Why do we not have a virtual constructor in C

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Why does C   not have a virtual constructor

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Interview answer is   virtual ptr and table are related to objects but not the class hence constructor builds the virtual table  hence we cant have virtual constructor as there is no Vtable before obj creation

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When a constructor is invoked  although there is no object created till that point  we still know the kind of object that is gonna be created because the specific constructor of the class to which the object belongs to has already been called   Virtual keyword associated with a function means the function of a particular object type is gonna be called   So  my thinking says that there is no need to make the virtual constructor because already the desired constructor whose object is gonna be created has been invoked and making constructor virtual is just a redundant thing to do because the object-specific constructor has already been    invoked and this is same as calling class-specific function which is achieved through the virtual keyword    Although the inner implementation won   t allow virtual constructor for vptr and vtable related reasons       Another reason is that C   is a statically typed language and we need to know the type of a variable at compile-time    The compiler must be aware of the class type to create the object  The type of object to be created is a compile-time decision   If we make the constructor virtual then it means that we don   t need to know the type of the object at compile-time that   s what virtual function provide  We don   t need to know the actual object and just need the base pointer to point an actual object call the pointed object   s virtual functions without knowing the type of the object  and if we don   t know the type of the object at compile time then it is against the statically typed languages  And hence  run-time polymorphism cannot be achieved   Hence  Constructor won   t be called without knowing the type of the object at compile-time  And so the idea of making a virtual constructor fails

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The virtual mechanism only works when you have a based class pointer to a derived class object  Construction has it s own rules for the calling of base class constructors  basically base class to derived  How could a virtual constructor be useful or called  I don t know what other languages do  but I can t see how a virtual constructor could be useful or even implemented  Construction needs to have taken place for the virtual mechanism to make any sense and construction also needs to have taken place for the vtable structures to have been created which provides the mechanics of the polymorphic behaviour

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Hear it from the horse s mouth      From Bjarne Stroustrup s C   Style and Technique FAQ Why don t we have virtual constructors      A virtual call is a mechanism to get work done given partial   information  In particular   virtual  allows us to call a function   knowing only any interfaces and not the exact type of the object  To   create an object you need complete information  In particular  you   need to know the exact type of what you want to create  Consequently    a  call to a constructor  cannot be virtual    The FAQ entry goes on to give the code for a way to achieve this end without a virtual constructor

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When people ask a question like this  I like to think to myself  what would happen if this were actually possible   I don t really know what this would mean  but I guess it would have something to do with being able to override the constructor implementation based on the dynamic type of the object being created   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

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C   virtual constructor is not possible For example you can not mark a constructor as virtual Try this code   include lt iostream h gt  using namespace std  class aClass       public          virtual aClass                               int main         aClass a       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    include lt iostream h gt  using namespace std  class aClass       public          aClass                         cout lt  lt  aClass contructor n                      aClass                         cout lt  lt  aClass destructor n                 class anotherClass public aClass        public          anotherClass                         cout lt  lt  anotherClass Constructor n                      anotherClass                         cout lt  lt  anotherClass destructor n                 int main         aClass  a      a new anotherClass      delete a         getchar         In main a new anotherClass  allocates a memory for anotherClass in a pointer a declared as type of aClass 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 e first the base and then the derived classes   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 Thanks

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Summary  the C   Standard could specify a notation and behaviour for  virtual constructor s that s reasonably intuitive and not too hard for compilers to support  but why make a Standard change for this specifically when the functionality can already be cleanly implemented using create     clone    see below    It s not nearly as useful as many other language proposal in the pipeline   Discussion  Let s postulate a  virtual constructor  mechanism   Base  p   new Derived       Base  p2   new p- gt Base        possible syntax      In the above  the first line constructs a Derived object  so  p s virtual dispatch table can reasonably supply a  virtual constructor  for use in the second line    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    The second line postulates the notation new p- gt Base   to request dynamic allocation and default construction of another Derived object   Notes    the compiler must orchestrate memory allocation before calling the constructor - constructors normally support automatic  informally  stack   allocation  static  for global namespace scope and class- function-static objects   and dynamic  informally  heap   when new is used   the size of object to be constructed by p- gt Base   can t generally be known at compile-time  so dynamic allocation is the only approach that makes sense   it is possible to allocate runtime-specified amounts of memory on the stack - e g  GCC s variable-length array extension  alloca   - but leads to significant inefficiencies and complexities  e g  here and here respectively    for dynamic allocation it must return a pointer so memory can be deleted later  the postulated notation explicitly lists new to emphasise dynamic allocation and the pointer result type    The compiler would need to    find out how much memory Derived needed  either by calling an implicit virtual sizeof function or having such information available via RTTI call operator new size t  to allocate memory invoke Derived   with placement new    OR   create an extra vtable entry for a function that combines dynamic allocation and construction   So - it doesn t seem insurmountable to specify and implement virtual constructors  but the million-dollar question is  how would it be better than what s possible using existing C   language features      Personally  I see no benefit over the solution below      clone    and  create     The C   FAQ documents a  virtual constructor  idiom  containing virtual create   and clone   methods to default-construct or copy-construct a new dynamically-allocated object   class Shape     public      virtual  Shape          A virtual destructor     virtual void draw     0     A pure virtual function     virtual void move     0                 virtual Shape  clone   const   0     Uses the copy constructor     virtual Shape  create   const   0     Uses the default constructor    class Circle   public Shape     public      Circle  clone   const     Covariant Return Types  see below     Circle  create   const     Covariant Return Types  see below               Circle  Circle  clone   const   return new Circle  this     Circle  Circle  create   const   return new Circle        It s also possible to change or overload create   to accept arguments  though to match the base class   interface s virtual function signature  arguments to overrides must exactly match one of the base class overloads   With these explicit user-provided facilities  it s easy to add logging  instrumentation  alter memory allocation etc

