473,289 Members | 1,875 Online
Bytes | Software Development & Data Engineering Community
Post Job

Home Posts Topics Members FAQ

Join Bytes to post your question to a community of 473,289 software developers and data experts.

a few questions about iterators

Hello,

Iterators are typically put into five different categories, namely
input iterator, output iterator, forward iterator, bidirectional
iterator and random iterator. The differences come from the
requirements each kind of iterator has to meet. Therefore, I think
the five categories are kind of conceptual thing, i.e. they are not
really C++ structs/classes etc, is this correct?

There are some functions that return iterators. For example, the
"back_inserter" function returns an iterator when given a container.
Is the returned iterator an object of class type? For this particular
function, is the returned iterator a forward iterator only, or is it
random access iterator?

Another question is that if I implement a container, which has a
function "end()", then it should return one past the last element.
However, the one past element isn't in the container, if the "end()"
is

iterator end(){
return begin() + size();}

then the result may be a pointer pointing to some other structure. On
one hand, it looks a bit unsafe, hence I should reserve the last
element in my container (and not store any real value at that
position) to represent the one-past. On the other hand, since
deferencing an iterator to "end()" is undefined, perhaps I can just
return "begin()+size()". Which strategy is better?

Thanks,
Jess

Jul 6 '07 #1
3 1517
On Jul 6, 12:23 pm, Jess <w...@hotmail.comwrote:
Hello,

Iterators are typically put into five different categories, namely
input iterator, output iterator, forward iterator, bidirectional
iterator and random iterator. The differences come from the
requirements each kind of iterator has to meet. Therefore, I think
the five categories are kind of conceptual thing, i.e. they are not
really C++ structs/classes etc, is this correct?
They are C++ classes, thats why a code like
vector<intv;
vector<int>::iterator s = v.begin();
compiles
There are some functions that return iterators. For example, the
"back_inserter" function returns an iterator when given a container.
Is the returned iterator an object of class type?
Yeah, back_inserter() returns a back_insert_iterator for the container
for which it is invoked.
template <class Cback_insert_iterator<Cback_inserter (C& x)
For this particular
function, is the returned iterator a forward iterator only, or is it
random access iterator?
The returned iteator is a back_insert_iterator which is an adaptor
that functions as output iterator.
Another question is that if I implement a container, which has a
function "end()", then it should return one past the last element.
However, the one past element isn't in the container, if the "end()"
is

iterator end(){
return begin() + size();}

then the result may be a pointer pointing to some other structure. On
one hand, it looks a bit unsafe, hence I should reserve the last
element in my container (and not store any real value at that
position) to represent the one-past. On the other hand, since
deferencing an iterator to "end()" is undefined, perhaps I can just
return "begin()+size()". Which strategy is better?
C++ guarantees that you can have an iterator to one-past-last element
of a container. Dereferencing such an itertaor is not allowed however.
>
Thanks,
Jess

Jul 6 '07 #2
On 2007-07-06 09:23, Jess wrote:
Hello,

Iterators are typically put into five different categories, namely
input iterator, output iterator, forward iterator, bidirectional
iterator and random iterator. The differences come from the
requirements each kind of iterator has to meet. Therefore, I think
the five categories are kind of conceptual thing, i.e. they are not
really C++ structs/classes etc, is this correct?
Yes, and no. Input, Output, ... iterators are concepts, the actual
iterators you use are objects instantiating these concepts. So
std::list<int>::iterator is a bidirectional iterator, but it is also a
class, with members etc. just like any other class.

Consider std::list<int>::iterator and std::set<int>::iterator, they are
both bidirectional iterators, since they full fill all the requirements
that comes with that concept. They are also classes that can be
instantiated just like any other class. An important point though is
that they are not related through inheritance or some other OO concept,
the only relationship they have is that they are C++ Iterators as
defined by the standard.
There are some functions that return iterators. For example, the
"back_inserter" function returns an iterator when given a container.
Is the returned iterator an object of class type? For this particular
function, is the returned iterator a forward iterator only, or is it
random access iterator?
Output iterator.
Another question is that if I implement a container, which has a
function "end()", then it should return one past the last element.
However, the one past element isn't in the container, if the "end()"
is

iterator end(){
return begin() + size();}
That's how std::vector does it. For node-based containers another
solution have to be used.
then the result may be a pointer pointing to some other structure. On
one hand, it looks a bit unsafe, hence I should reserve the last
element in my container (and not store any real value at that
position) to represent the one-past. On the other hand, since
deferencing an iterator to "end()" is undefined, perhaps I can just
return "begin()+size()". Which strategy is better?
Reserving an extra element should not be needed.

