I have a question about how the stl map class works. I have a subscription
class that I use to manage client application subscriptions. These are
stored in a map. Periodically, I need to update the subscriptions in the map
to reflect changes to data.  When I iterate over the map, I expect to get a
reference to the object stored in the map using the myclass mc =
*iter->second notation;. What I am finding is that the copy constructor is
getting invoked resulting in a copy getting returned instead of a reference.
I tried the alternate approach using the operator[] but get the same
results.

At this point the workaround is to put the modified object back into the map
which seems terribly inefficient.

I checked the SGI website and it looks like it should be returning a
reference and not a copy. But I am not certain.

This is on Solaris 2.8 with 6.2 compiler.

Does this sound correct? Should I expect this to be the defined behavior or
is there another approach?

Thanks,

Thomas




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If you use
             myclass mc = *iter->second;
the copy constructor is invoked and mc will be a copy of the stored element.
You want to modify the element in the map, not the copy it, so you must get
a reference of that element. The correct version is:

             myclass& mc = *iter->second;

Now the modification on mc will affect the element in the map the iterator
points to.

Bolo.

""Thomas"" <tdineen@erols.com> wrote in message
news:bl0bca$k2r$1@bob.news.rcn.net...
> I have a question about how the stl map class works. I have a subscription
> class that I use to manage client application subscriptions. These are
> stored in a map. Periodically, I need to update the subscriptions in the
map
> to reflect changes to data.  When I iterate over the map, I expect to get
a
> reference to the object stored in the map using the myclass mc =
> *iter->second notation;. What I am finding is that the copy constructor is
> getting invoked resulting in a copy getting returned instead of a
reference.
> I tried the alternate approach using the operator[] but get the same
> results.
>
> At this point the workaround is to put the modified object back into the
map
> which seems terribly inefficient.
>
> I checked the SGI website and it looks like it should be returning a
> reference and not a copy. But I am not certain.
>
> This is on Solaris 2.8 with 6.2 compiler.
>
> Does this sound correct? Should I expect this to be the defined behavior
or
> is there another approach?
>
> Thanks,
>
> Thomas
>
>
>
>
> ---
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> [ your news-reader.  If that fails, use mailto:std-c++@ncar.ucar.edu    ]
> [              --- Please see the FAQ before posting. ---               ]
> [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html                       ]
>


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> I have a question about how the stl map class works. I have a subscription
> class that I use to manage client application subscriptions. These are
> stored in a map. Periodically, I need to update the subscriptions in the
map
> to reflect changes to data.  When I iterate over the map, I expect to get
a
> reference to the object stored in the map using the myclass mc =
> *iter->second notation;. What I am finding is that the copy constructor is
> getting invoked resulting in a copy getting returned instead of a
reference.
> I tried the alternate approach using the operator[] but get the same
> results.
>

You should write myclass &mc = iter->second;

That is you want a reference on left side... Also on the right side
iter->second already returns a reference to its member so you should
not need an * (except if you are storing pointers).

Philippe

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"Thomas" wrote:
> I have a question about how the stl map class works. I have a
> subscription class that I use to manage client application
> subscriptions. These are stored in a map. Periodically, I need to
> update the subscriptions in the map to reflect changes to data.  When
> I iterate over the map, I expect to get a reference to the object
> stored in the map using the myclass mc = *iter->second notation;.
> What I am finding is that the copy constructor is getting invoked
> resulting in a copy getting returned instead of a reference. I tried
> the alternate approach using the operator[] but get the same results.
[SNIP]

Rerefence?  So why don't you ask for one?

myclass &mc = *iter->second notation;
        ^
        |
--------+

Now it is a reference.

--
WW aka Attila


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On Fri, 26 Sep 2003 17:33:51 +0000, Thomas wrote:

> I have a question about how the stl map class works. I have a subscription
> class that I use to manage client application subscriptions. These are
> stored in a map. Periodically, I need to update the subscriptions in the map
> to reflect changes to data.  When I iterate over the map, I expect to get a
> reference to the object stored in the map using the myclass mc =
> *iter->second notation;. What I am finding is that the copy constructor is
> getting invoked resulting in a copy getting returned instead of a reference.
> I tried the alternate approach using the operator[] but get the same
> results.
>

If you do mc = (*iterator)->second, then the copy ctor will be invoked,
but if you directly assign to it without the mc = part, then it's fine,
something like: *iterator->second = <something>. Here, only the assignment
operator will be invoked.


> At this point the workaround is to put the modified object back into the map
> which seems terribly inefficient.
>
> I checked the SGI website and it looks like it should be returning a
> reference and not a copy. But I am not certain.
>
> This is on Solaris 2.8 with 6.2 compiler.
>
> Does this sound correct? Should I expect this to be the defined behavior or
> is there another approach?

Regards,
-Dhruv.






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On Fri, 26 Sep 2003 17:33:51 +0000 (UTC), tdineen@erols.com ("Thomas")
wrote:

> myclass mc = iter->second notation;

I removed a * in the above.

> What I am finding is that the copy constructor is
> getting invoked resulting in a copy getting returned instead of a reference.

You asked for a copy.

   myclass& mc(iter->second);

Now you have a reference.

John

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Hello,

tdineen@erols.com ("Thomas") wrote in message news:<bl0bca$k2r$1@bob.news.rcn.net>...
> I have a question about how the stl map class works. I have a subscription
> class that I use to manage client application subscriptions. These are
> stored in a map. Periodically, I need to update the subscriptions in the map
> to reflect changes to data.  When I iterate over the map, I expect to get a
> reference to the object stored in the map using the myclass mc =
> *iter->second notation;. What I am finding is that the copy constructor is
> getting invoked

I think your map is declared as

std::map < Something, myclass >

so you should write

myclass mc = iter->second

And if your class doesn't have an assignment operator, '=' calls the
copy constructor. Try using a reference instead:

myclass & mc = iter->second;


Hope that helps.
Wagner

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