As a beginner in C++, I am practicing C++ with an algorithm assignment. Along the way, I have some questions that I have difficulty getting through. Pardon me if the questions sound entry-level since I am still learning.
The goal is to find collinear points in a given vector of points with three classes. The following briefly describes the three classes' purposes:
Point: Representing a point with x and y values.
LineSegment: Representing a line segment with two points at ends.
Collinear: Containing the segments found in a vector of Points. The main part of the algorithm.
So, I would expect the client code to look like this:
std::vector<Point> points; // may become huge
// populate points
// ...
Collinear collinear_points(points);
std::vector<LineSegment> segments_in_points = collinear_points.GetSegments();
Since the class Collinear depends on a certain vector of points to get the segments correspondingly, I think it would need it as a data member. The question that keeps haunting me is, should it hold a copy of the vector or hold a raw pointer/reference to the vector outside the object. I think a smart pointer would be an overkill here. According to the old answer here, maybe it is better to go with reference which also avoids potential expensive copying? What is the common practice for this kind of dependency between classes if any exists?
If points gets modified after the construction of collinear_points, then the data collinear_points is referencing will be inconsistent with the segments it contains. Is it common to leave the responsibility to users for making sure the validness of an object depending on other ones? Is there a way to let collinear_points know the content has been modified and put it in an invalid state?
To answer your actual question from the title: A non-owning raw pointer would still be the usual choice, mainly because that’s what we’ve been doing since the old C days. Of course, a pointer has the problem that it can be nullptr. So using a reference communicates more clearly that null is not an allowed value. Because of that I tend to use the reference, although it still feels a tiny bit weird even to myself. But imo it’s the better design decision overall
That said, I believe the real question here is one of ownership. If Collinear does not own the vector, the user of your API has to make sure that the vector lives at least as long as the associated Collinear object. Otherwise you’ll access a dangling pointer/reference and things tend to go downhill from there. ;)
Is there a way to let collinear_points know the content has been modified and put it in an invalid state?
Yes, there is. Own everything. That includes the points vector and the segments vector. Following this approach Collinear could look something like this:
class Collinear {
public:
// usings because I’m a lazy typer
using PointsVec = std::vector<Point>;
using SegmentsVec = std::vector<LineSegment>;
// Take ownership of the points vector by either copying
// or moving it into a member.
explicit Collinear(const PointsVec& p): m_points(p) {}
explicit Collinear(PointsVec&& p): m_points(std::move(p)) {}
// Retain ownership of the segments vector by returning a const ref.
const SegmentsVec& GetSegments(); // Check if you can make it const.
// access functions for the two vectors ...
private:
PointsVec m_points;
SegmentsVec m_segments;
}
Now Collinear controls access to the points vector. You’ll have to write the functions for the permitted operations as members of Collinear. The important thing is never to return a non-const pointer or non-const ref to m_points, because then you might miss write accesses.
The segments vector is similar. Your provide the write access member functions and Collinear retains ownership, which means it can re-calculate it when necessary and the user doesn’t need to be concerned with that. Depending on how expensive the calculation is you can now go wild with lazy evaluation and every optimization you can think of.
There is a completely different design approach, though. Own nothing. Does Collinear have to be a class at all? Could it be a bunch of free functions in a namespace?
namespace Collinear {
std::vector<LineSegment> GetSegments(const std::vector<Point>& points);
}
// ...
auto segments_in_points = Collinear::GetSegments(points);
That’s the opposite of the own-everything approach. Before, you had full control. Now your user has full control. On the other hand, they now have to take care of any laziness/optimizations/update detection.
Which approach is appropriate is a question of a) API design philosophy and b) your conrete situation. What are your users? What do they expect? Which approach makes their lives easier? Since this is an assignment, you probably won’t have any real users. So imagine a group of people that might want to use your code and decide based on that. Or just use the approach you’ll have more fun implementing. The important thing imo: Pick one of the two approaches. Don’t mix them, because such an API is inconsistent. That increases confusion, decreases ease of use, and makes errors more likely.
Btw:
I think a smart pointer would be an overkill here.
Using smart pointers is not a question of overkill. It’s a question of ownership. If you have an owning pointer never use a raw pointer for it. … Unless a legacy API forces you to. Even then it’s a great idea to mark it as owning with a transparent wrapper like gsl::owner<T>.
