Does the delete() operator destroy all the sub-objects of an object too?
Or do I need to call the delete of sub-object before deleting the parent object?
class equipment
{
public:
int model_id;
...
}
class player
{
public:
int x, y;
equipment * Equipment;
player(void) { Equipment = new equipment[2];};
~player(void) { delete [] Equipment; }
};
int main (....)
{
player = new Player;
...
...
delete Player;
}
you need to delete the dynamically allocated subobjects in the destructor of the main object and you have properly done this by deleting array Equipment
Delete operator does delete all "subobjects" as you say, but usually and in your case you need to delete not only "sub objects" but also memory, where member pointers point to.
So if you used vector object, for example, it would be deleted automatically, but in this case you need to delete it explicitly, which you did correctly
To put it simply, during destruction of the object :
Member objects are themselves destroyed
Objects pointed by member pointers aren't
That means that you have to delete them manually (like your Equipement pointer), if you own them. There might be case when your class ends up with a pointer it does not own (for example, a handle to some other object you need in this class), and thus should not delete.
You can use smart pointers (i.e.:std::shared_ptr, std::unique_ptr, etc.) to manage memory in a safer way (they, notably, automatically delete the pointers they manage, following the RAII principle ).
One thing you are missing is the destructor in class equipment
The stuff you do in your example is sufficient, you
delete[] Equipment;
in the destructor of player, so all contained equipments will be deleted as soon as a player gets deleted.
Nicely done!
Related
In my class I wrote
class Game {
private:
mtm::Dimensions dimensions;
std::vector<Character*> board;
public:
explicit Game(int height, int width):dimensions(height,width), board(height*width){
}
~Game() {}
};
But how should I free my vector, I think it leaks memory if I leave the d'tor empty.
If Game is not the owner it must not free the memory. The owner has to clean up.
If Game is the owner it can delete all elements in the destructor
~Game() {
for (auto &character : board) {
delete character;
}
}
The better way is to use smart pointers and remove the destructor
std::vector<std::unique_ptr<Character>> board;
You should try to follow the rule of 0
Assuming that your Game owns the objects, then your Game destructor would need to free up the memory. Presumably, the Characters are allocated with new. The below will delete all the Characters in the board, then the class variables (like the vector) will automatically be freed afterward.
~Game() {
for ( Character *character : board )
delete character;
}
Unless this is for an exercise with pointers, it's generally recommended not to use bare-pointers, and instead use a smart pointer, such as unique_ptr or shared_ptr.
std::vector<std::shared_ptr<Character>> board;
In this case, there will be no leak, since the Characters will automatically be freed once no one points to them anymore.
Use of shared_ptr vs unique_ptr is dependent on what you're doing.
The vector is a member variable. Member variables, like all sub objects are automatically destroyed when the super object is destroyed. The implicitly generated destructor does the right thing for vector.
Note that the vector contains pointers. If those pointers point to dynamically allocated objects, and are the only pointers to those objects, then those objects would have to be deleted, or else they would leak. If something else owns the pointed objects, then there is nothing that needs to be done in this destructor.
An example where the implicit destructor is correct. I pretend that the member is public for this example:
{
Character c;
Game g;
g.board.push_back(&c);
} // no leaks
Avoid owning bare pointers. Prefer to use RAII containers or smart pointers if you need ownership of dynamic objects.
the vector destructor won't leak memory you should write destructors for the types used in the vector. Also I wouldn't recommend a pointer to something which dosen't destruct itself.
I think what you are looking for are smart pointers.
I can't help reading the bulk of forum posts on destructors and getting totally confused.
Some say to call the destructor (with delete) once for each call to new. Some say the destructor automatically gets called in a variety of circumstances i.e. when the pointer is reassigned, when the object goes out of scope. Some suggest the pointer going out of scope while being a return value where the object exists as a copy of its former self, (does this then need explicit destruction as it was originally created with a new?
There seems to be some suggestion that calling the same destructor more than once will corrupt memory so all delete calls should be partnered with *pointer = NULL; to avoid corruption. If not then some more advanced object management system would require implementing, or an iron-fisted rigour of ownership.
