copying a vector of objects at runtime

**Julian** · Jul 10 '08, 08:05 AM

Re: copying a vector of objects at runtime

"Julian" <julvar@nospamt amu.eduwrote in message
news:g54akk$c32 $1@news.tamu.ed u...

Hi,
I have a vector defined like this:
std::vector<Loa d*LoadList;
which is populated with different objects of classes that are derived from
'Load'.
I need to make a copy of this list during runtime.
I imagine I'd have to loop through the list, make a copy of each of the
objects in the vector using the copy constructor... but I don't know the
class name during run time so that I can call the appropriate copy
constructor.
actually, I read up on 'typeid' which I supposed would give me the class
name... but I am still confused as how I can call the appropriate copy
constructor for that class during run time.
One option would be to do a string compare of the class names... but that
requires for me to be aware of all the classes derived from 'Load'.
Is there some other way to go about this?
thanks,
Julian.

I did some more searching and looks like I found the solution from a 10 yr
old post!

http://groups.google.com/group/comp.lang.c++/browse_thread/thread/1d418158864335d0/ea75e39ab5004cfc?lnk=st&q=copying+objects+c%2B%2B#

do let me know if this isn't the right solution
sorry for the inconvenience

**Joe Greer** · Jul 10 '08, 03:05 PM

Re: copying a vector of objects at runtime

"Julian" <julvar@nospamt amu.eduwrote in
news:g54akk$c32 $1@news.tamu.ed u:

Hi,
I have a vector defined like this:
std::vector<Loa d*LoadList;
which is populated with different objects of classes that are derived
from 'Load'.
I need to make a copy of this list during runtime.
I imagine I'd have to loop through the list, make a copy of each of
the objects in the vector using the copy constructor... but I don't
know the class name during run time so that I can call the appropriate
copy constructor.

Hmmm, if copying the objects in the vector is the answer, then I have to
wonder why there are pointers in the vector to begin with. That is,
wouldn't vector<Loadhave been a better place to start? Then copying
would be as simple as LoadList2 = LoadList. IME the only time you
really want a vector of pointers is if the objects have identity
(can't/shouldn't be copied at all) or are horrendously expensive to
copy.

If you still want to copy the vector of pointers (after considering
making them value types instead), you can do something like:

#include <iostream>
#include <sstream>
#include <vector>

template <typename T>
T * clone(T * a)
{
return new T(*a); //assuming copy constructor
}

template <typename T>
class clone_insert_it erator : public std::iterator
<std::output_it erator_tag, typename T::value_type>
{
T& m_Container;
public:
explicit clone_insert_it erator(T & container) : m_Container
(container) {}
clone_insert_it erator<T& operator=(typen ame T::value_type const &
a)
{
m_Container.pus h_back(clone(a) );
return *this;
}
clone_insert_it erator<T& operator*()
{
return *this;
}
clone_insert_it erator<T& operator++()
{
return *this;
}
clone_insert_it erator<Toperato r++(int)
{
return *this;
}
};

int main()
{
std::vector<int *v;
std::vector<int *v1;

v.push_back(new int(5));
v.push_back(new int(1));
v.push_back(new int(10));
v.push_back(new int(7));

clone_insert_it erator<std::vec tor<int * cii(v1);
std::copy(v.beg in(), v.end(), cii);

for (int ix = 0; ix < v1.size(); ++ix)
{
std::cout << *v[ix] << std::endl;
}
std::cout << std::endl;
std::cout << std::endl;
for (int ix = 0; ix < v1.size(); ++ix)
{
std::cout << *v1[ix] << std::endl;
}
}

Which seems to work, but I made no attempt at robustness nor
completeness for any particular use.

joe

**Daniel T.** · Jul 11 '08, 02:55 AM

Re: copying a vector of objects at runtime

"Julian" <julvar@nospamt amu.eduwrote:

"Julian" <julvar@nospamt amu.eduwrote:

I have a vector defined like this:
std::vector<Loa d*LoadList;
which is populated with different objects of classes that are derived from
'Load'.
I need to make a copy of this list during runtime.
I imagine I'd have to loop through the list, make a copy of each of the
objects in the vector using the copy constructor... but I don't know the
class name during run time so that I can call the appropriate copy
constructor.
actually, I read up on 'typeid' which I supposed would give me the class
name... but I am still confused as how I can call the appropriate copy
constructor for that class during run time.
One option would be to do a string compare of the class names... but that
requires for me to be aware of all the classes derived from 'Load'.
Is there some other way to go about this?
thanks,
Julian.

