Simen Kjaeraas wrote:
Sam Huwrote:
Q4.In the delegate somFnExp:front(),popFront,empty() are all not
defined??Anyway it is not an interface ,so why it is allowed?
Basically, is(typeof(X)) is D magic.
One could interpret it as 'is X a valid type', or perhaps more
correctly as 'does X compile'. So if SomeFnExp does something it
is not allowed to (e.g. call popFront on something that has no
popFront), it will return false.
If I were to write this code without is(typeof()) around it:
R r;
if (r.empty) {}
r.popFront;
auto h = r.front;
It might seem a strange piece of code, but there is nothing
inherently wrong with it. empty, popFront and front are
expected members of R, and will give a compilation error if R
does not follow the interface (note: not as in a D interface, but
as in exposing the correct functions to the outside world).
--
Simen
IOW,
is(typeof({ XXXXXX }())
Evaluates to a boolean meaning "does a function containing XXXXXX
compile". Its purpose here is to find out if r has "empty", "popFront"
and "front". This is placed inside a template that has one member that
is the name of the template, so it ends up that:
isInputRange!(R)
just becomes a boolean that's true if R has "empty", "popFront", and
"front", and false if it doesn't.
... C++ template hacks are worse. Really. Usually.