The question that keeps haunting me is, should it hold a copy of the vector or hold a raw pointer/reference to the vector outside the object
I think you should keep a copy of vector here to keep things generic.You don't want your collinear class depend on a particular vector<Points>, instead it should be like while creating instance of collinear class you just tell it on what vector<Points>, it has to work on.Then if you change this vector and you want collinear also to work on this data set(which you might not want), its your responsibility to tell collinear to work on new data set.If you want collinear to be updated automatically when you update vector<points>, you can do so, but you have to answer questions like what happens to the state of collinear(which would be depending on the vector<points>) when the data set changes.
Related
First time I'm posting to stack so if I'm not following the correct procedure, instruct me to do better and I will.
Ok, I have quite a big project with lots of classes and I cannot share code at this point, but if necessary I will write a dummy file to further explain. Hopefully I will be clear enough without that.
I have this class method that receives as params a vector: std::vector< myClass > &objects_1.
Some of these will be bound (logically) to other objects in the class. The important thing to keep in mind is that I have to modify some of the received objects_1 without using the vector indexing (as it may change along the way)
Also unfortunately it is relevant for the function to have this exact signature.
The way I do it now is by having several pointers initialized with nullptr that, sooner or later point to elements of the objects_1 vector.
My questions are: is this a bad practice? Do I have to delete the pointer or does the destructor takes care of this? Is there a better way to do this? I tried std::shared_ptr but as someone pointed out these are intended more for allocating memory dynamically.
I have been given some code to read which does some geometric operations on meshes.
A mesh data structure, by definition, should contain at least the information
regarding the coordinates of points, edge connectivity and face information.
So, the code given to me has classes to define vertex, edge and face data structure,
named respectively as Vertex, Edge and Face.
However the mesh class looks like this.
class basemesh
{
public:
/* Methods to operate on the protected data below.*/
protected:
/*! list of edges */
std::list<Edge*> m_edges;
/*! list of vertices */
std::list<Vertex*> m_verts;
/*! list of faces */
std::list<Face*> m_faces;
}
My question: Why does the mesh data structure store a list of pointers rather than a
list of the corresponding objects themselves.
e.g why not say directly std::list<Vertex>
I have seen this construct being used in a couple of other C++ codes
Does this have something to do with inheritance of classes? Or is it something to do
with performance with regards to iterating on the list?
This basemesh class is, as the name suggests, a base class from which
other specialized meshes are derived.
There is no performance reasons here. Its simply a case of ownership sharing. Remember this as a rule of thumb: Pointers in C++ are used to share/pass ownership of a resource, or to provide polymorphic behaviour through dynamic binding.
People is talking about performence because you avoid copying the things. Blah, blah, blah.
If you need to copy, you should copy. The only reason why its using pointers is because the author didn't want to copy the things when he/she copies the list of things, in other words, he/she wants to maintain the same things in two locations (lists): Ownership sharing, as I said before.
On the other hand, note that the class is called basemesh. So the real point of the pointers here could be to work with polymorphic vertices, edges, etc (Dynamic binding).
NOTE: If performance was the point here, I'm pretty sure the author would be using compact and aligned non-cache-miss-prone std::vector instead of std::list. In this case, the most presumable reason about the use of pointers is polymorphism, not performance. Anything related to pointers, dereferencing, and transversing linked lists will always have less performance than compact data, exactly what std::vector<Vertex> is, for example. Again, if the use of pointers is not for polymorphism, is for ownership related things, not performance.
Other note: Copying Yes, you are copying. But note what and how are copying. Vertices are, except of a very rare implementation, pairs of floats/ints. There is no gain at all about copying 64bits of floats vs 32/64bits of pointers.
Also note that, except you don't be so lucky, you are copying things stored at the same cache line, or almost at the cache.
A good rule about optimization nowadays is: Try to optimize memory accesses, not CPU cicles. I recommend this thread: What is "cache-friendly" code?, and this for a practical case: Why are elementwise additions much faster in separate loops than in a combined loop?. Finally, this thread contains good notes about optimizing using modern compilers.
My guess is that it's either made for a very unusual specific case, but more likely, it's written by a programmer who doesn't know how heap allocations or std::list actually work, and just blindly use pointers.
It seems very unlikely a std::list of pointers to single vertices was the best option performance- or designwise.
On a practical level if a method changes a point it does not need to reproduce the change in the other data structures. They will all point to the same thing.
