In many sources, books etc. are written "don't call this->virtualFunction in child class constructor" and in some sources explain why you mustn't do it. Because at the moment of construction class isn't created at all. So virtual function that will be called from child constructor will be function of base class. It means that in child constructor body VPTR of that class pointed to base VTABLE.
So my question is,
what is the moment in object construction when VPTR of child class will be overriden to address on it's virtual table? I guess that some automatically generated code do it at end of constructor body or after constructor body will be executed.
And second question is,
why is VPTR overriden at the end of construction? Maybe for it have some important reasons?
Why can't override VPTR at beginning constructor body or after base class constructed?
Child::Child() :
Base()
//<----- Why not here?
//members initialization
{
//<----- Why not here?
//code
}
In many sources, books etc. are written "don't call this->virtualFunction in child class constructor"
I doubt that. It's often advised not to call virtual functions from the base class constructor, to avoid confusion if you expect them to call the final overrides, not the base-class versions. And you must certainly not call them from the base class if they are pure virtual there - that gives undefined behaviour.
Within the derived class constructor, they are well-defined and do what you would expect.
So virtual function that will be called from child constructor will be function of base class.
No, during the child's constructor body, the dynamic type is Child, and virtual functions calls will use the Child overrides.
what is the moment in object construction when VPTR of child class will be overriden to address on it's virtual table?
After all base class constructors have completed, and before child class members are initialised. Child member functions, including virtual functions, can be called from member initialisers (but not base class initialisers) or the constructor body.
You need to be careful calling them from member initialisers, since they might access uninitialised members. But within the constructor body, all base objects and members are initialised, so they are quite safe.
why is VPTR overriden at the end of construction?
It isn't. It happens at the first point you indicate with <----- Why not here?.
I disagree with your overly simplified reason to why you shouldn't call a virtual function. First of all, a VTABLE is not actually defined by the C++ standard, and is, in fact, implementation specific:
When is VTable in C++ created?
Calling a virtual function from a constructor is allowed by the standard, and should actually should call correctly within the hirearchy at that level (with some limitations)
C++ constructors: why is this virtual function call not safe?
http://www.parashift.com/c%2B%2B-faq-lite/calling-virtuals-from-ctors.html
However, there are many, many reasons not to do this.
The constructor for the derived class has not been called yet. Any access of any members of the derived class will yield undefined behavior.
There is a rule of thumb that you should keep constructors as simple and dumb as possible. This is due to the fact that error handling in constructors can be a giant pain due to destructor not being called and the only way to return a sensical error state is to throw an exception.
This will often break the dependency inversion principal, and may cross the line of demeter, and will usually introduce high coupling to the code.
Related
Just out of curiosity I tried to do something like the example below to see if the compiler gives me a warning or so instead of calling an endless loop which ends up in a stack overflow. I thought maybe there is a different behavior than just calling normal functions or methods. But that's not the case. Is there a special explanation for that or is it just handled as normal function calls since I am explicitely calling the base class destructor by using the this operator?
Example:
class A {
virtual ~A();
};
class B : A {
virtual ~B() { this->~A(); }
};
#M.M's comment hit it. You are calling the destructor twice. This is undefined behavior and anything may happen, including the behavior you observe.
(In reality, most likely one of those destructor calls modifies the vptr of the object, meaning that subsequent destructor calls no longer go to the most derived object. But that's just a guess.)
The correct thing to do is to not call the destructor manually.
Virtual destructors in a derived class will always call parent-class destructors first, in a recursive order such that the most "ancestral" base-class' destructor will be called, and then the second-most-"ancestral", etc. Imagine that Child inherits from Parent, which inherits from GrandParent. The Child class' destructor will actually call GrandParent's destructor, then Parent's destructor, then Child's destructor.
Indeed your derived class constructors also call their parent-class constructors in the same recursive order. You have to imagine derived classes like a "layer cake": each instance of inheritance adds a layer to your object. So the Child class has 3 layers {GrandParent, Parent, Child}, each layer's construction/destruction is handled by the corresponding class.
What you're attempting will attempt to call the parent destructor twice, which is a bad idea. Generally you do not have to explicitly call destructors, except in the case where you overloaded the new operator. See this answer for more details: Is calling destructor manually always a sign of bad design?
Calling derived class virtual destructor causes call of base class destructor. But not vise versa.
