dev-cpp-users

[Dev-C++] STL, iterators and pointers?

[Phil S. <fy...@qw...>]

Some questions about the STL containers and iterators.

 When you insert an object into a multiset (or list, etc.) does it
create a copy of the original Object, or does it connect to the
original? 

and I'm not sure how to ask this next one:
If you have an iterator for the above multiset, can you get a generic
Object* out of it that points to the Object?  I tried taking the
addressof the dereferenced iterator ( &(*iter) ) but that didn't seem to
work out quite right...


Why I'm asking:
I have two classes, Book and Author that each contain a pointer to the
other.

I have a multiset of Books, and a multiset of Authors. When I add a new
Book, I need to be able to link it to an Author, and vice versa. The
Book* I get when I make a new object doesn't correspond to where the
iterator is pointing. 

I guess that I could do this easier if I replaced the reciprical
pointers with multiset iterators, but I want to make the Book and Author
classes as basic as possible.  I'm experimenting with the different STL
containers. and I want to not have to adapt the classes for each one.

Hoping this makes sense,
Phil


-- 
Fy...@qw...

RE: [Dev-C++] STL, iterators and pointers?

[Ioannis V. <no...@ya...>]

> -----Original Message-----
> From: dev...@li... 
> [mailto:dev...@li...] On Behalf 
> Of Phil Schulz
> Sent: Friday, July 06, 2001 4:17 AM
> To: dev...@li...
> Subject: [Dev-C++] STL, iterators and pointers?
> 
> 
> 
> Some questions about the STL containers and iterators.
> 
>  When you insert an object into a multiset (or list, etc.) 
> does it create a copy of the original Object, or does it 
> connect to the original? 


When you say "multiset" i consider that you mean standard library
container. When you create an object of a container type containing a
standard object type then the number specified of objects are created.

E.g..

vector<string> l(10); creates an array (vector) of 10 string objects
with the name l; These 10 string objects are created with the creation
of the vector.

so if we have string s="Test";

l[0]=s;

s is COPIED in l[0];


Now if you create a vector of pointers e.g.

vector<string *> pointers(10);

pointers[0]=&s;

pointer[0] is pointing to our string s;


> 
> and I'm not sure how to ask this next one:
> If you have an iterator for the above multiset, can you get a generic
> Object* out of it that points to the Object?  I tried taking 
> the addressof the dereferenced iterator ( &(*iter) ) but that 
> didn't seem to work out quite right...
> 

iterators act like pointers but they are not pointers. the &(*iter) of
yours should work. Perhaps the container contained pointers and not
objects?

e.g. vector<int *>integers(5);
integers[0]=new int(5); // it is like we assign then integers[0]=5;

vector<int *>::iterator iter=integers.begin();

int *p;

p=*iter // and NOT &(*iter) since the container contains pointers and
our iterator behaves like int **



> 
> Why I'm asking:
> I have two classes, Book and Author that each contain a 
> pointer to the other.
> 
> I have a multiset of Books, and a multiset of Authors. When I 
> add a new Book, I need to be able to link it to an Author, 
> and vice versa. The
> Book* I get when I make a new object doesn't correspond to 
> where the iterator is pointing. 


Probably you made some mistakes on the assignment, check what i wrote
above.


Ioannis

* Ioannis Vranos
* Programming pages: http://www.noicys.f2s.com
* Alternative URL: http://run.to/noicys

RE: [Dev-C++] STL, iterators and pointers?

[Ioannis V. <no...@ya...>]

a small correction


> e.g. vector<int *>integers(5);
> integers[0]=new int(5); // it is like we assign then integers[0]=5;

it is like we assign then *integers[0]=5;


Ioannis

* Ioannis Vranos
* Programming pages: http://www.noicys.f2s.com
* Alternative URL: http://run.to/noicys

Re: [Dev-C++] STL, iterators and pointers?

[Phil S. <fy...@qw...>]

I wrote:
> > Some questions about the STL containers and iterators.
> >
> >  When you insert an object into a multiset (or list, etc.)
> > does it create a copy of the original Object, or does it
> > connect to the original?
> 
Ioannis Vranos wrote:
> When you say "multiset" i consider that you mean standard library
> container. When you create an object of a container type containing a
> standard object type then the number specified of objects are created.
> 
> E.g..
> 
> vector<string> l(10); creates an array (vector) of 10 string objects
> with the name l; These 10 string objects are created with the creation
> of the vector.
> 
> so if we have string s="Test";
> 
> l[0]=s;
> 
> s is COPIED in l[0];

okay, that's pretty clear.  I had been looking at examples that start
with defining an empty container and then adding to it using pushback()
or insert(), so it wasn't clear to me...  I forgot that I probably have
to define copy constructors for my classes. Haven't tested this yet, but
this might be the problem...  
 

> >
> > and I'm not sure how to ask this next one:
> > If you have an iterator for the above multiset, can you get a generic
> > Object* out of it that points to the Object?  I tried taking
> > the addressof the dereferenced iterator ( &(*iter) ) but that
> > didn't seem to work out quite right...
> >
> 
> iterators act like pointers but they are not pointers. the &(*iter) of
> yours should work. Perhaps the container contained pointers and not
> objects?
nope, definitely defined as objects.
My problem is when I try to assign  &(*iter) to a pointer, I get compile
errors 
"initialization to `Book *' from `const Book *' discards qualifiers"
I'll make the new pointer a const, but that'll require another function
to be declared a CONST and on and on , to a point of uselessness. 


> Ioannis
> 
> * Ioannis Vranos
> * Programming pages: http://www.noicys.f2s.com
> * Alternative URL: http://run.to/noicys
> 

Thanks for your input.
Phil

RE: [Dev-C++] STL, iterators and pointers?

[Ioannis V. <no...@ya...>]

> -----Original Message-----
> From: dev...@li... 
> [mailto:dev...@li...] On Behalf 
> Of Phil Schulz
> Sent: Saturday, July 07, 2001 10:21 AM
> To: dev...@li...
> Subject: Re: [Dev-C++] STL, iterators and pointers?
> 
> okay, that's pretty clear.  I had been looking at examples 
> that start with defining an empty container and then adding 
> to it using pushback() or insert(), so it wasn't clear to 
> me...  I forgot that I probably have to define copy 
> constructors for my classes. Haven't tested this yet, but 
> this might be the problem...  

When you use pushback() and vector is full, new objects within the
vector are created.


> nope, definitely defined as objects.
> My problem is when I try to assign  &(*iter) to a pointer, I 
> get compile errors 
> "initialization to `Book *' from `const Book *' discards 
> qualifiers" I'll make the new pointer a const, but that'll 
> require another function to be declared a CONST and on and on 
> , to a point of uselessness. 


Code that works:

A)
#include <vector>

class A
{
};

int main()
{
vector<A> test(10);
A *p;
p=&test[0];
}


B)

#include <vector>

class A
{
};

int main()
{
vector<A> test(10);
vector<A>::iterator iter=test.begin();
A *p;
p=&(*iter);
}



Code that does not work (of course):

#include <vector>

class A
{
};

int main()
{
vector<A> test(10);
vector<A>::const_iterator iter=test.begin();
A *p;
p=&(*iter);
}



You probably were using const_iterator or doing something else wrong.


Ioannis

* Ioannis Vranos
* Programming pages: http://www.noicys.f2s.com
* Alternative URL: http://run.to/noicys