I can't seem to make any sense of discussions on calling sequence of destructors, i.e. does the call 1) originate at the base superclass and cascade down to the specific class, calling all virtualised destructors on the way, 2) originate at the instantiated class and move up to the superclass, or 3) originate at the particular cast the class has when it goes out of scope, then traverse both toward the instantiated and base class. Do cascading destructors
Ultimately I simply don't know strictly how or when to delete objects if ever, whether objects are responsible for deleting all objects they reference, how to cleanly handle a proper object-oriented deletion routine where an object is referenced multiple times, it's just a mess in my head. As you can see I can't really formulate a single solid question to ask, am really hoping someone can offer a clean and concise discussion of if not the single 'correct' approach, at least the industry best practice to object deletion.
There are 3 types of allocation for which destructors are called in different ways:
Automatic allocation
These objects reside in automatic memory (trivially, the stack):
int main()
{
A a;
//...
}
The destructor of a is automatically called when a goes out of scope (closing }).
Dynamic allocation
Objects reside in dynamic memory (the heap). They are allocated with new and in order for the dstructor to be called, you need to call delete:
int main()
{
A* a = new A;
delete a; //destructor called
}
In this case it was probably suggested you should assign NULL to a after the delete. There are two schools of thought regarding this (I personally wouldn't suggest it). The motivation would be that you could possibly call delete again on a and crash the program if you don't set it to NULL. Which is correct. But if you do call delete again, that's already a bug or something wrong in the logic, which shouldn't be masked by making the code appear to run correctly.
Static allocation
Objects reside in static memory. Regardless of where they are allocated, the destructor is automatically called when the program ends:
A a; //namespace scope
int main()
{
}
Here, As destructor is called when the program terminates, after main finishes.
The C++ language leaves memory management in the hand of the programmer, that is the reason for which you can find that level of confusion.
Repeating what Luchian Grigore said there are three main types of memory
automatic storage (stack)
dynamic storage (heap)
static storage
If you allocate an object in automatic storage the the object will be destroyed once the scope is terminated; for example
void foo() {
MyClass myclass_instance;
myclass_instance.doSomething();
}
in the above case when the function terminates myclass_instance is destroyed automatically.
If you instead allocate an object in the heap with new then it's your responsibility to call the destructor with delete.
In C++ also an object can have sub-objects. For example:
class MyBiggerClass {
MyClass x1;
MyClass x2;
...
};
those sub-objects are allocated in the same memory the containing object is allocated to
void foo() {
MyBiggerClass big_instance;
MyBiggerClass *p = new MyBiggerClass();
...
delete p;
}
in the above case the two sub-objects big_instance.x1 and big_instance.x2 will be allocated in automatic storage (stack), while p->x1 and p->x2 are allocated on the heap.
Note however that you don't need in this case to call delete p->x1; (compile error, p->x1 is not a pointer) or delete &(p->x1); (syntactically valid, but logical mistake because that it wasn't allocated explicitly on the heap, but as a sub-object of another object). Deleting the main object p is all that is needed.
Another complication is that an object may keep pointers to other objects instead of including them directly:
class MyOtherBigClass {
MyClass *px1;
MyClass *px2;
};
in this case it will be the constructor of MyOtherBigClass that will have to find the memory for the sub-objects and it will be ~MyOtherBigClass that will have to take care of destroying the sub-objects and freeing the memory.
In C++ destroying a raw pointer doesn't automatically destroy the content.
Base classes in simple cases can be seen just as hidden embedded sub-objects. I.e. it's like if an instance of the base object is embedded in the derived object.
class MyBaseClass {
...
};
class MyDerivedClass : MyBaseClass {
MyBaseClass __base__; // <== just for explanation of how it works: the base
// sub-object is already present, you don't
// need to declare it and it's a sub-object that
// has no name. In the C++ standard you can find
// this hidden sub-object referenced quite often.
...
};
This means that the destructor of the derived object doesn't need to call the destructor of the base object because this is taken care by the language automatically.
The case of virtual bases is more complex, but still the invocation of base destructors is automatic.
Given that memory management is in the control of the programmer there are a few strategies that have emerged to help programmers avoiding making a mess of intricate code that always ends up in object leaks or multiple destruction.
Plan carefully how you are going to handle lifetime of the instances. You cannot just leave this as an afterthought because it will be impossible to fix later. For every object instance it should be clear who creates and who destroys it.
When it's impossible to plan ahead of time when an object should be destroyed then use reference counters: for every object keep track how many pointers are referencing it and destroy the object once this number reaches zero. There are smart pointers that can take care of this for you.