>
I did some more searching and looks like I found the solution from a 10 yr
old post!

http://groups.google.com/group/comp.lang.c++/browse_thread/thread/1d4181588643

35d0/ea75e39ab5004cf c?lnk=st&q=copy ing+objects+c%2 B%2B#
do let me know if this isn't the right solution
sorry for the inconvenience

Yes, this is an FAQ:

Inheritance -- virtual functions, C++ FAQ

http://www.parashift.com/c++-faq-lite/virtual-functions.html#faq-20.8

Be advised though, this may not be the best solution for you. You should
also look into holding a vector of smart pointers. If that will work for
you (i.e., if it's ok if both vectors actually point to the same set of
objects,) then copying will be a simple matter of assignment:

vector<smart_pt r<Load loadList;
vector<smart_pt r<Load copy = loadList;

**spacemanspiff** · Jul 11 '08, 04:45 AM

Re: copying a vector of objects at runtime

On Jul 10, 9:46 pm, "Daniel T." <danie...@earth link.netwrote:

"Julian" <jul...@nospamt amu.eduwrote:

I have avectordefined like this:
std::vector<Loa d*LoadList;
which is populated with differentobject sof classes that are derived from
'Load'.
I need to make a copy of this list during runtime.
I imagine I'd have to loop through the list, make a copy of each of the
>objectsin thevectorusing the copy constructor... but I don't know the
class name during run time so that I can call the appropriate copy
constructor.
actually, I read up on 'typeid' which I supposed would give me the class
name... but I am still confused as how I can call the appropriate copy
constructor for that class during run time.
One option would be to do a string compare of the class names... but that
requires for me to be aware of all the classes derived from 'Load'.
Is there some other way to go about this?
thanks,
Julian.

>

I did some more searching and looks like I found the solution from a 10yr
old post!
http://groups.google.com/group/comp....ead/thread/1d4...
35d0/ea75e39ab5004cf c?lnk=st&q=copy ing+objects+c%2 B%2B#
do let me know if this isn't the right solution
sorry for the inconvenience

>
Yes, this is an FAQ:
>

Inheritance -- virtual functions, C++ FAQ

http://www.parashift.com/c++-faq-lite/virtual-functions.html#faq-20.8

>
Be advised though, this may not be the best solution for you. You should
also look into holding avectorof smart pointers. If that will work for
you (i.e., if it's ok if both vectors actually point to the same set ofobjects,) thencopyingwill be a simple matter of assignment:
>
vector<smart_pt r<Load loadList;vector <smart_ptr<Lo ad copy = loadList;

thanks for the response...I will look into smart pointers. but in this
case, I actually want to create a copy of the set of objects. not just
a set of pointers that point to the original set of objects.
I am trying to parallelize some of the steps in my Finite element
analysis program and the reasoning for doing this is so that each
processor can have its own set of objects so that it doesn't cause
conflicts with what the other processor is doing. hope that makes
sense.

Julian.

**Julian** · Jul 11 '08, 04:45 AM

Re: copying a vector of objects at runtime

On Jul 10, 11:35 pm, spacemanspiff <jul...@gmail.c omwrote:

On Jul 10, 9:46 pm, "Daniel T." <danie...@earth link.netwrote:
>
>
>

"Julian" <jul...@nospamt amu.eduwrote:

"Julian" <jul...@nospamt amu.eduwrote:
I have avectordefined like this:
std::vector<Loa d*LoadList;
which is populated with differentobject sof classes that are derivedfrom
'Load'.
I need to make a copy of this list during runtime.
I imagine I'd have to loop through the list, make a copy of each ofthe
objectsin thevectorusing the copy constructor... but I don't know the
class name during run time so that I can call the appropriate copy
constructor.
actually, I read up on 'typeid' which I supposed would give me the class
name... but I am still confused as how I can call the appropriate copy
constructor for that class during run time.
One option would be to do a string compare of the class names... but that
requires for me to be aware of all the classes derived from 'Load'.
Is there some other way to go about this?
thanks,
Julian.

>

I did some more searching and looks like I found the solution from a 10 yr
old post!
>http://groups.google.com/group/comp....ead/thread/1d4....
35d0/ea75e39ab5004cf c?lnk=st&q=copy ing+objects+c%2 B%2B#
do let me know if this isn't the right solution
sorry for the inconvenience

>

Yes, this is an FAQ:

>

http://www.parashift.com/c++-faq-lit....html#faq-20.8

>

Be advised though, this may not be the best solution for you. You should
also look into holding avectorof smart pointers. If that will work for
you (i.e., if it's ok if both vectors actually point to the same set ofobjects,) thencopyingwill be a simple matter of assignment:

>

vector<smart_pt r<Load loadList;vector <smart_ptr<Lo ad copy = loadList;

>
thanks for the response...I will look into smart pointers. but in this
case, I actually want to create a copy of the set of objects. not just
a set of pointers that point to the original set of objects.
I am trying to parallelize some of the steps in my Finite element
analysis program and the reasoning for doing this is so that each
processor can have its own set of objects so that it doesn't cause
conflicts with what the other processor is doing. hope that makes
sense.
>
Julian.

Sorry for the confusion... i am the original poster. I just started
using Google groups now

**Julian** · Jul 11 '08, 04:55 AM

Re: copying a vector of objects at runtime

On Jul 10, 10:02 am, Joe Greer <jgr...@doublet ake.comwrote:

"Julian" <jul...@nospamt amu.eduwrote innews:g54akk$c 32$1@news.tamu. edu:
>

Hi,
I have a vector defined like this:
std::vector<Loa d*LoadList;
which is populated with different objects of classes that are derived
from 'Load'.
I need to make a copy of this list during runtime.
I imagine I'd have to loop through the list, make a copy of each of
the objects in the vector using the copy constructor... but I don't
know the class name during run time so that I can call the appropriate
copy constructor.

