Object update in multiple STL containers via pointers

Question

I have a class named Group:

class Group
{
    int id;
    string addr;
    set<int> members;
    ...
};

I have pointers to multiple groups stored in these containers:

vector<Group*> grpVec
map<int, Group*> grpIdMap 
map<string, Group*> grpAdMap

I am storing the pointers like this:

//create and populate group object pointer Group *grp
grpVec.push_back(grp)
grpIdMap.insert(std::pair<int,Group*>(grp->id, grp)) 
grpAdMap.insert(std::pair<string,Group*>(grp->addr, grp))

Now, I want to update a group object. If I update only the pointer in the grpIdMap container, will the same object pointer in all other containers get updated?

//Will this update the same pointer object in grpVec and grpAdMap?
grpIdMap.find(1)->second->members.insert(99)

Is there any issue in this approach?

Looks like you should be using shared pointers, i.e. std::shared_ptr<Group> and not raw pointers. — PaulMcKenzie
– PaulMcKenzie, Commented Jul 20, 2015 at 6:10
Here is the issue with your approach: If the grp is dynamically allocated, which container is responsible for deallocating the memory? — PaulMcKenzie
– PaulMcKenzie, Commented Jul 20, 2015 at 6:21
@PaulMcKenzie Like the way I am updating a single container to update everywhere, I would iterate through any single container and deallocate everything once the use is over. — cppcoder
– cppcoder, Commented Jul 20, 2015 at 6:49
@cppcoder Your design is flawed. You will deallocate everything in a single container, but then you have pointers to invalid memory in the other containers. What if you forget to remove all those pointers from these containers? To spare all of this work, use std::shared_ptr, which quite honestly is what it looks like you should be using. — PaulMcKenzie
– PaulMcKenzie, Commented Jul 20, 2015 at 9:26

Some programmer dude · Accepted Answer · 2015-07-20 06:17:53Z

2

If the pointers all point to the same object, then yes you can use any pointer to change that object.

Lets look at it a little "graphically":

+-------------------+
| pointer in vector | -----\
+-------------------+       \
                             \
+--------------------+        \     +---------------+
| pointer in one map | -------->--- | actual object |
+--------------------+        /     +---------------+
                             /
+----------------------+    /
| pointer in other map | --/
+----------------------+

answered Jul 20, 2015 at 6:17

Some programmer dude

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1 Comment

paddy Over a year ago

It may be worth highlighting to @cppcoder that if you were to modify the id or addr members of the Group instance, it would not change the indexing in the grpIdMap or grpAdMap objects. The old index would need to be removed and a new index added.

Community · Accepted Answer · 2017-05-23 12:14:09Z

1

This approach will update the object. If there is any pointers to this object (not copies of it) then they will get updated info.

I recommend you to stop using raw pointers and read how to use smart pointers here: Which kind of pointer do I use when?

And I think you should look this one: http://en.cppreference.com/w/cpp/memory/shared_ptr

P.S. Use encapsulation in your classes. Other objects should not know about how the data is stored inside of you class, just provide interface.

P.P.S. If you want all members to be public, maybe you want to use struct rather than class

edited May 23, 2017 at 12:14

CommunityBot

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answered Jul 20, 2015 at 6:20

Nikolay Zhulikov

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4 Comments

paddy Over a year ago

shared_ptr is the safe option but does come with a performance hit which might not be desirable in this particular scenario. I like your point about encapsulation here, which is the best advice. In that case, a vector of unique_ptr may be more appropriate, and the encapsulating class can then deal with raw pointers in the maps.

Nikolay Zhulikov Over a year ago

@paddy, I agree with you, unique_ptr must be a default option. Looking for other types of pointers should be based on the time measurements and concrete reasons.

cppcoder Over a year ago

I did not get how to use unique_ptr. Can you show an example for my code?

cppcoder Over a year ago

I do not have C++11/boost. So I would prefer a solution without them.

user2477 · Accepted Answer · 2015-07-20 06:15:28Z

0

It is changing the object's value that the pointer point to, not pointer value.So as long as all pointers point to the same object, it'll works

answered Jul 20, 2015 at 6:15

user2477

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