virtual constructor in c++ [duplicate]

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Possible Duplicate:

Why do we not have a virtual constructor?

why we dont have virtual constructor in c++?

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C++ classes are not first class objects - there's no way, like in java, to make a variable that refers to a class and invoke construction based on this variable. So, a virtual constructor does not make sense, since you always know the type of the object you create.

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How would it work? What would you want to achieve with it?

You need to have an object in order to have a virtual-table / function-table, but when you are in the constructor you don't have an object yet, as it is under construction.

What you probably do want is an abstract factory. That will create a different class in an "abstract" way, eg:

class Base { public: virtual ~Base() {} };

class Derived1 : public Base
{
};

class Derived2 : public Base
{
};

class BaseFactory
{
public:
   virtual ~BaseFactory() {}
   virtual Base * create() const = 0;
};

class Derived1Factory : public BaseFactory
{
public:
   // could also return Derived1*
   Base * create() const { return new Derived1; }
};

class Derived2Factory : public BaseFactory
{
public:
   // could also return Derived2*
   Base * create() const { return new Derived2; }
};

// abstractly construct a Base
Base * createABase( const BaseFactory & fact )
{
   return fact.create();
}