In either case, f must be the final overrider, guaranteeing consistent behavior of the function being called. Avoid calling virtual functions from constructors Avoid calling virtual functions from destructors Do not invoke class's virtual functions from any of its constructors Do not invoke class's virtual functions from its destructor. Search for other vulnerabilities resulting from the violation of this rule on the CERT website. There are some nonprintable characters that have crept in to the citation for Lockheed Martin Pages Boards.
Page tree. Browse pages. A t tachments 0 Page History People who can view. Jira links. Created by Stephen C. Dewhurst , last modified by Anirban Gangopadhyay on Jul 19, Noncompliant Code Example In this noncompliant code example, the base class attempts to seize and release an object's resources through calls to virtual functions from the constructor and destructor.
Aaron Ballman. Permalink Mar 13, David Svoboda. Fixed, thanks. Profile Answers by kalayama. I agree with ShruthiMitra. It doesn't make sense to have virtual constructor and constructed never needs to overridden. Since all the functions in the Public, protected are inherited we may have virtual functions. That is U will never need to call a derived class constructor using the Base class Pointer As our folks told, we don't really required virtual at declaration stage of construction, but might required at creational stage to avoid duplicate.
From mypoint of view: A fully constructed object is required inorder to invoke virtual member function of class. As we know constructor gets called during object getting constructed, If constructor is marked as virtual mean that it also required fully constructed object to invoke but object is still under construction.
A virtual-table vtable is made for each Class having one or more 'virtual-functions'. Whenever an Object is created of such class, it contains a 'virtual-pointer' which points to the base of corresponding vtable.
Whenever there is a virtual function call, the vtable is used to resolve to the function address. Constructor can not be virtual, because when constructor of a class is executed there is no vtable in the memory, means no virtual pointer defined yet.
Hence the constructor should always be non-virtual. Profile Answers by YogitaSabnis. A constructor cannot be virtual because at the time when the constructor is invoked the virtual table would not be available in the memory.
Hence we cannot have a virtual constructor. Profile Answers by paul. Profile Answers by chethanhb. Profile Answers by masterofcse.
For argument sake, Virtual functions are same for all objects in a class. Why can't those functions be maintained as static in nature. Please don't be confuse with static here. My idea is to keep virtual table is in common location and access since this is same across all objects.
Why do we need to create a VTable for each object. One VTable is sufficient for a class. We can have VPTR which is static and be constructed by compiler. Profile Answers by surendrap. What if I want my constructor to have different behavior then its default?
What about parameterized constructor It also contain the same name. The constructor initializes the object and thereby the vtable. When constructor of a class is invoked, the vtable is not yet created, thereby negating the point of having a virtual constructor. Virtual table is created at compile time. The FAQ entry goes on to give the code for a way to achieve this end without a virtual constructor.
Virtual functions basically provide polymorphic behavior. That is, when you work with an object whose dynamic type is different than the static compile time type with which it is referred to, it provides behavior that is appropriate for the actual type of object instead of the static type of the object.
Now try to apply that sort of behavior to a constructor. When you construct an object the static type is always the same as the actual object type since:. To construct an object, a constructor needs the exact type of the object it is to create [ The class does not exist as an object at runtime, so you can't call a virtual method on it.
The object exists only after the constructor ends. In order for the constructor to be dispatched using the virtual table , there has to be an existing object with a pointer to the virtual table , but how can a pointer to the virtual table exist if the object still doesn't exist? You use the virtual keyword when you want to declare a somewhat polymorphic behaviour. Since virtual tables and polymorphism in general are all about polymorphic behaviour rather on polymorphic data , There is no sense with declaring a virtual constructor.
It's not nearly as useful as many other language proposal in the pipeline. Dozens of answers on this page stating "the object doesn't yet exist so virtual construction is impossible" are unnecessarily myopically focused on the to-be-constructed object. Personally, I see no benefit over the solution below.
With these explicit user-provided facilities, it's easy to add logging, instrumentation, alter memory allocation etc.. Semantic reasons aside, there is no vtable until after the object is constructed, thus making a virtual designation useless. It will work only when object exists. Whereas constructors are used to create the objects. Constructors will be called at the time of object creation. So if you create the constructor as virtual , as per the virtual keyword definition, it should have existing object to use, but constructor is used to to create the object, so this case will never exist.
So you should not use the constructor as virtual. You can find an example and the technical reason to why it is not allowed in stefan 's answer. Now a logical answer to this question according to me is:. The major use of virtual keyword is to enable polymorphic behaviour when we don't know what type of the object the base class pointer will point to.
But think of this is more primitive way, for using virtual functionality you will require a pointer. And what does a pointer require? An object to point to! So, we basically require an object that already exists somewhere in the memory we are not concerned with how the memory was allocated, it may be at compile time or either runtime so that our pointer can correctly point to that object. So we can see that we don't actually need to worry about the constructor being virtual, because in any of the cases you wish to use a polymorphic behaviour our constructor would have already been executed making our object ready for usage!
When people ask a question like this, I like to think to myself "what would happen if this were actually possible? I see a number of potential problems with this. For one thing, the derived class will not be fully constructed at the time the virtual constructor is called, so there are potential issues with the implementation. Secondly, what would happen in the case of multiple inheritance?
Your virtual constructor would be called multiple times presumably, you would then need to have some way of know which one was being called. Thirdly, generally speaking at the time of construction, the object does not have the virtual table fully constructed, this means it would require a large change to the language specification to allow for the fact that the dynamic type of the object would be known at construction time.
This would then allow the base class constructor to maybe call other virtual functions at construction time, with a not fully constructed dynamic class type.
Finally, as someone else has pointed out you can implement a kind of virtual constructor using static "create" or "init" type functions that basically do the same thing as a virtual constructor would do. Although the concept of virtual constructors does not fit in well since object type is pre-requisite for object creation, its not completly over-ruled. GOF's 'factory method' design pattern makes use of the 'concept' of virtual constructor, which is handly in certain design situations.
Virtual functions are used in order to invoke functions based on the type of object pointed to by the pointer, and not the type of pointer itself. But a constructor is not "invoked". It is called only once when an object is declared. Interview answer is : virtual ptr and table are related to objects but not the class.
You shouldn't call virtual function within your constructor either. In addition I'm not sure that you really need a virtual constructor. You can achieve polymorphic construction without it: you can write a function that will construct your object according to the needed parameters. A virtual-table vtable is made for each Class having one or more 'virtual-functions'.
Whenever an Object is created of such class, it contains a 'virtual-pointer' which points to the base of corresponding vtable. Whenever there is a virtual function call, the vtable is used to resolve to the function address.
Constructor can not be virtual, because when constructor of a class is executed there is no vtable in the memory, means no virtual pointer defined yet. Hence the constructor should always be non-virtual. For example you can not mark a constructor as virtual.
Try this code. It causes an error. This code is trying to declare a constructor as virtual. Now let us try to understand why we use virtual keyword.
Virtual keyword is used to provide run time polymorphism. For example try this code. This causes both the constructor In aClass and anotherClass to call automatically. So we do not need to mark constructor as virtual. Because when an object is created it must follow the chain of creation i. But when we try to delete a delete a; it causes to call only the base destructor. So we have to handle the destructor using virtual keyword. So virtual constructor is not possible but virtual destructor is.
Cant we simply say it like..
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