But in terms of memory management it would be wise to use smart pointers,
At a guess I'd say it's so that these objects can have pointers to each other (e.g. an Edge can have pointers to two Vertices, each of which can have a pointer back to the Edge).
If all the Vertices lived in a std::list in basemesh, then pointers to them would not be reliable, although list::iterators might work well enough.
Using pointers is less efficient when retrieving inner data in general because you will have to dereference the value every time you access it.
But at the same time it will be more efficient when passing data around, since you are just passing pointers. I guess the solution chosen is related to the fact that data is shared between multiple objects by composition. Eg: multiple Edge instances could refer to same Vertex.
Now std::list guarantees that addresses to values contained are consistent until the element itself is removed so actually doing something like
Edge(const Vertex *v1, const Vertex *v2) { .. }
std::list<Vertex>::iterator it = std::advance(vertices.begin(), 3);
std::list<Vertex>::iterator it2 = std::advance(vertices.begin(), 5);
new Edge(&(*it), &(*it2));
Would work since addresses won't be invalidated so there is no real necessity to use pointers to store objects. Actually by using this solution you don't need to care about memory management of single objects since you won't need to delete them or wrap them into smart pointers.
It's using pointers for performance reasons and to reduce the chance of an error.
Imagine the alternative of not using pointers. Every insertion into class basemesh would cause a copy of the object to be created, and every time you access an object, if you aren't careful, you'll get a copy as well.
For example, imagine this statement:
Edge e = m_edges[0];
e.doSomethingThatModifiesState();
In this example, without pointers, you'll have a copy of the object, and any operations you perform on it will not affect the actual edge object stored in m_edges.
With pointers, you don't have this issue:
Edge* e = m_edges[0];
e->doSomethingThatModifiesState();
In this example, no copy of the object is made, and when you do something, you get the intended behavior.
As many others said the speed is the most obvious reason. Another reason is to get polymorphic behavior through pointers to the base class.
C++
Is it better to update the value of a pointer or to change the pointer to point to something else?
Let's say we have two classes: ball and coordinates.
class ball
{
Coordinates *ballCurrent;
public:
ball(int);
~ball();
void setLoc(Coordinates&); // OR void setLoc(int, int);
};
class Coordinates
{
int x, y;
public:
Coordinates(int, int);
void setCoordinates(int, int);
};
For the setLoc method in the ball class, which parameters should is better? Would it be better to just setLoc by using (*ballCurrent).setCoordinates(int, int) OR by using (ballCurrent).setLoc((new Coordinates(int, int)))? Please go into detail the reason for each case if possible.
Is it better to update the value of a pointer or to change the pointer
to point to something else?
That's not the sort of question that can be answered without some context. It's often useful to know that an object is immutable -- that as long as you have a pointer to that object, the object won't change under your nose. For example, if a function or method takes a pointer to an immutable type as a parameter, you know that you can pass objects to that function without worrying that the function will change the object.
On the other hand, mutable objects also have their place. For one thing, you sometimes want to be able to build an object up in several steps rather than doing it all at once, and you may not want to have to create several intermediate objects along the way just to get to the final object.
So, it depends on context. I see that you've provided a simple example, but I don't think there's necessarily a correct answer even for your example. It depends on what's important to you and how the objects in question will interact with other parts of the system. I'd lean toward mutable objects, I think, because in a game it's likely that several objects will need to know about the ball. If the only way to move the ball is to create a new ball and pass it around to all the other objects that care about it, well, that could easily get to be a huge problem. The coordinates object could easily be immutable, though -- anyone who needs to know the location of the ball should ask the ball for it's location. Once they get it, they'll probably expect the location not to change every few milliseconds. If you don't make the coordinates object immutable, the other way to handle that problem is for the ball to make a copy of its location whenever someone asks, and hand out that copy instead of returning a pointer to its own location object.
I might write this way if I could:
class ball
{
Coordinates coordinate_; // better name than ballCurrent?
public:
explicit ball(int) // add explicit to avoid implicit converting
: coordinate_(0,0) // initialize corrdinate_ properly
{
}
~ball();
void setLoc(const Coordinates& co) // add const
{
coordinate_ = co;
}
void setLoc(int x, int y)
{
coordinate_.setCoordinates(x,y);
}
};
No need to worry about dynamic memory issue, could setLoc in two ways by function overloading. No wild pointer, no worries.
