Creating a new object from dynamic type info

Question

In C++, is there any way to query the type of an object and then use that information to dynamically create a new object of the same type?

For example, say I have a

Answer 1

You can use e.g. typeid to query an object's dynamic type, but I don't know of a way to directly instantiate a new object from the type information.

However, apart from the clone approach mentioned above, you could use a factory:

#include <typeinfo>
#include <iostream>

class Base
{
public:
    virtual void foo() const
    {
        std::cout << "Base object instantiated." << std::endl;
    }
};


class Derived : public Base
{
public:
    virtual void foo() const
    {
        std::cout << "Derived object instantiated." << std::endl;
    }
};


class Factory
{
public:
    static Base* createFrom( const Base* x )
    {
        if ( typeid(*x) == typeid(Base) )
        {
            return new Base;
        }
        else if ( typeid(*x) == typeid(Derived) )
        {
            return new Derived;
        }
        else
        {
            return 0;
        }
    }
};


int main( int argc, char* argv[] )
{
    Base* X = new Derived;
    if ( X != 0 )
    {
        std::cout << "X says: " << std::endl;
        X->foo();
    }

    Base* Y = Factory::createFrom( X );
    if ( Y != 0 )
    {
        std::cout << "Y says: " << std::endl;
        Y->foo();
    }

    return 0;
}

P.S.: The essential part of this code example is of course the Factory::createFrom method. (It's probably not the most beautiful C++ code, since my C++ has gone a little rusty. The factory method probably shouldn't be static, on second thought.)

Answer 2

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

class Foo : public Base
{

};

class Bar : public Base
{

};

template<typename T1, typename T2>
T1* fun(T1* obj)
{
 T2* temp = new T2();
 return temp;
}

int main()
{
  Base* b = new Foo();
  fun<Base,Foo>(b);
}