I' ve read some articles, and as they say, main virtual destructor use cases are:
derived classes may have dynamic data allocation from heap, i.e. "own" that data object. So, they need some deletion routine in destructor. Deletion through base class pointer requires virtual declaration of destructors in all derived class till those with dynamic data allocation (base class also requires it)
this class has virtual methods. But this is unclear for me. Just calling virtual methods through base class pointer always results in the-most-derived-realization calls. They only exclusion for that rule is construction phase. Literaly, during it this object is not a derived type yet, even if later will be. Ok, what about destruction phase? As I understood, the rule is in the backward order. No matter, was destructor of some class in hierarchy declared as virtual, during each destructor this pointer is used as if of this class type, any derived were already been destroyed due to virtual d-r, or not destroyed (and that hypothetically may be ok in some design). Maybe this is the case, why d-r must be virtual? Vtable will have entries for derived class, and calling virtual methods in some base class from d-r will result in UB? Ok, but this rule applies only to case when this class calls some virtual methods in d-r, and they do have realization in derived classes.
My opinion, that there is also may be a case with no dynamic data allocation, no virtual methods in all hierarchy, but still derived destructors may do some critical tasks upon deletion (syncs, unlocks and so on). And we need virtual d-r in base class. May be such cases are results of bad design.
But anyway, developer of some public class cannot 100% know, if derived class will or not use some virtual methods in d-r, or allocate dynamic data. So, am I correct to say, that any public class, not declared as final, has to declare d-r as virtual? Only final keyword guarantees that any pointer to this class will always be of this type, and so, could be safely deleted non-virtually.
If a derived object is deleted through a pointer to the base class, then (and only then) the base class destructor must be virtual. Otherwise it is undefined behaviour. There are no other relevant rules.
If the class had a virtual function anyway, then no overhead is introduced. If the class did not have any other virtual functions, then the base class designer has to consider the trade between adding the runtime penalty of a virtual destructor, vs. the risk that a user of the class might try to delete a derived object through the base class pointer.
Here is a link to a similar discussion, with Standard quotes
Currently, I have the "lots of code" as noted in the diagram inside each child's constructor. My goal is to move it to the parent's constructor.
Is it possible to call a virtual method from an abstract class constructor?
Technically it is possible, but it won't work as you expect, so don't do it, because the virtual table for derived classes has not been constructed yet.
The implementation of the class being constructed is going to be called when a virtual function is invoked from a constructor, and if the virtual function you are calling is pure, you get undefined behavior.
Per paragraph 10.4/6 of the C++11 Standard:
Member functions can be called from a constructor (or destructor) of an abstract class; the effect of making a
virtual call (10.3) to a pure virtual function directly or indirectly for the object being created (or destroyed)
from such a constructor (or destructor) is undefined.
If you try to invoke initializeFiles() from the FileContainer constructor it will invoke FileContainer::initializeFiles(). This is because the constructor for the derived class has not been executed yet and therefore the v-table for the derived class has not been built.
Also if FileContainer::initializeFiles() is a pure-virtual function then you will get a crash.
I'm currently writing a program with the following polymorphic hierarchy: Base: Multinumber. Derived: Pairs, Complex, Rational. Multinumber is a virtual class and is never instantiated.
During the course of my program I frequently dynamically manage the base classes so I need a destructor. My question is this: how do I make the destructor virtual? In my Multinumber.h file right now I have this:
virtual ~Multinumber();
In my Multinumber.cpp:
Multinumber::~Multinumber()
{
}
And in all of my derived classes I have this:
Rational::~Rational()
{
}
I have nothing in any of my derived.h files. Unfortunately, this does not compile. Rather, I get this error:
Complex.cpp|75|error: definition of implicitly-declared 'virtual Complex::~Complex()'
What is wrong with my syntax? Thanks for any help you can give me.
Just declaring the virtual destructor in the base, and giving it the empty definition, is sufficient. For the other classes you do not need to do anything at all, unless there's actual work for those destructors to do.
The point of the virtual declaration in the base class is to ensure that the destructor can be invoked polymorphically (so that Base* d = new Derived(); delete d; works correctly, calling the Derived destructor instead of the Base destructor). You then have to define that destructor (even if it does no work) because you declared it.
However, for all of the derived classes, if you don't specify anything, the default "call destructors for members and bases" destructor gets generated for them, and everything works as you need it to. Unless, again, you need to do anything else to destruct the object properly.
As Dark Falcon noted, you need a declaration in the base for every member that you define, including destructors. So if you do write Complex::~Complex, then it must be declared in the Complex class definition, even though you inherit from a class that declares and defines a destructor. (Destructors, like constructors, aren't actually inherited anyway; the default "call recursively on members and bases" behaviour isn't really the same thing. These functions are special, since they manage the object lifetime, rather than using the object.)