Never keep around a pointer to an object that has already been destroyed.
Use containers that are classes designed explicitly to handle the lifetime of contained objects. Examples are std::vector or std::map.
If your code calls new, then it should call delete as well, yes. Except if you are using smart pointers (which will call delete for you when the pointer gets destroyed). Whenever possible, you should use smart pointers and use vector or string to avoid having to manually allocate memory using new - if you don't call new, you don't need to worry about making sure delete is called -> no memory leaks, and no problems with objects being destroyed at the wrong time, etc.
Calling delete multiple times for the same instance is definitely a bad idea.
If we have this:
class A
{
int *p;
public:
A() { p = new int[10]; }
~A() { delete [] p; }
};
class B
{
A a;
~B() { ... }
...
};
class C : public B
{
...
~C() { ... }
}
...
C *cp = new C;
....
delete cp;
then the destructor of C gets called by delete. The destructor of B is called by the C destructor, and the destructor of A gets called by the B destructor. This is automatic, and the compiler will "make sure this happens".
And if we don't call new:
...
{
C c;
...
} // Destructor for C gets called here (and B and A as describe above)
Im new to this and just wanted to ask a quick question about deleting objects.
I have an object called MyClass1 and from it I have a number of other classes, MyClassA, MyClassB etc.
Now should I do this in MyClass1:
MyClass1::~MyClass1()
{
delete MyClassA;
delete MyClassB;
}
Or will everything created in MyClass1 automatically be deleted when I delete MyClass1?
Also, if I have more objects created in MyClassA and MyClassB, will these also have to be deleted manually in their respective class?
Thanks
If you're asking this, you're just learning C++, so the best advice is - neither. You should know about this stuff (dynamic allocation & memory management - see Guillaume's answer for this), but what you really should do is use RAII (google this). The proper C++ way of doing it would be:
struct MyClass1
{
MyClassA mA;
std::shared_ptr<MyClassB> mB;
MyClass1() : mB(new MyClassB)
{
}
};
See? No more destructor, which means you also don't need a copy constructor or copy assignment operator (which is where Guillaume's answer is flawed - it's missing the last two).
call delete operator only if you have created your objects with new operator
struct MyClass1
{
MyClassA mA;
MyClassB * mB;
MyClass1()
{
mB = new MyClassB;
}
~MyClass1()
{
delete mB;
}
};
You can't delete objects that aren't pointers because that's not the purpose of delete. It's meant to free dynamic memory associated with an object. That is, whatever is created with new must be deleted. You can have pointers to a class, and they can be deleted. But since nothing was allocated with new, there's no need to use delete. The class will in fact be destructed from memory at the end of the scope in which it is created. Those objects are allocated on the stack while dynamic memory is on the heap. Objects on the stack have automatic storage duration (deleted at the end of its scope, unless its declared static in which case it has "static" storage duration); moreover, objects on the heap have dynamic storage duration. Dynamic memory in C++ is controlled by you, that's why we are given new and delete (because C++ expects us to handle the memory ourselves). And otherwise deleting an object not constructed with new is undefined behavior and may lead to a crash.
If Qt, use QPointer! It is a smart pointer: nothing needed in destructor.
#include <QPointer>
class MyClass1
{
QPointer<MyClassA> pA;
QPointer<MyClassB> pB;
};
delete is applied to objects, not to classes. As a rule, calling delete (or arranging to have it called automatically, via a shared pointer, or with the RAII idiom in general) is necessary only if you called new to create the object. The exception is the return value of some (library?) call being an object that the (library's?) documentation states explicitly that the caller has to dispose of with delete (in which case, think of the call as a wrapper around a new that you have become responsible for.) Of course, APIs like that should be avoided if at all possible.
So I have a class called List which stores a vector of pointers to classes of type Object. List has a function called add which initialises an Object class and adds it's pointer to the vector. I thought that once the add function ended that the Object class would be destroyed and accessing the pointer would cause an exception. When I wrote a test program to test this it turned out that the Object class was never destroyed.
Are classes initialised inside a function ever destroyed once the function ends?
When are classes automatically destroyed?
Depends how you're creating the object. If you are doing it like this:
void add() {
Object obj;
vec.push_back(&obj);
}
Then you are creating obj with automatic storage duration. That means it will be destroyed when the add function ends. The pointer you have pushed into the vector will no longer point to a valid Object, so definitely don't do this.