If you use a new object, it will need memory allocated for itself, and it will also need to be initialized (constructor/s run, properties assigned, etc). That is a lot of overhead compared to just change the value of an object you already have (and that you created for that very purpose).
Besides, if you create a new object, you need to free the memory of your previous pointer, which again is overhead... so just change your existing object.
Just change the values of the object that is being pointed to. It doesn't make sense to create a whole new object every time you want to update a value.
One situation where you might have to create a whole new object rather just change the values is if Coordinates had a const x and y value and there was no setCoordinates function. But then that's mostly likely a poor design decision if you are needing to be update the co-ordinates often.
Also rather than this:
(*ballCurrent).setCoordinates(int, int);
you can use this:
ballCurrent->setCoordinate(int, int);
Which does the same thing but is just easier to write and a lot of people will find it more readable.
In the case of SetLoc(int,int), you are creating a dependency between ball and Coordinates on the Coordinates constructor, and more importantly, on what it represents. If the constructor signature changes, you will have to carry these changes over to ball::SetLoc. This argument is valid in the general case, where you really want to avoid passing along a set of parameters to initialize an object component.
In this particular case, Coordinates embodies an position in space, and probably should come with a set of methods to manipulate it, in order to preserve its implementation details from pervading the rest of the programme. If you wish to move to a 3D coordinate system, ideally, all you would have to change are the constructors invocations and the class implementation. So you really want to reduce the necessary changes outside the class to a minimum, which means avoiding the issue explained earlier.
You present two different solutions to set the location of the ball object. In practice would these solutions appear? In a trivial programme, where the ball position hardcoded, that is a possibility, and either solution could work. But in a more sophisticated software, the ball position would probably be initially set by a configuration file, and any subsequent update of the position done after some non trivial computation. In this situation, it much more maintainable to encapsulate all the logic pertaining to coordinates in the class, and avoid the (int, int) parameter passing.
If you are concerned by memory allocation overheads (e.g. temporaries generated by function returned results), there are ways to alleviate them by using wisely consts, references, or even pointers, and define sensible copy constructors.
My question is best illustrated with a code sample, so let's just start off with that:
class Game
{
// All this vector does is establish ownership over the Card objects
// It is initialized with data when Game is created and then is never
// changed.
vector<shared_ptr<Card> > m_cards;
// And then we have a bunch of pointers to the Cards.
// All these pointers point to Cards from m_cards.
// These could have been weak_ptrs, but at the moment, they aren't
vector<Card*> m_ptrs;
// Note: In my application, m_ptrs isn't there, instead there are
// pointers all over the place (in objects that are stored in member
// variables of Game.
// Also, in my application, each Card in m_cards will have a pointer
// in m_ptrs (or as I said, really just somewhere), while sometimes
// there is more than one pointer to a Card.
}
Now what I want to do is to make a deep copy of this Game class. I make a new vector with new shared_ptrs in it, which point to new Card objects which are copies of the original Card objects. That part is easy.
Then the trouble starts, the pointers of m_ptrs should be updated to point to the cards in m_cards, which is no simple task.
The only way I could think of to do this is to create a map and fill it during the copying of m_cards (with map[oldPtr] = newPtr) and then to use that to update m_ptrs. However, this is only O(m * log(n)) (m = m_ptrs.size(); n = m_cards.size()). As this is going to be a pretty regular operation* I would like to do this efficiently, and I have the feeling that it should be possible in O(m) using custom pointers. However, I can't seem to find an efficient way of doing this. Anybody who does?
*it's used to create a testbed for the AI, letting it "try out" different moves
Edit: I would like to add a bit on accepting an answer, as I haven't yet. I am waiting until I get back to this project (I got on a side track as I had worked too much on this project - if you do it for fun it's got to stay fun), so it may be a while longer before I accept an answer. Nevertheless, I will accept an answer some time, so don't worry :P
Edit nr 2: I still haven't gotten back to this project. Right now, I am thinking about just taking the O(m * log(n)) way and not complaining, then seeing later if it needs to be faster. However, as I have recently taken some time to learn my patterns, I am also thinking that I really need to refactor this project some time. Oh, and that I might just spend some time working on this problem with all the new knowledge I have under my belt. Since there isn't an answer that says "just stick with the hashmap and see later if it really needs to be faster" (and I would actually be pretty disappointed if there was, as it's not an answer to my question), I am postponing the picking of an answer yet a bit more till I do get back to this project.
