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From: Ulrich Eckhardt on 8 Dec 2009 08:53
Andrew wrote:
> Subject says it all really.

A common baseclass and virtual functions require a certain overhead which is
paid for just dereferencing or stepping an iterator. That overhead is
unacceptable to the STL philosophy, which had efficiency as a major goal.

> I realise that in the upcoming standard the use of auto to declare the
> iterator will make it slightly easier.

That doesn't change anything, it just saves you some typing. The STL or its
descendants in the C++ standard library will remain compile-time
polymorphic only.

Uli

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From: Pedro Lamarão on 8 Dec 2009 08:53
On 7 dez, 17:52, Andrew <marlow.and...(a)googlemail.com> wrote:

> Subject says it all really. I have been in Javaland for a while where
> you do have this capability. I am aware of Meyers Effective STL item 2
> ("beware of the illusion of container-independent code") but I don't
> think item 2 applies when all you want to do is iterate over a
> container. This is the simple case that people want to do most of the
> time (IMHO). Of course if you want to do something else like reverse
> iterate etc then with C++ you can. But it does seem mysterious to me
> why it is more awkward to do a generic iteration for the most common
> case.

Do you mean you want to use one name for the type of "iterator
variable" and use that name with all container types?

Like this?

-- x --

std::vector<int> v;
std::list<double> l;
std::set<char> s;

general_iterator* i = v.begin();
// etc.

i = l.begin();
// etc.

i = s.begin();
// etc.

-- x --

In C++ an array is a very primitive type whose "iterator" cannot be an
instance of general_iterator. Therefore, this scheme would never be
truly uniform -- as the current scheme actually is.
As you pointed out, the notational inconvenience will go away with the
new "auto" syntax.

This is just no the C++ way of things.
It may sound silly but goes all the way down to compiler optimizers.

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P.

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From: CornedBee on 8 Dec 2009 08:53
On Dec 7, 8:52 pm, Andrew <marlow.and...(a)googlemail.com> wrote:
> Subject says it all really. I have been in Javaland for a while where
> you do have this capability.

The runtime polymorphism of Java is often not available in C++ in
favor of static polymorphism using generic programming. for_each can
iterate over any container.

Sebastian


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From: Kirill_Lykov on 8 Dec 2009 08:52
STL - is a template library. Template is an implementation of static
polymorphism. It makes it possible to use containers in common way in
different algorithms and so on but in another way than using dynamics
polymorphism. I believe you should read more about difference in these
two kinds of polymorphism.
In Java there is no static polymorphism at all(generics is just a
syntactical sugar) that's why dynamic polymorphism is used.

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From: Kenneth 'Bessarion' Boyd on 8 Dec 2009 08:51
On Dec 7, 1:52 pm, Andrew <marlow.and...(a)googlemail.com> wrote:
> Subject says it all really. I have been in Javaland for a while where
> you do have this capability.

Well, C++ does have generic iteration, it just uses the tools
available (template functions and #include <algorithm>) to generically
iterate with any iterator (as defined by the standards), even raw
pointers.

Raw pointers are bad news for generically iterating by deriving from
the One True Abstract Base Class Iterator of the One True Abstract
Base Class.
* In C++, iterators are generalizations of raw C-style pointers. So
generic iteration has to generically iterate raw C-style pointers.
* raw C-style pointers can't inherit from anything, by definition. In
particular, they can't inherit from the One True Abstract Base Class
Iterator.

As Java does not have raw pointers by design, it has more flexibility
in how to provide generic iteration.

Someone who actually knows the history could review what other options
were rejected for C++.


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