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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       Furthermore      you cannot have a pointer to a constructor    Bjarne Stroustup  P424 The C   Programming Language SE

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Virtual functions in C   are an implementation of run-time polymorphism  and they will do function overriding   Generally the virtual keyword is used in C   when you need dynamic behavior  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   So  if we try to declare virtual constructor compiler throw an Error        Constructors cannot be declared virtual

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If you think logically about how constructors work and what the meaning usage of a virtual function is in C   then you will realise that a virtual constructor would be meaningless in C    Declaring something virtual in C   means that it can be overridden by a sub-class of the current class  however the constructor is called when the objected is created  at that time you cannot be creating a sub-class of the class  you must be creating the class so there would never be any need to declare a constructor virtual   And another reason is  the constructors have the same name as its class name and if we declare constructor as virtual  then it should be redefined in its derived class with the same name  but you can not have the same name of two classes  So it is not possible to have a virtual constructor

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Cant we simply say it like   We cannot inherit constructors  So there is no point declaring them virtual because the virtual provides polymorphism

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Unlike object oriented languages such as Smalltalk or Python  where the constructor is a virtual method of the object representing the class  which means you don t need the GoF abstract factory pattern  as you can pass the object representing the class around instead of making your own   C   is a class based language  and does not have objects representing any of the language s constructs  The class does not exist as an object at runtime  so you can t call a virtual method on it   This fits with the  you don t pay for what you don t use  philosophy  though every large C   project I ve seen has ended up implementing some form of abstract factory or reflection

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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

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two reasons I can think of   Technical reason  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      Logic reason  You use the virtual keyword when you want to declare a somewhat polymorphic behaviour  But there is nothing polymorphic with constructors   constructors job in C   is to simply put an object data on the memory   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

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You shouldn t call virtual function within your constructor either  See   http   www artima com cppsource nevercall html  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

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The Vpointer is created at the time of object creation  vpointer wont exists before object creation  so there is no point of making the constructor as virtual

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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  So  a constructor cannot be made virtual in C

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There s a very basic reason  Constructors are effectively static functions  and in C   no static function can be virtual   If you have much experience with C    you know all about the difference between static  amp  member functions  Static functions are associated with the CLASS  not the objects  instances   so they don t see a  this  pointer  Only member functions can be virtual  because the vtable- the hidden table of function pointers that makes  virtual  work- is really a data member of each object    Now  what is the constructor s job  It is in the name- a  T  constructor initializes T objects as they re allocated  This automatically precludes it being a member function  An object has to EXIST before it has a  this  pointer and thus a vtable  That means that even if the language treated constructors as ordinary functions  it doesn t  for related reasons I won t get into   they d have to be static member functions    A great way to see this is to look at the  Factory  pattern  especially factory functions  They do what you re after  and you ll notice that if class T has a factory method  it is ALWAYS STATIC  It has to be

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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

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We do  it s just not a constructor  -   struct A     virtual  A        virtual A   Clone     return new A        struct B   public A     virtual A   Clone     return new B        int main         A   a1   new B     A   a2   a1- gt Clone          virtual construction    delete a2     delete a1

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Semantic reasons aside  there is no vtable until after the object is constructed  thus making a virtual designation useless

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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   considering case for correct execution of the program   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   Now  think of the situation about the moment when the object of the class to be pointed is being assigned some memory -  Its constructor will be called automatically at that instance itself   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

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