Edit nr 3: I still didn't get back to this project. More precisely, it has been shelved indefinitely. I am pretty sure I just wouldn't get my head too bent over the O(m * log(n))right now, and then perhaps look at it later if it turned out to be a problem. However, that would just not have been a good answer to my question, as I explicitly asked for better performance. Not wanting to leave the answers unaccepted any longer, I chose the most helpful answer and accepted it.
Store the pointers as indexes.
As you say they all point to m_Cards which is a vector that can be indexed (is that correct English?).
Either you do that only for storing and convert them back to pointers at loading.
Or you may think of using indices generally instead of pointers.
What about keeping cards elements index instead of pointer:
vector<int> m_indexes;
...
Card* ptr = &m_cards[m_indexes[0]];
Vector with indexes can be copied without changes.
I recently encountered the very similar problem: cloning the class internal structure implemented by pointers and std::vector as an objects storage.
First of all (unrelated to the question though), I'd suggest either stick with smart pointers or with plain structures. In your case it means that it makes much more sense to use vector<weak_ptr<Card> > m_ptrs instead of raw pointers.
About the question itself - one more possible workaround is using pointer differences in the copy constructor. I will demonstrate it for vector of objects, but working with shared pointers will utilize the same principle, the only difference will be in copying of m_cards (you should not simply use assignment if you want objects clones but copy the m_cards element-by-element).
It is very important that the method works only for containers where the elements are guaranteed to be stored consequently (vector, array).
Another very important moment is that the m_ptrs elements should represent only internal Card structrure, i. e. they must point only to the internal m_cards elements.
// assume we store objects directly, not in shared pointers
// the only difference for shared pointers will be in
// m_cards assignment
// and using m_cards[...].get() instead of &m_cards[...]
vector<Card> m_cards;
vector<Card*> m_ptrs;
In that case your array of pointers can be easily computed by using pointers arithmetic taking linear time. In that case your copy constructor will look like this:
Game::Game(const Game &rhs) {
if (rhs.m_cards.empty())
return;
m_cards = rhs.m_cards;
// if we have vector of shared pointers
// and we need vector of pointers to objects clones
// something like this should be done
// for (auto p: rhs.m_cards) {
// // we must be certain here that dereferencing is safe,
// // i. e. object must exist. If not, additional check is required.
// // copy constructor will be called here:
// m_cards.push_back(std::make_shared<Card>(*p));
// }
Card *first = &rhs.m_cards[0];
for (auto p: rhs.m_ptrs) {
m_ptrs.push_back(&m_cards[p - first]);
}
}
Basically in this deepcopy method you will be still working with indexes, but you preserve the convenience of working with pointers in other class methods without storing your indexes separately.
Anyway, for using that kind of structure you should exactly know what you are doing with the class members and why, that requires much more manual control (for example, at least adding/removing elements to/from m_cards should be done consciously, in other case m_ptrs can easily become broken even without copying the object).
class MyContainedClass {
};
class MyClass {
public:
MyContainedClass * getElement() {
// ...
std::list<MyContainedClass>::iterator it = ... // retrieve somehow
return &(*it);
}
// other methods
private:
std::list<MyContainedClass> m_contained;
};
Though msdn says std::list should not perform relocations of elements on deletion or insertion, is it a good and common way to return pointer to a list element?
PS: I know that I can use collection of pointers (and will have to delete elements in destructor), collection of shared pointers (which I don't like), etc.
I don't see the use of encapsulating this, but that may be just me. In any case, returning a reference instead of a pointer makes a lot more sense to me.
In a general sort of way, if your "contained class" is truly contained in your "MyClass", then MyClass should not be allowing outsiders to touch its private contents.
So, MyClass should be providing methods to manipulate the contained class objects, not returning pointers to them. So, for example, a method such as "increment the value of the umpteenth contained object", rather than "here is a pointer to the umpteenth contained object, do with it as you wish".
It depends...
It depends on how much encapsulated you want your class to be, and what you want to hide, or show.
The code I see seems ok for me. You're right about the fact the std::list's data and iterators won't be invalidated in case of another data/iterator's modification/deletion.
Now, returning the pointer would hide the fact you're using a std::list as an internal container, and would not let the user to navigate its list. Returning the iterator would let more freedom to navigate this list for the users of the class, but they would "know" they are accessing a STL container.
