I have a vector of pointers to other "objects" in a class.
Is there any way, after/when one of those objects gets deleted to remove any pointer that might have been pointing to it from the vector?
The best thing would be to propagate the deletion somehow but, it would also be okay to just know an object has been deleted whenever the vector gets traversed.
Do not use vanilla pointers. Use the classes std::shared_ptr and std::weak_ptr instead.
These classes implement automatic memory management. When you don't need an object anymore, you don't delete the object, you delete all the shared_ptrs which reference that object. Only when no shared_ptrs to an object exist anymore, the actual object will get deleted.
Any weak_ptrs which still exist will return an object which will evaluate as false. When that happens, you know the weak_ptr can be removed from the vector.
That's why std::weak_ptr exists. It doesn't stop deletion, but it does detect it.
Related
I have a vector of pointers to an object (I have them as pointers because I will be swamping the positions around a lot, and I would imagine it would be a lot faster to just swap the pointer, rather than swapping around the whole object.
Anyway, I will ultimately need to delete the vector, but the objects that it points to still need to be valid. The documentation seems to say that it will call the destructor on every object in the vector. This makes sense when it's an array of objects, but if the array is an array of pointers to objects, will the objects that the pointers point to also be deleted, or do I need to delete them manually?
If they are deleted automatically, is the only way to keep the objects around (say they were used in a different vector) is to actually copy the objects to another location, and have the pointers in the vector point to those objects (rather than the originals)?
Thank you.
Calling a destructor on a pointer value does nothing. (On the other hand, calling delete on a pointer value runs the destructor for the pointed-to object, and frees the memory.)
In the case of an array of pointers to objects, you must free the objects manually if that's what you want.
If you have a vector of pointers, the actual objects should still be around if you delete (or clear) the vector.
You could use smart pointers in your vector, such as Boost shared_ptr.
It will indeed destruct any objects in the container. However, since all the objects in your container are pointers, that won't do much of anything.
Reading your question, that sounds like exactly what you want it to do, so you are good.
It does not matter - that's the reason for the delete keyword. If you go out of scope, then the object's destructor is called. If a pointer goes out of scope, then it tends to be a memory leak. The same applies here, so you will not have to do anything special.
They will continue to exist.
Anyway, I will ultimately need to delete the vector, but the objects that it points to still need to be valid. The documentation seems to say that it will call the destructor on every object in the vector. This makes sense when it's an array of objects, but if the array is an array of pointers to objects, will the objects that the pointers point to also be deleted, or do I need to delete them manually?
First, read this: http://crazyeddiecpp.blogspot.com/2010/12/pet-peeve.html
Now ask yourself, does the documentation say that vector deletes every object that every object it contains points at?
If you can answer that question with 'No' then there you have it.
If you can answer that question with 'Yes'...well...try different documentation.
I have a class that contains a vector of object pointers. I have a GetObject(..) function in that class that looks through the vector, finds the object desired, and returns a pointer to it. However, if the user of the class does a delete() on that returned pointer, my program will crash because the vector still has a pointer to that, now invalid, object. So, in my GetObject() class, I can return a const pointer, but that doesn't solve the problem because you can still delete the object. The object is mutable so I can't return a pointer to a const object. I suppose I could prevent deletion by returning a reference to the object but I have my function returning NULL if there is an error. I guess I can pass back the object reference via the parameters and then return and error number like this
//-1 on object on found. 0 for success. Object is passed back in
// the function parameter result.
int MyObject::GetObject(int object_id, Item& result)
Is this the best solution for such a situation?
The best way to solve this problem is to use a shared-ownership smart pointer like shared_ptr, which you can find in Boost, C++ TR1, and C++0x.
A smart pointer is a container that manages the lifetime of your dynamically allocated object for you. It takes responsibility for deleteing the object when you are done using it.
With a shared ownership smart pointer, you can have multiple smart pointers that all share ownership of the dynamically allocated object. A reference count is kept that keeps track of how many smart pointers have ownership of the object, and when the last owning smart pointer is destroyed, the dynamically allocated object is deleted.
