allowed to pass null unless we ready the comment or
documentation. I thought there could be an “optional” or “opt” modifier that
serves to 1) document the usage of the function and enforce nil checks on the
pointer when dereferencing. It’s kind of like “const” for read-only parameters
but it says “must-check” instead.
Basically, if a param (or result even?) was marked optional then you would get
a compiler error if you tried to dereference the pointer, unless you were in an
assigned block (or some other intrinsic since “assigned” is just a function).
What do you think of that? Sounds like an easy way to get some support for nil
pointers deref’s and provides self documenting code.
====================
program test;
procedure FreeThis(optional var obj: TObject);
begin
// must check assigned or we get a compiler error
if assigned(obj) then
begin
obj.Free;
obj := nil;
end;
end;
procedure DoThis(optional obj: TObject);
begin
// must check assigned or we get a compiler error
if assigned(obj) then
obj.DoIt;
end;
// can return nil so return value must be checked
function MakeObject: TObject; optional;
begin
result := TObject.Create;
end;
var
obj: TObject;
begin
// MakeObject result is optional so we need to check for assigned
obj := MakeObject;
// must check assigned or we get a compiler error when we read the class name
if assigned(obj) then
begin
writeln(‘new object:’, obj.ClassName);
DoThis(obj);
FreeThis(obj);
// "obj" is "optional var" so we get a compiler error unless we check for
assigned
writeln(obj.ClassName);
end;
end.
Regards,
Ryan Joseph
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