ATL / WTL / STL

5-Feb-03 21:34

Hi, everyone!

I am puzzled at some words taken from my STL
tutorial. Here are the words at which I am puzzled.

--------
Note: For two input iterators a and b, a == b implies *a == *b.
For istream iterators, this condition doesn't hold.
--------

My opinion is,

--------
Two istream iterators are equal if :

both are end-of-stream iterators and this can no longer be read, or
both can read and use the same stream

from "The C++ Standard Library" by Josuttis.
--------

So I think whether a and b are equal has nothing to do
with *a and *b. Another question, "input iterators" is
different from "istream iterators"? I think they are the same.

Am I correct? Who can give me an explanation?
Better with an example. Smile | :)

Thanks in advance,
George

Joaquín M López Muñoz5-Feb-03 22:38

5-Feb-03 22:38

Note: For two input iterators a and b, a == b implies *a == *b.

This is just false. The only requisite about == for input iterators is that it should be an equivalence relation.

both are end-of-stream iterators and this can no longer be read, or
both can read and use the same stream.

This is absolutely right.

Another question, "input iterators" is different from "istream iterators"?

No, an istream_iterator is an input iterator, but an input iterator is not necessarily an istream_iterator (for instance, a pointer is an input iterator).

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

George26-Feb-03 0:03

6-Feb-03 0:03

Hi, Joaquín buddies!

As you said,

--------
This is just false. The only requisite about == for input iterators is that it should be an equivalence relation.
--------

Do you mean that what tutorial is saying is not correct?
What puzzled me most is the meaning of "equivalence relation". What do you mean "equivalence relation"?

Because I think I can give you an example when a == b, while the same time *a != *b.

Thanks in advance,
George

Joaquín M López Muñoz6-Feb-03 7:01

6-Feb-03 7:01

Of course you can give that example Smile | :)

An equivalence relation, if you remember from your Math classes, is just a relation being reflexive, symmetric and transitive. With regard to our particular problem, this means that

i==i always evaluates to true,
if i==j then j==i,
if i==j and j==k then i==k.

As you can see, expressions of the type *i are not involved in this definition.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

George26-Feb-03 15:22

6-Feb-03 15:22

Hi, Joaquín buddies!

I think you mis-understand my question. My
origin question is, the tutorial said,

--------
if( (i == j) && (i and j are both input iterators) )
then
(*i == *j)
endif
--------

My question is not that I do not understand what means
"equivalence relation". Smile | :)

What is your opinion about the tutorial? Is it correct?

Thanks in advance,
George

Joaquín M López Muñoz6-Feb-03 19:48

6-Feb-03 19:48

--------
if( (i == j) && (i and j are both input iterators) )
then
(*i == *j)
endif
--------

This is false. The tutorial is wrong. The only requisite about == for input iterators in general is the equivalence relation stuff, as you pointed out before.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

George27-Feb-03 2:52

Re: Tab Control quick hiding

7-Feb-03 2:52

Thanks, Joaquín buddies!

George

Tab Control quick hiding

Leo Smith5-Feb-03 11:59

Leo Smith

5-Feb-03 11:59

I have what is probably a simple question, but I am stuck and need help. I am trying to create an Options dialog box for an app. I am using WTL to create the app. I have added the tab control, but I need a quick way to select the controls for each page. In VB I would create a group control and add the controls to the group, then show and hide the group as needed. I can't seem to make this work in WTL (probably doing something wrong with adding the controls to the group). I also, think that I might be able to use a dialog resource like a property sheet, but I am at a loss at how to get this added to the Tab Control. What is the best way to proceed?

Thanks for any help you can give.

Leo T. Smith

Ben Burnett7-Feb-03 13:37

Ben Burnett

7-Feb-03 13:37

This is probably not the answer you’re looking for but, as far as I know, the way groups work in VB is just that, a VB thing. There is no parallel functionality in the rest of windows programming. The closest you could come would be to store a list of all the controls in the 'group' you want to modify and simply hide/shoe them when necessary.

Besides, the thing you probably want to be doing, as you mentioned, is create a property sheet. There are some samples of this on this site:

Using DDX and DDV with WTL[^]

It does concern itself with a more advance topic, but the code for using both property sheets and property pages is in there.

cheers,
-B

Question about output iterator.

George25-Feb-03 3:16

5-Feb-03 3:16

Hi, everyone!

Here are the sentences taken from my STL tutorial,

--------
Any value of an output iterator may have at most one active copy at any given time.

// i and j are output iterators
// a and b are values written to a iterator position
i = j;
*i++ = a;
*j = b;

is not a valid code sequence.
--------

I write a piece of code by myself tried the codes and
run it. I find it runs OK! So I doubted what is the tutorial
talking about. Can anyone explain it to me? (What means "at most one active copy at any given time" ?)