>
Hmmm, if copying the objects in the vector is the answer, then I have to
wonder why there are pointers in the vector to begin with. That is,
wouldn't vector<Loadhave been a better place to start? Then copying
would be as simple as LoadList2 = LoadList. IME the only time you
really want a vector of pointers is if the objects have identity
(can't/shouldn't be copied at all) or are horrendously expensive to
copy.
>

I'm not really sure if I understand the first part of your response.
from what I understand (and please let me know if i'm on the wrong
track), vector<Loadcan only store only Load objects. even if i try
to store an object of a class derived from Load, it still stores only
a Load object because it calls Load(const Load&) when you do a
push_back.
In my case, I want this vector to 'store' a combination of Load
objects AND other objects that are derived from Load.
hope this helps in describing my situation... and please let me know
if it better for me to use some other c++ strategy.
thanks,
Julian.

**red floyd** · Jul 11 '08, 05:35 AM

Re: copying a vector of objects at runtime

Joe Greer wrote:

"Julian" <julvar@nospamt amu.eduwrote in
news:g54akk$c32 $1@news.tamu.ed u:
>

>Hi,
>I have a vector defined like this:
>std::vector<Lo ad*LoadList;
>which is populated with different objects of classes that are derived
>from 'Load'.
>I need to make a copy of this list during runtime.
>I imagine I'd have to loop through the list, make a copy of each of
>the objects in the vector using the copy constructor... but I don't
>know the class name during run time so that I can call the appropriate
>copy constructor.

>
Hmmm, if copying the objects in the vector is the answer, then I have to
wonder why there are pointers in the vector to begin with. That is,
wouldn't vector<Loadhave been a better place to start? Then copying
would be as simple as LoadList2 = LoadList. IME the only time you
really want a vector of pointers is if the objects have identity
(can't/shouldn't be copied at all) or are horrendously expensive to
copy.
>

You forgot the other case, where your vector contains polymorphic
objects. But then you should probably be using a container of your
favorite smart pointer.

**Joe Greer** · Jul 11 '08, 12:25 PM

Re: copying a vector of objects at runtime

Julian <julvar@gmail.c omwrote in
news:00c0d939-e3d2-4c2b-85b3-f49efec35285@59 g2000hsb.google groups.com:

On Jul 10, 10:02 am, Joe Greer <jgr...@doublet ake.comwrote:

>"Julian" <jul...@nospamt amu.eduwrote
>innews:g54akk$ c32$1@news.tamu .edu:
>>

>
I'm not really sure if I understand the first part of your response.
from what I understand (and please let me know if i'm on the wrong
track), vector<Loadcan only store only Load objects. even if i try
to store an object of a class derived from Load, it still stores only
a Load object because it calls Load(const Load&) when you do a
push_back.
In my case, I want this vector to 'store' a combination of Load
objects AND other objects that are derived from Load.
hope this helps in describing my situation... and please let me know
if it better for me to use some other c++ strategy.
thanks,
Julian.

Ah yes, I forgot polymorphism... In that case, if your objects have
identity, that is, you really only want one instance of it, then you might
look at a smart_ptr (from either TR1 or boost) to manage lifetime of the
object and just copy the vector normally. Or you make your Load interface
contain a clone() method of some sort which has the object create a copy of
itself. A clone_inserter iterator can then be made which invokes this
method to get the clone of the object and use std::copy() as I described
elsewhere.

joe

**Joe Greer** · Jul 11 '08, 12:25 PM

Re: copying a vector of objects at runtime

red floyd <no.spam.here@e xample.comwrote in
news:qiCdk.1209 1$cW3.7840@nlpi 064.nbdc.sbc.co m:

Joe Greer wrote:

>"Julian" <julvar@nospamt amu.eduwrote in
>news:g54akk$c3 2$1@news.tamu.e du:
>>

>>Hi,
>>I have a vector defined like this:
>>std::vector<L oad*LoadList;
>>which is populated with different objects of classes that are
>>derived from 'Load'.
>>I need to make a copy of this list during runtime.
>>I imagine I'd have to loop through the list, make a copy of each of
>>the objects in the vector using the copy constructor... but I don't
>>know the class name during run time so that I can call the
>>appropriate copy constructor.

>>
>Hmmm, if copying the objects in the vector is the answer, then I have
>to wonder why there are pointers in the vector to begin with. That
>is, wouldn't vector<Loadhave been a better place to start? Then
>copying would be as simple as LoadList2 = LoadList. IME the only
>time you really want a vector of pointers is if the objects have
>identity (can't/shouldn't be copied at all) or are horrendously
>expensive to copy.
>>

>
You forgot the other case, where your vector contains polymorphic
objects. But then you should probably be using a container of your
favorite smart pointer.

Yep. Somehow, the way the question was worded, It didn't occur to me that
these were pointers to a base/interface. Sigh.

joe

copying a vector of objects at runtime

copying a vector of objects at runtime

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