It's your choice, there, I guess.
Note that if it == std::list<>.end(), then you'll have a problem with this code, but I guess you already know that, and that this is not the subject of this discussion.
Still, there are alternative I summarize below:
Using const will help...
The fact you return a non-const pointer lets the user of you object silently modify any MyContainedClass he/she can get his/her hands on, without telling your object.
Instead or returning a pointer, you could return a const pointer (and suffix your method with const) to stop the user from modifying the data inside the list without using an accessor approved by you (a kind of setElement ?).
const MyContainedClass * getElement() const {
// ...
std::list<MyContainedClass>::const_iterator it = ... // retrieve somehow
return &(*it);
}
This will increase somewhat the encapsulation.
What about a reference?
If your method cannot fail (i.e. it always return a valid pointer), then you should consider returning the reference instead of the pointer. Something like:
const MyContainedClass & getElement() const {
// ...
std::list<MyContainedClass>::const_iterator it = ... // retrieve somehow
return *it;
}
This has nothing to do with encapsulation, though..
:-p
Using an iterator?
Why not return the iterator instead of the pointer? If for you, navigating the list up and down is ok, then the iterator would be better than the pointer, and is used mostly the same way.
Make the iterator a const_iterator if you want to avoid the user modifying the data.
std::list<MyContainedClass>::const_iterator getElement() const {
// ...
std::list<MyContainedClass>::const_iterator it = ... // retrieve somehow
return it;
}
The good side would be that the user would be able to navigate the list. The bad side is that the user would know it is a std::list, so...
Scott Meyers in his book Effective STL: 50 Specific Ways to Improve Your Use of the Standard Template Library says it's just not worth trying to encapsulate your containers since none of them are completely replaceable for another.
Think good and hard about what you really want MyClass for. I've noticed that some programmers write wrappers for their collections just as a matter of habit, regardless of whether they have any specific needs above and beyond those met by the standard STL collections. If that's your situation, then typedef std::list<MyContainedClass> MyClass and be done with it.
If you do have operations you intend to implement in MyClass, then the success of your encapsulation will depend more on the interface you provide for them than on how you provide access to the underlying list.
No offense meant, but... With the limited information you've provided, it smells like you're punting: exposing internal data because you can't figure out how to implement the operations your client code requires in MyClass... or possibly, because you don't even know yet what operations will be required by your client code. This is a classic problem with trying to write low-level code before the high-level code that requires it; you know what data you'll be working with, but haven't really nailed down exactly what you'll be doing with it yet, so you write a class structure that exposes the raw data all the way to the top. You'd do well to re-think your strategy here.
#cos:
Of course I'm encapsulating
MyContainedClass not just for the sake
of encapsulation. Let's take more
specific example:
Your example does little to allay my fear that you are writing your containers before you know what they'll be used for. Your example container wrapper - Document - has a total of three methods: NewParagraph(), DeleteParagraph(), and GetParagraph(), all of which operate on the contained collection (std::list), and all of which closely mirror operations that std::list provides "out of the box". Document encapsulates std::list in the sense that clients need not be aware of its use in the implementation... but realistically, it is little more than a facade - since you are providing clients raw pointers to the objects stored in the list, the client is still tied implicitly to the implementation.
If we put objects (not pointers) to
container they will be destroyed
automatically (which is good).
Good or bad depends on the needs of your system. What this implementation means is simple: the document owns the Paragraphs, and when a Paragraph is removed from the document any pointers to it immediately become invalid. Which means you must be very careful when implementing something like:
other objects than use collections of
paragraphs, but don't own them.
Now you have a problem. Your object, ParagraphSelectionDialog, has a list of pointers to Paragraph objects owned by the Document. If you are not careful to coordinate these two objects, the Document - or another client by way of the Document - could invalidate some or all of the pointers held by an instance of ParagraphSelectionDialog! There's no easy way to catch this - a pointer to a valid Paragraph looks the same as a pointer to a deallocated Paragraph, and may even end up pointing to a valid - but different - Paragraph instance! Since clients are allowed, and even expected, to retain and dereference these pointers, the Document loses control over them as soon as they are returned from a public method, even while it retains ownership of the Paragraph objects.