Within the class Complex, you also need to have a declaration for the destructor:
~Complex();
Note that virtual is optional here. The destructor will be virtual because the base's destructor is virtual.
When to declare a destructor virtual?
I recommend you to follow this algorithm to decide whether you should declare a destructor virtual or not.
Is your class intended to be used as a base class?
No: Declare non-virtual destructor (avoids v-pointer on each object of the class) and remember not to derive from concrete classes.
Yes: Go to next question.
Is your base class abstract? (i.e. any virtual pure methods?)
No: Try to make you base class abstract by redesigning your hierarchy [1] (i.e. don't allow you base class to be instantiated).
Yes: Go to next question.
Do you want to allow polymorphic deletion though a base pointer?
No: Declare protected virtual destructor to avoid the unwanted usage.
Yes: Declare public virtual destructor (no overhead in this case).
References:
[1]: S. Meyers. More Effective C++, Item 33 (Addison-Wesley, 1996).
[2]: "Virtuality" http://www.gotw.ca/publications/mill18.htm
What is wrong with my syntax?
The following:
And in all of my derived classes I
have this:
Rational::~Rational() { }
No you don't. You didn't do it for Complex.
The question is: When to not declare the destructor virtual? There are no drawbacks to declaring a destructor virtual.
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
Why do we not have a virtual constructor?
I know this has been asked before but I didn't understand the complex technical words used in the other answers.
I read on a community the reason that constructors cannot be virtual is
The ‘virtual’ mechanism works on a logically complete (completely constructed) object. We know that we use constructors to logically initialize our objects. In other words, the object is not completely constructed until the constructor has finished executing. Thus, we can’t have virtual constructors.
There is a misconception that by then virtual table is incomplete so we can’t have virtual constructors. Just before the constructor starts executing the virtual table is properly constructed and the ‘this’ pointer passed to the constructors. Moreover, virtual table mechanism is implementation depended, and finds no place in the C++ standard. And hence, to argue over this issue using the virtual table concept is illogical.
Now, as the constructor finishes executing any other function can be virtual. Destructor is no exception to this rule as it is a function. Virtual destructors are required in case we use a base class pointer to refer to a derived class object, use it, and then delete it. If we have virtual destructor, using ‘delete’, a chain of destructors is called starting from the derived to the base. But, had there been no ‘virtual’ in destructor only the base class destructor is called (and not the derived). This (may) generate inconsistencies in the program.
Is the above reason correct? The answer doesn't talk about the static and dynamic types of objects.
Virtual constructors don't make sense and aren't necessary. The only time you call a constructor is when creating an object. You need to know the type of an object in order to create it, so the static and dynamic types are the same, and the correct constructor to call is the one for that type.
That's why they aren't necessary. Why they don't make sense, is that when creating an object, base class constructors are called as well as derived class constructors. If the base class constructor was overridden in the derived class, is that supposed to mean that the base class constructor isn't called after all?
Other languages have virtual constructors, perhaps because constructors in those languages are methods, and they only have virtual invocation for non-static methods. But those other languages (Java and Python spring to mind) have to introduce special rules that constructors must/should construct their base class explicitly as a call from the constructor. C++ just does it (perhaps in an initializer list, if the base class constructor requires parameters), using non-virtual constructors, and there is no option to enter the body of the constructor with uninitialized base class sub objects.
To quote from The C++ Programming Language:
To construct an object, a constructor
needs the exact type of the object it
is to create. Consequently, a
constructor cannot be virtual.
Furthermore, a constructor is not
quite an ordinary function. In
particular, it interacts with memory
management routines in ways ordinary
member functions don’t. Consequently,
you cannot have a pointer to a
constructor.
It must be interesting to note that C++ do have an idiom called virtual constructor. You can read more about that here.
Yes, the reason is you need to have a pointer/reference to a complete object already before you can call a virtual function. When the constructor is being invoked there's no complete object yet. Even more, when you do new SomeClass() you don't even have a pointer yet - the pointer is returned upon the new statement is completed successfully.
The virtual keyword cannot be applied to a constructor since a constructor turns raw bits into a living object, and until there is a living object against which to invoke a member function, the member function cannot possibly work correctly. Instead of thinking of constructors as normal member functions on the object, imagine that they are static member functions that create objects. - C++ FAQs