A Question about std::list

[JustSomeGuy]

When you iterate through a list of objects in a list.

list<object> mylist;
list<object>::c onst_iterator iter;
object ob;

for (iter=mylist.be gin(); iter != mylist.end(); ++iter)
{
ob = *iter;
ob.value = 10;
}

So here I am iterating through the list
but is the statement ob = *iter making a copy of an element in my list?
or am I getting a refrence to an element in mylist?
It seems inefficient to me to make a copy of an element in the list, when
all I really want is a pointer to the object or a refrence to it so that
I can modify it...

I assume that iter->value is not going to work...

Comments?

[Kai-Uwe Bux]

JustSomeGuy wrote:

When you iterate through a list of objects in a list.

list<object> mylist;
list<object>::c onst_iterator iter;
object ob;

for (iter=mylist.be gin(); iter != mylist.end(); ++iter)
{
ob = *iter;
You get a copy;
ob.value = 10;
You modify the copy. Now, you probably want to insert:

*iter = ob;
}

So here I am iterating through the list
but is the statement ob = *iter making a copy of an element in my list?
or am I getting a refrence to an element in mylist?

It seems inefficient to me to make a copy of an element in the list, when
all I really want is a pointer to the object or a refrence to it so that
I can modify it...
Yep, that would be inefficient.

I assume that iter->value is not going to work...

You did not actually try, did you?
Best

Kai-Uwe

[John Harrison]

"JustSomeGu y" <no**@nottellin g.com> wrote in message
news:DmtAc.7577 59$Pk3.612408@p d7tw1no...

When you iterate through a list of objects in a list.

list<object> mylist;
list<object>::c onst_iterator iter;
object ob;

for (iter=mylist.be gin(); iter != mylist.end(); ++iter)
{
ob = *iter;
ob.value = 10;
}

So here I am iterating through the list
but is the statement ob = *iter making a copy of an element in my list?
or am I getting a refrence to an element in mylist?
It seems inefficient to me to make a copy of an element in the list, when
all I really want is a pointer to the object or a refrence to it so that
I can modify it...

I assume that iter->value is not going to work...

Comments?

Curious, if you want to modify the list why did you use a const_iterator? If
you want a pointer or reference to an element in the list, why didn't you
just declare one?

list<object>::i terator iter;
object& ref;
object* ptr;

for (iter=mylist.be gin(); iter != mylist.end(); ++iter)
{
ref = *iter;
ptr = &*iter;
ref.value = 10;
ptr->value = 10;

But as Kai-Uwe said simply 'iter->value = 10' works provided you use an
iterator not a const iterator.

john

[David Harmon]

On Fri, 18 Jun 2004 06:53:44 +0100 in comp.lang.c++, "John Harrison"
<jo************ *@hotmail.com> wrote,

object& ref;
References must be initialized to refer to an object.
ref = *iter;

References may not be reseated after initialization.
Assignment assigns to the underlying object.

for (iter=mylist.be gin(); iter != mylist.end(); ++iter)
{
object &ref(*iter);
ref.value = 10;
}

[John Harrison]

"David Harmon" <so****@netcom. com.invalid> wrote in message
news:40******** *******@news.we st.earthlink.ne t...

On Fri, 18 Jun 2004 06:53:44 +0100 in comp.lang.c++, "John Harrison"
<jo************ *@hotmail.com> wrote,

object& ref;

References must be initialized to refer to an object.

ref = *iter;

References may not be reseated after initialization.
Assignment assigns to the underlying object.

for (iter=mylist.be gin(); iter != mylist.end(); ++iter)
{
object &ref(*iter);
ref.value = 10;
}

Yes, my mistake.

john

[JustSomeGuy]

John Harrison wrote:

"David Harmon" <so****@netcom. com.invalid> wrote in message
news:40******** *******@news.we st.earthlink.ne t...

On Fri, 18 Jun 2004 06:53:44 +0100 in comp.lang.c++, "John Harrison"
<jo************ *@hotmail.com> wrote,

object& ref;

References must be initialized to refer to an object.

ref = *iter;

References may not be reseated after initialization.
Assignment assigns to the underlying object.

for (iter=mylist.be gin(); iter != mylist.end(); ++iter)
{
object &ref(*iter);
ref.value = 10;
}

Yes, my mistake.

john

Is there any speed difference between the refrence vs the pointer?

[JustSomeGuy]

John Harrison wrote:

Curious, if you want to modify the list why did you use a const_iterator? If
you want a pointer or reference to an element in the list, why didn't you
just declare one?

At the risk of sounding like a novice... uhmmm... I didn't know there was
any other type of iterator but a const_iterator. .. doh! :)

[JustSomeGuy]

>

Curious, if you want to modify the list why did you use a const_iterator? If
you want a pointer or reference to an element in the list, why didn't you
just declare one?

this doesn't want to compile....

class image : public std::list<eleme nt>
{
element getElement(key k) const
{
image::iterator iter;
for (iter=begin(); iter != end(); ++iter)
{
element &elem(*iter) ;
if (key == k)
{
return(elem);
}
}
throw("Not found");
}
I think the reason this isn't working is due to the const declaration of the
method.
So what am I to do?

[David Harmon]

On Fri, 18 Jun 2004 11:26:12 -0600 in comp.lang.c++, JustSomeGuy
<No***@ucalgary .ca> wrote,

Is there any speed difference between the refrence vs the pointer?

Probably none. Only actual measurement can tell for sure.

[John Harrison]

"JustSomeGu y" <No***@ucalgary .ca> wrote in message
news:40******** *******@ucalgar y.ca...

Curious, if you want to modify the list why did you use a const_iterator? If you want a pointer or reference to an element in the list, why didn't you just declare one?

this doesn't want to compile....

class image : public std::list<eleme nt>
{
element getElement(key k) const
{
image::iterator iter;
for (iter=begin(); iter != end(); ++iter)
{
element &elem(*iter) ;
if (key == k)
{
return(elem);
}
}
throw("Not found");
}
I think the reason this isn't working is due to the const declaration of

the method.
So what am I to do?

const iterators and const references

class image : public std::list<eleme nt>
{
element getElement(key k) const
{
image::const_it erator iter;
for (iter=begin(); iter != end(); ++iter)
{
const element &elem(*iter) ;
if (key == k)
{
return(elem);
}
}
throw("Not found");
}

That's the thing about const, it's contagious. Start using it one place and
it ends up everywhere.

john