This... is bad. You've end up with an incomplete, superficial, encapsulation, a leaky abstraction, and in some ways it is worse than having no abstraction at all. Because you hide the implementation, your clients have no idea of the lifetime of the objects pointed to by your interface. You would probably get lucky most of the time, since most std::list operations do not invalidate references to items they don't modify. And all would be well... until the wrong Paragraph gets deleted, and you find yourself stuck with the task of tracing through the callstack looking for the client that kept that pointer around a little bit too long.
The fix is simple enough: return values or objects that can be stored for as long as they need to be, and verified prior to use. That could be something as simple as an ordinal or ID value that must be passed to the Document in exchange for a usable reference, or as complex as a reference-counted smart pointer or weak pointer... it really depends on the specific needs of your clients. Spec out the client code first, then write your Document to serve.
The Easy way
#cos, For the example you have shown, i would say the easiest way to create this system in C++ would be to not trouble with the reference counting. All you have to do would be to make sure that the program flow first destroys the objects (views) which holds the direct references to the objects (paragraphs) in the collection, before the root Document get destroyed.
The Tough Way
However if you still want to control the lifetimes by reference tracking, you might have to hold references deeper into the hierarchy such that Paragraph objects holds reverse references to the root Document object such that, only when the last paragraph object gets destroyed will the Document object get destructed.
Additionally the paragraph references when used inside the Views class and when passed to other classes, would also have to passed around as reference counted interfaces.
Toughness
This is too much overhead, compared to the simple scheme i listed in the beginning. It avoids all kinds of object counting overheads and more importantly someone who inherits your program does not get trapped in the reference dependency threads traps that criss cross your system.
Alternative Platforms
This kind-of tooling might be easier to perform in a platform that supports and promotes this style of programming like .NET or Java.
You still have to worry about memory
Even with a platform such as this you would still have to ensure your objects get de-referenced in a proper manner. Else outstanding references could eat up your memory in the blink of an eye. So you see, reference counting is not the panacea to good programming practices, though it helps avoid lots of error checks and cleanups, which when applied the whole system considerably eases the programmers task.
Recommendation
That said, coming back to your original question which gave raise to all the reference counting doubts - Is it ok to expose your objects directly from the collection?
Programs cannot exist where all classes / all parts of the program are truly interdependent of each other. No, that would be impossible, as a program is the running manifestation of how your classes / modules interact. The ideal design can only minimize the dependencies and not remove them totally.
So my opinion would be, yes it is not a bad practice to expose the references to the objects from your collection, to other objects that need to work with them, provided you do this in a sane manner
Ensure that only a few classes / parts of your program can get such references to ensure minimum interdependency.
Ensure that the references / pointers passed are interfaces and not concrete objects so that the interdependency is avoided between concrete classes.
Ensure that the references are not further passed along deeper into the program.
Ensure that the program logic takes care of destroying the dependent objects, before cleaning up the actual objects that satisfy those references.
I think the bigger problem is that you're hiding the type of collection so even if you use a collection that doesn't move elements you may change your mind in the future. Externally that's not visible so I'd say it's not a good idea to do this.
std::list will not invalidate any iterators, pointers or references when you add or remove things from the list (apart from any that point the item being removed, obviously), so using a list in this way isn't going to break.
As others have pointed out, you may want not want to be handing out direct access to the private bits of this class. So changing the function to:
const MyContainedClass * getElement() const {
// ...
std::list<MyContainedClass>::const_iterator it = ... // retrieve somehow
return &(*it);
}
may be better, or if you always return a valid MyContainedClass object then you could use
const MyContainedClass& getElement() const {
// ...
std::list<MyContainedClass>::const_iterator it = ... // retrieve somehow
return *it;
}
to avoid the calling code having to cope with NULL pointers.
STL will be more familiar to a future programmer than your custom encapsulation, so you should avoid doing this if you can. There will be edge cases that you havent thought about which will come up later in the app's lifetime, wheras STL is failry well reviewed and documented.
Additionally most containers support somewhat similar operations like begin end push etc. So it should be fairly trivial to change the container type in your code should you change the container. eg vector to deque or map to hash_map etc.
Assuming you still want to do this for a more deeper reason, i would say the correct way to do this is to implement all the methods and iterator classes that list implements. Forward the calls to the member list calls when you need no changes. Modify and forward or do some custom actions where you need to do something special (the reason why you decide to this in the first place)
It would be easier if STl classes where designed to be inherited from but for efficiency sake it was decided not to do so. Google for "inherit from STL classes" for more thoughts on this.