It is extremely difficult to manage resources manually in C++, and it's very easy to write code that looks correct and works right most of the time but that is still not correct. By using smart pointers and other resource-owning containers (like the standard library containers), you no longer have to manage resource manually. It is significantly easier to write correct code when all of your resource management is automatic.
Automatic resource management in C++ is accomplished using a design pattern called Resource Acquisition is Initialization (RAII), which is arguably the most important design pattern you as a C++ programmer should become familiar with.
Anybody in your code could also try to de-reference NULL. You going to stop them doing that too? If your container owns the object, and you make this clear (returning a raw pointer is usually pretty clear or mention in docs), then anyone who deletes it, the result is their own fault. The only way that you could guarantee the prevention of the deletion of the object is to prevent the user from ever gaining a native reference or pointer - in which case they just can't access the object.
Who are clients of your class? If they are not your mortal enemies you could just ask them nicely not to delete the object. Otherwise, there will always be a way for "them" to mess you up.
Okay, one possible solution is to make destructor private. That will prevent everyone from deleting the object. But then the object has to delete itself (delete this) somehow, maybe through some function called DeletObjectButDontBlameMeIfAppCrashes. If the owner is some other class then you can set the destructor to protected and owner class as friend of this class.
You should return a reference to the object. There are two ways to handle the case when there is no object found.
First, you can use the Null Object Pattern in order to implement a special value for that case. That might not make sense for your case though.
The other way is to split it up into two methods, one which can be used to check if an appropriate element exists, and one to retrieve it.
Many STL-containers or algorithms implement both of these, either by returned a past-the-end iterator, or by having empty() returns false as a prerequisite of calling a method like front or back.
If you can use the boost libraries, smart pointers can be used to ensure that pointers stay valid. Essentially, each reference to a smart pointer increases its "use count" by 1. When the reset function is called on a reference, the reference goes away and the use counter decrements. As long as some object is still holding on to a smart pointer, the reference will be valid. Note that the other classes will be able to change what its pointing to, but can't delete it.
(My description deals mainly with smart pointers specifically - the different types of pointers vary a little, but the general idea remains the same).
Im working on this project,
The problem Im having is that the an object, does not really get deleted when I need it to be because it has a couple of shared pointers pointing to it.
How do I solve this,please help.
This is by design. As long as an object is owned by one or more shared_ptr smart pointers, it will not be destroyed. Ownership of the object is shared by all of the smart pointers that have ownership and the object can't be destroyed until all of the owners are done with it. This is what shared ownership is.
If you want to be able to destroy the object while there are still some references to it, you can use weak_ptr for those references.
You can decrease the use_count of a shared_ptr by using its reset() method.
If you do this for every pointer which holds the instance, the last reset() will destroy the object it points to.
shared_ptr<Class> myPointer1( new Class() ); //myPointer holds an instance of Class
shared_ptr<Class> myPointer2 = myPointer1; //use_count == 2
myPointer1.reset(); //use_count == 1
myPointer2.reset(); //instance of class will be destroyed
But you probably have a problem with you design, shared_ptr should automatically go out of focus when certain objects are destroyed or methods end. Perhaps you should have a look at the points where the shared_ptrs still hold pointers to the object and check if they shouldn't hold the object anymore.
What I am trying to do is essentially create two vectors of objects, where some objects are entered into both lists and some are just entered into one. The first problem I found was that when I used push_back() to add an object into both lists the object was copied so that when I changed it from one list the object did not change in the other. To get around this I tried to create a list of pointers to objects as one of the lists. However when I accessed the pointer later on the data seemed to be corrupted, the data member values were all wrong. Here are some snippets of my code:
Definition of vectors:
vector<AbsorbMesh> meshList;
vector<AbsorbMesh*> absorbList;
...
Adding an object to both:
AbsorbMesh nurbsMesh = nurbs.CreateMesh(uStride, vStride);
// Add to the absorption list
absorbList.push_back(&nurbsMesh);
// Store the mesh in the scene list
meshList.push_back(nurbsMesh);
Accessing the object:
if (absorbList.size() > 0)
{
float receivedPower = absorbList[0]->receivedPower;
}
What am I doing wrong?
There's some details missing, but at a guess.
nurbsMesh goes out of scope between the push_back and the absorbList[0]->receivedPower.