You may, however, be doing this:
void add() {
Object* obj = new Object();
vec.push_back(obj);
}
If you are, you are creating the Object with dynamic storage duration and it will not be destroyed at the end of the function. The pointer you push into the vector will remain valid. However, if you do this, you need to remember to delete the object at a later time. If you don't, you'll have a leak.
The best option is to avoid using pointers at all, if you can. Just make the vector a std::vector<Object> and copy objects into it:
void add() {
vec.push_back(Object());
}
Or in C++11:
void add() {
vec.emplace_back();
}
If you really need pointers, prefer smart pointers.
In C++, if you don't use any memory manager, smart pointers or some kind of "managed" environment, classes created using keyword new. To avoid misunderstanding could you please post constructor of Object and method List.Add?
So, when you are creating new object like:
Object* o = new Object();
you reserving space in the heap and store pointer to that space in pointer 'o'. If you never calling
delete o;
explicitly your object will never deleted.
This is merely for curiosity sake because I have not used new and delete in c++ except for the most basic uses.
I know that delete releases memory. The thing I'm wondering is how does it handle more complex cases?
For instance, if I have a user-defined class like this:
class MyClass
{
public:
MyClass();
~MyClass()
{
delete [] intArray;
}
//public members here
private:
int* intArray;
};
Assuming the class allocates memory somehow for intArray, and then release it in the destructor, What if I used the class like this: MyClass* myClass = new MyClass(); and released it later with delete myclass;
How does delete handle the releasing of all the memory? Does the class destructor get called first to release all of the memory allocated by the class (ie int* intArray) and then release the memory allocated to hold the class?
What if I had a class like this:
class MyClass
{
public:
MyClass();
~MyClass()
{
delete anotherMyClass;
}
//public members here
private:
MyClass* anotherMyClass;
};
Assuming anotherMyClass is not allocated with the constructor, which would use up memory very quickly, what if there was a chain of MyClasses attached to each other like a linked-list? Would the delete statement in the destructor work in this case? Would each anotherMyClass be recursively released when the destructor gets called?
Are there any specific weird tricks or caveats with the new and delete statements that you know about?
Given a pointer (p) to a dynamically allocated object, delete does two things:
It calls the destructor of the dynamically allocated object. Note that when ~MyClass() completes, the destructors for any member variables of class type are called.
It frees the memory occupied by the dynamically allocated object.
It doesn't search the member variables of the object for other pointers to free; it doesn't free any other memory and doesn't do anything else.
If you need to free the memory pointed to by intArray, you need to delete it in the destructor of MyClass.
However, in almost all C++ code, you don't need to worry about this. You should be using smart pointers like shared_ptr, unique_ptr, auto_ptr, and scoped_ptr to automatically manage dynamically allocated objects. Manual resource management is difficult at best and should be avoided wherever possible.
This is part of a broader idiom, Scope-Bound Resource Management (SBRM, also called Resource Acquisition is Initialization, or RAII). This is by far the most important design pattern to understand and to use everywhere in your C++ code.
If in your class you had declared this instead:
boost::scoped_ptr<int> intArray;
then when the scoped_ptr<int> object is destroyed, it will free the pointer that it holds. You then do not have to do any work to manually destroy the object.
In well-written, modern C++ code, you should rarely need to manually use delete. Smart pointers and other SBRM containers should be used to manage any type of resource that needs cleanup, including dynamically allocated objects.
In your second example, given a linked list that looks like:
x -> y -> z -> 0
you would have an order of operations that looks like this:
delete x;
x.~MyClass();
delete y;
y.~MyClass();
delete z;
z.~MyClass();
delete 0;
[free memory occupied by z]
[free memory occupied by y]
[free memory occupied by x]
The objects in the linked list would be destroyed in reverse order.
delete intArray;
I assume intArray points to the first element of an int array? In that case, delete intArray yields undefined behavior. If you allocate via new[], you must release via delete[].
delete[] intArray;
Yes I know, delete intArray might appear to work just fine on certain systems with certain compiler versions under certain compiler options -- or it might not. That's undefined behavior for ya.
Also, you should follow the Rule of Three. Defining your own destructor but relying on the implicitly-defined copy constructor and copy assignment operator is a recipe for disaster.