Here is my sample,

--------
#include <iterator>
#include <fstream>
#include <iostream>

using namespace std;

void main()
{
ofstream ofile ("c:\\file1.txt");
ostream_iterator<int> i (ofile);
ostream_iterator<int> j (ofile);
// i and j are output iterators
// a and b are values written to a iterator position
*i++ = 1;
*j = 2;
}
--------

Thanks in advance,
George

Joaquín M López Muñoz5-Feb-03 8:01

5-Feb-03 8:01

The semantics of output iterators are simple, and it reduces to the following rule: a given position has to be accessed once and only once. Let us examine why the following pieces of code are incorrect:

*i=5;
++i;
++i;

This is incorrect as you're "skipping" a value.

*i=5;
*i=6;

Wrong, as you're assigning to the same position twice.

j=i;
*i++=1;
*j=2;

Wrong, for the same reason as before.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

George25-Feb-03 19:58

5-Feb-03 19:58

Hi, Joaquín buddies!

In the last sample,

--------
j=i;
*i++=1;
*j=2;
--------

i and j are referencing to the same stream but
they are different iterators. I still doubted
why they are assigning to the same position twice?
Since they are different iterators and I think it
is like two pointers pointing to the same memory.

Can you give me an explanation?

Thanks in advance,
George

Joaquín M López Muñoz5-Feb-03 22:40

5-Feb-03 22:40

No, wait. I've reread the standard and now I think the above piece of code is correct. Sorry.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

George25-Feb-03 23:53

5-Feb-03 23:53

Hi, Joaquín buddies!

Do you mean that the tutorial is
not correct? Since the tutorial says
that the piece of code is not correct.

Thanks in advance,
George

George27-Feb-03 4:54

Question about input iterator on the same input stream.

7-Feb-03 4:54

Hi, Joaquín buddies!

Have you found the answer?

Thanks in advance,
George

George24-Feb-03 19:26

Re: Question about input iterator on the same input stream.

4-Feb-03 19:26

Hi, everyone!

Here is a simple STL program. It uses two input iterators
to use on the same input file. I do not understand why the
output is looking like this.

Here are the source codes,

--------
#include <iterator>
#include <fstream>
#include <iostream>

using namespace std;

void main()
{
// c:\\file1.txt: 0 1 2 3
ifstream ifile ("c:\\file1.txt");

istream_iterator<int> r (ifile);
istream_iterator<int> s (ifile);

(r==s) ? cout << "equal" : cout << "not equal";
cout << endl;

++r;
++s;

cout << *r << endl;
cout << *s << endl;

(r==s) ? cout << "equal" : cout << "not equal";
cout << endl;
}
--------

Here are the outputs,

--------
equal
2
3
equal
--------

Thanks in advance,
George

Joaquín M López Muñoz4-Feb-03 20:14

Re: Question about input iterator on the same input stream.

4-Feb-03 20:14

You're twisting the semantics of istream_iterators: You only can do the following with such an iterator:

Check for the end of stream with the expression it==istream_iterator<int>().

retrieve a value from the associated istream with an expression of the form x=*it;.

Increment the iterator with ++it or it++.Moreover, after retrieving a value, you must increment the iterator previous to further retrievals. That's it, do not assume you can do much else with such iterators. Do no think these iterators are something like pointers to the data contained in the file, because they are not, hence the odd results when testing r and s for equality. istream_iterator is designed as a convenient adaptor to take advantage of generic algoritms, as such its functionality is strictly limitied to what these algorithms require.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

George25-Feb-03 1:17

Question about copy algorithm of STL.

5-Feb-03 1:17

Thanks, Joaquín buddies!

Such piece of codes are taken from a turorial
and it is a concept-proof sample.

It really doubted me. So, if you can explain it
to me, I will be really thankful. I am a newbie
of STL.

Cheers,
George

George24-Feb-03 19:23

4-Feb-03 19:23

Hi, everyone!

Here is a simply STL program deals with copy
algorithm of STL. I do not understand what is the
meaning of the socond parameter of my case, i.e.

--------
istream_iterator<int> ()
--------

What does it mean? Here are the source codes
and related output.

Source codes,

--------
#include <iterator>
#include <fstream>
#include <iostream>

using namespace std;

void main()
{
//0 1 2 3 are in c:\\file1.txt
ifstream ifile ("c:\\file1.txt");

copy (istream_iterator <int> (ifile),
istream_iterator<int> (),
ostream_iterator<int> (cout) );

}
--------

Output,

--------
0123
--------

Thanks in advance,
George

valikac4-Feb-03 20:08

valikac

4-Feb-03 20:08

istream_iterator() indicates end of a stream.

Kuphryn

George25-Feb-03 1:09

5-Feb-03 1:09

Hi, Kuphryn buddies!

As you mentioned, istream_iterator<int> () is an "end-of-stream" iterator. But it is the end of which stream? Why it is assigned as
the end of the ifile stream? Since there are no parameter in istream_iterator<int> (), it really doubted me a lot. (If it has a parameter identifies the ifile, it would be clear, I think.)

Thank in advance,
George

Ryan_Roberts5-Feb-03 3:29

Ryan_Roberts

5-Feb-03 3:29

Its the end of all istream_iterator<int>'s. Think of it as a constant.

Ryan.

They read good books, and quote, but never learn
a language other than the scream of rocket-burn.
Our straighter talk is drowned but ironclad;
elections, money, empire, oil and Dad.

George25-Feb-03 19:39