So now your vector of pointers contains a pointer to an object that doesn't exist anymore.
Try adding a copy constructor to your AbsorbMesh class and adding to your vector like this.
absorbList.push_back(new AbsorbMesh(nurbsMesh));
meshList.push_back(nurbsMesh);
don't forget to delete the objects in absorbList, like this
for(vector<AbsorbMesh*>::iterator it = absorbList.begin(); it != absorbList.end(); it++) {
delete it;
}
Or store a shared pointer in your vector instead of a bare pointer. Boost has a good shared pointer implementation if you're interested. See the docs here
If you want to have updates to items in one vector modify objects in the other vector, then you'll need to store pointers in both vectors.
Using your original requirements (updating an item in one vector affects items in the other vector, here's how I'd do it with a boost shared pointer. (WARNING, untested code)
vector<boost::shared_ptr<AbsorbMesh> > meshList;
vector<boost::shared_ptr<AbsorbMesh> > absorbList;
boost::shared_ptr<AbsorbMesh> nurb = new AbsorbMesh(nurbs.CreateMesh(uStride, vStride));
meshList.push_back(nurb);
absorbList.push_back(nurb);
...
...
if (absorbList.size() > 0)
{
float receivedPower = absorbList[0].get()->receivedPower;
}
You are storing the address of an object allocated on stack. The nurbsMesh object gets destroyed as soon as your method which does push_back() ends. If you try to access this pointer afterwards the object is already destroyed and contains garbage. What you need is to retain the object which remains even after the function goes out of scope. To do this allocate the memory for the object from heap using new. But for every new you should have a corresponding delete. But in your case you'll have problems deleting it as you are pushing the same pointer into two vectors. To solve this, you would require some type reference counting mechanism.
Object is deleted when you try get pointer from vector.
Try do
vector.push_back(new Object);
Once you have fixed the problem that others mentioned (storing a pointer to an object that's on the stack), you're going to run into another issue. A vector may reallocate, which results in its contents moving to another location.
The safe way to do this, then, is to store pointers in both vectors. Then, of course, you need to ensure that they get deleted... but that's C++ for you.
absorbList.push_back(&nurbsMesh); is wrong
absorbList save pointer to local object. When nurbMesh is destroyed you can not write
absorbList[0]->
However when I accessed the pointer later on the data seemed to be corrupted
When you put something on a vector, the vector might move it from one physical location to another (especially e.g. when the vector is resized), which invalidates any pointer to that object.
To fix that, you'll need to store a pointer (possibly a 'smart pointer') in both vectors (instead of having one vector contain the object by value).
If you're going to do this, it might be a good idea to disable the object's copy constructor and assignment operator (by declaring them as private, and not defining them) to ensure that after an object is created it cannot be moved.
There are several things wrong with your example
AbsorbMesh nurbsMesh = nurbs.CreateMesh(uStride, vStride);
This object is allocated on the stack. It is a purely local object. This object is going to be destroyed when you reach the end of the current block surrounded by {}.
absorbList.push_back(&nurbsMesh);
Now you get the pointer to the object that most likely will be destroyed.
meshList.push_back(nurbsMesh)
And this copies an entirely new object on the vector.
It is equally wrong to push the object on the vector first and then push a pointer to the object on the vector using absorbList.push_back( &meshList.back() ) because vector::push_back will reallocate the whole vector, invalidating all pointers.
You might be able to create all AbsorbMesh objects first, push them onto a vector, and then get the pointers to these objects in the vector. As long as you don't touch the vector, you'll be fine.
Alternatively, create objects on the heap using new AbsorbMesh() but make sure to call delete on each pointer thus created. Otherwise you have a memory leak.
Third solution, avoid the trouble and use smart pointers that take care of object destruction for you.
First, as everybody else points out, you can't allocate objects on the stack (i.e., other than by new or something similar), and have them around after leaving the scope.
Second, having objects in an STL container and maintaining pointers to them is tricky, since containers can move things around. It's usually a bad idea.
Third, auto_ptr<> simply doesn't work in STL containers, since auto_ptrs can't be copied.
Pointers to independently allocated objects work, but deleting them at the right time is tricky.
