Simulation of virtual Constructor!

This is a discussion on Simulation of virtual Constructor! within the C++ Programming forums, part of the General Programming Boards category; Hi Guys, I have a quick doubt on virtual contructor! Though, We cannot write virtual constructor in C++. But I ...

  1. #1
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    Simulation of virtual Constructor!

    Hi Guys,

    I have a quick doubt on virtual contructor!

    Though, We cannot write virtual constructor in C++. But I read it we can achieve the effect of virtual contructor in C++ deisgn pattern called "Factory Design pattern".

    Can somebody explain me with a nice and easy example of virtual contructor with factory design pattern?

    Your help will be highly appreciated!

    Thanks,
    Vaibhav

  2. #2
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    Code:
    #include <iostream>
    class foo
    {
      public:
      foo()
      {
        std::cout << "foo";  
      }
    };
    
    class bar : public foo
    {
      public:
      bar():
        foo()
      {
        std::cout<<"bar";
      }
    };
    
    class factory
    {
        public:
           virtual foo& make()
          {
            return *new foo();
          }
    };
    class barFactory : public factory
    {
      public:
           virtual bar& make()
          {
            return *new bar();
          }
    };
    i suppose you could do something like the above. i don't see the point though.

  3. #3
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    How would we call the same from main?
    Please elaborate

  4. #4
    C++ Witch laserlight's Avatar
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    m37h0d's example is a little unconventional in that it returns a reference to a dynamically allocated object. A canonical example of a "virtual constructor" would be with a virtual clone() function, allowing one to clone an object of the derived class type without knowing the actual derived class type (i.e., only the base class type is known).

    Quote Originally Posted by vaibhavs17
    How would we call the same from main?
    What would your attempt be?
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  5. #5
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    Thanks Laser Light!
    Could you please explain with an example?
    Thanks
    Vaibhav

  6. #6
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by vaibhavs17
    Could you please explain with an example?
    As you wish:
    Code:
    #include <ostream>
    #include <memory>
    #include <iostream>
    
    class X
    {
    public:
        virtual ~X() {}
    
        virtual X* clone() const = 0;
    
        virtual void identify(std::ostream& out) const = 0;
    };
    
    class Y : public X
    {
    public:
        explicit Y(int n) : n(n) {}
    
        virtual Y* clone() const
        {
            return new Y(*this);
        }
    
        virtual void identify(std::ostream& out) const
        {
            out << "I am of type Y with n = " << n << std::endl;
        }
    private:
        int n;
    };
    
    class Z : public X
    {
    public:
        explicit Z(int n) : n(n + n) {}
    
        virtual Z* clone() const
        {
            return new Z(*this);
        }
    
        virtual void identify(std::ostream& out) const
        {
            out << "I am of type Z with n = " << n << std::endl;
        }
    private:
        int n;
    };
    
    X* foo(int n)
    {
        if (n % 2 == 0)
        {
            return new Y(n);
        }
        else
        {
            return new Z(n);
        }
    }
    
    int main(int argc, char* argv[])
    {
        std::auto_ptr<X> p1(foo(argc));
        std::auto_ptr<X> p2(p1->clone());
        p2->identify(std::cout);
    }
    Notice that p2 in the global main function points to a copy of the object pointed to by p1, but all we know is that the type of object that p1 points to is a subtype of X.
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