Never seen this usage before
l h
that I don't understand the purpose of. If someone could tell me what
kind of construct this is and its purpose, I'd appreciate it.
class A
{
public:
typedef A Adesc;
typedef B::C<Adesc> B1;
typedef B::D<Adesc> B2;
};
What is the intent of this class of typedefs and what is it used for?
Thanks.
Les
Ian Collins
They are shorthand. If you uses a template type often, typing the full
type becomes tedious.
--
Ian Collins
tonydee
> class A
> {
> public:
> typedef A Adesc;
> typedef B::C<Adesc> B1;
> typedef B::D<Adesc> B2;
>
> };
>
> What is the intent of this class of typedefs and what is it used for?
Typedefs provide alternative names, sometimes for convenience,
sometimes to hide lower level implementation choices. Here, A wants
to define types Adesc, B1 and B2 so people can use them as "A::Adesc,
A::B1, A::B2, though they actually refer to A, B::C<A> and B::D<A>
respectively. Clearly, C and D are templates in a scope (namespace or
class/struct) "B". Such code is common, for instance - classes often
typedef instantiations of STL containers (e.g. "std::vector<int>
Container"), making it both easier to refer to the Container type and
its iterators and easier to change to say a list later without needing
so many changes elsewhere in the class and client code.
Cheers,
Tony
l h
I understand the basics of typedefs. I've just never seen them nested
within a class before, particularly since the class is just a
collection of typedefs. So you're saying that this is using the outer
class as a namespace for the typedefs?
Les
Victor Bazarov
To know exactly what *this* class is for, you need to ask the person who
*wrote* the code. However, we could try *guessing* the purpose. Often
in generic programming types are used to express some relationships and
to control generation of code (like conditional expressions in
run-time). Often the types need to conform to a certain specification,
like in the standard library every standard container has 'data_type'
defined and so one. It means that some generic algorithm is looking for
'T::data_type', which will resolve to something *iff* the 'T' class has
the proper typedef for 'data_type'. Want more? Get a decent book on
template programming, like the book by Vandevoorde and Josuttis.
V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
tonydee
> I understand the basics of typedefs. I've just never seen them nested
> within a class before, particularly since the class is just a
> collection of typedefs. So you're saying that this is using the outer
> class as a namespace for the typedefs?
Some people use classes as a scoping mechanism, preferring them over
namespaces. Professor John Lakos (author of Large Scale C++ Design),
encouraged this in the past (and may still), on the grounds that class
scope can't be reopened - if you've found the class/struct, then you
know exactly what's inside it, whereas namespaces can be put together
from many pieces - making it harder to get an accurate idea of their
content. On the other hand, this very ability means namespaces can
unify a logical interface across the bounds of multiple "physical"
files, which IMHO is more important. Another aspect is that classes
force the use of the scope (i.e. A::B1 must be used), whereas
namespaces can be "used" so "B1" is sufficient. Again, opinions vary
as to whether this is good or bad. Anyway, don't be thrown by the
class wrapping the typedefs... it's just being used like a namespace
as you say....
Cheers,
Tony
James Kanze
I don't think that's a very frequent use today. The most common
use is probably as a sort of traits class, as Victor described,
configuring some template class. Another possibility would be
to use it as a base class, so that the derived class would get
all of the typedef's, immediately. This is often done when the
derived class needs a lot of typedef's; e.g. when it's a
container, or an iterator, or such.
--
James Kanze