What will probably work best is shared_ptr<>. Make each vector a vector<shared_ptr<AbsorbMesh> >, allocate through new, and at a slight cost in performance you avoid a whole lot of hassle.
All the documentation I can find on the STL containers (both queue and list) say that for any of the remove functions the removed object's destructor is called. This means that I can't use std::queue any time I want a queue that's simply a list of objects needing some operation performed on them.
I want to be able to add objects to the queue when they are waiting in line for me to do something to them. Then I want to remove them from it when I've finished with them, without destroying the object in question. This doesn't appear to be possible from the documentation I've read. Am I misreading the documentation? Is there another type of queue in the STL other than the basic "queue" that doesn't call the removed object's destructor on a call to pop_front?
Edit to clarify: In my case I'm using a list of pointers. Something like this:
dbObject *someObject;
queue<dbObject *> inputQueue;
inputQueue.push_back(someObject);
...
dbObject *objectWithInput = inputQueue.front();
//handle object's input...
inputQueue.pop_front(); // Remove from queue... destroyed now?
If you put pointers to objects in the queue (and any other STL container), the pointers won't get deleted when you remove them.
To elaborate: when you use std::queue and remove an object the destructor of some_obj* is called. But the destructor for plain pointer (or any POD type - int, char, etc) is empty, no-op. The fine line here is that the destructor for some_obj* is very different from the destructor for some_obj.
STL containers have value semantics. When you push an object into an STL container, the STL container keeps it's own copy of the object, and when the object (internal copy) is removed from the container it is destroyed.
If you used a container of a proxy type, as raw pointers, smart pointers (shared_ptr, weak_ptr), or adapters (as boost::reference_wrapper), then the STL container will destroy the proxy but not the type. Choosing one over the others is usually a matter of how you want to deal with resources.
The most common idiom is using raw pointers, but they don't explicit who is in charge of destruction (the code that pulls from the container should delete the pointer, or the resource is handled somewhere else?).
Modern usage moves towards the shared_ptr approach, as it dilutes the ownership problem. The object will be guaranteed to be alive when you take it out of the container, and if nobody else holds a shared_ptr then the object will automatically be deleted when the local shared_ptr goes out of scope. Using a weak_ptr will keep the ownership in the original code, but will allow you to check for validity of the pointer (if it was deleted) before usage. This could allow you to avoid performing the operation on an object that will be removed right away.
The problem with the shared_ptr/weak_ptr approach is that it forces you to use shared_ptr to hold the original resource. This means that you will not be able to put a pointer into a subobject (member attribute) of another class without redesigning the class to hold the attribute through a shared_ptr, and that will have other implications (the attributes will no longer be contiguous in memory, more dynamic allocation operations will be required...)
A technique that is hardly seen is using adapters as boost::reference_wrapper<>. A reference wrapper is a proxy object that contains a reference to the original object and is itself copyable. The advantage over plain raw pointers is that reading the code it is clear that the resource is managed outside of the queue: the code that pulls data from the queue does not need to delete the object. The advantage over the smart pointer approach is that you do not need to redesign other parts of your system to use smart pointers. The disadvantage is that, as in the raw pointer approach, you must ensure that the lifetime of the referred object outlives the reference in the container manually.
class someobj_t {};
std::queue<someobj_t> q;
...
someobj_t ppd = q.front(); // ppd is not a reference
q.pop();
// ppd now contain removed object
If you don't want someobj_t to be copied you could use std::queue< shared_ptr<someobj_t> >.
How about using a list of pointers to the objects?
Think of item's in a container being "in-scope" of that container, when an item is removed from a container it is just like leaving the scope of a function. If the variable is a pointer nothing happens to the item when leaving the scope. If variable is a stack local then destructor will will be automatically called upon leaving the scope.
Storing pointers in containers have the same downfalls as allocating into a local raw pointer, the memory is not cleaned up automatically. In a function if you don't delete the pointer or transfer ownership by returning it then you have a memory leak.
When storing raw pointers in a container the ownership can become a bit ambiguous and leaks can easily happen. Take a look at tr1::shared_ptr and store those in the containers instead.
std::unique_ptr in C++0x would also be a good solution for storing a pointer in a stdlib container when it is available.