--
Erik Wikström
Jul 6 '07 #3
On Jul 7, 2:50 pm, Jess <w...@hotmail.comwrote:
On Jul 7, 12:21 am, James Kanze <james.ka...@gmail.comwrote:
[...]
However, the one past element isn't in the container, if the "end()"
is
iterator end(){
return begin() + size();}
This supposes that the iterator is random access. It won't work
with most iterators. Even with random access iterators, it's at
least as likely that begin() and end() are the primitives, and
that size() is implemented:
size_type size() const
{
return end() - begin() ;
}
then the result may be a pointer pointing to some other structure.
Thanks. Do you mean I should implement begin() and end() first as
primitives and then define size() using end() - begin()?
No. It means that some implementations do it this way. You can
do it whichever way is most convenient for you.
If I have a
container that has an underlying array as a member, I think I can
probably return the "pointer-to-arrays-last-element + 1" as the result
of "end()". Would this work?
Yes.

--
James Kanze (Gabi Software) email: ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

Jul 7 '07 #4

This thread has been closed and replies have been disabled. Please start a new discussion.

Similar topics

4
by: matthurne | last post by:
I am working through exercise 8-2 in Accelerated C++...I am implementing the function equal(b, e, b2) where b is an iterator for the first element in a container, e is an iterator pointing to one...
18
by: deancoo | last post by:
I have gotten into the habit of often using copy along with an insert iterator. There are scenarios where I process quite a lot of data this way. Can someone give me a general feel as to how much...
15
by: Pelle Beckman | last post by:
Hi all, I have a few newbie questions: In function declaration what does a 'const' mean inside the parameter list ? That it won't modify the value? void MemberFunction(const int x);
3
by: codefixer | last post by:
Hello, I am trying to understand if ITERATORS are tied to CONTAINERS. I know the difference between 5 different or 6(Trivial, on SGI). But what I fail to understand is how can I declare all 5...
24
by: Lasse Vågsæther Karlsen | last post by:
I need to merge several sources of values into one stream of values. All of the sources are sorted already and I need to retrieve the values from them all in sorted order. In other words: s1 = ...
2
by: ma740988 | last post by:
typedef std::vector < std::complex < double > > complex_vec_type; // option1 int main() { complex_vec_type cc ( 24000 ); complex_vec_type dd ( &cc, &cc ); } versus
90
by: John Salerno | last post by:
I'm a little confused. Why doesn't s evaluate to True in the first part, but it does in the second? Is the first statement something different? False print 'hi' hi Thanks.
18
by: desktop | last post by:
1) I have this code: std::list<intmylist; mylist.push_back(1); mylist.push_back(2); mylist.push_back(3); mylist.push_back(4);
8
by: Krypto | last post by:
Hi, I have used Python for a couple of projects last year and I found it extremely useful. I could write two middle size projects in 2-3 months (part time). Right now I am a bit rusty and trying...
0
by: MeoLessi9 | last post by:
I have VirtualBox installed on Windows 11 and now I would like to install Kali on a virtual machine. However, on the official website, I see two options: "Installer images" and "Virtual machines"....
0
by: DolphinDB | last post by:
Tired of spending countless mintues downsampling your data? Look no further! In this article, you’ll learn how to efficiently downsample 6.48 billion high-frequency records to 61 million...
0
by: Aftab Ahmad | last post by:
Hello Experts! I have written a code in MS Access for a cmd called "WhatsApp Message" to open WhatsApp using that very code but the problem is that it gives a popup message everytime I clicked on...
0
by: Aftab Ahmad | last post by:
So, I have written a code for a cmd called "Send WhatsApp Message" to open and send WhatsApp messaage. The code is given below. Dim IE As Object Set IE =...
0
by: ryjfgjl | last post by:
ExcelToDatabase: batch import excel into database automatically...
0
isladogs
by: isladogs | last post by:
The next Access Europe meeting will be on Wednesday 6 Mar 2024 starting at 18:00 UK time (6PM UTC) and finishing at about 19:15 (7.15PM). In this month's session, we are pleased to welcome back...
0
by: jfyes | last post by:
As a hardware engineer, after seeing that CEIWEI recently released a new tool for Modbus RTU Over TCP/UDP filtering and monitoring, I actively went to its official website to take a look. It turned...
0
by: ArrayDB | last post by:
The error message I've encountered is; ERROR:root:Error generating model response: exception: access violation writing 0x0000000000005140, which seems to be indicative of an access violation...
1
by: PapaRatzi | last post by:
Hello, I am teaching myself MS Access forms design and Visual Basic. I've created a table to capture a list of Top 30 singles and forms to capture new entries. The final step is a form (unbound)...

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.