Create a variable to hold objects of different types C++

Question

I have 3 different objects A, B and C. Depending on the the parameter given, I would like to choose among these different objects. In

Answer 1

It seems to me that you should use a virtual common base class/struct and a pointer to this base class/struct.

The following is a full working example

#include <iostream>

struct Base
 { virtual void printHello () = 0; };

class A : public Base { 
  public: 
    void printHello() { std::cout << "HELLO A" << std::endl; }
}; 

class B  : public Base{ 
  public: 
    void printHello() { std::cout << "HELLO B" << std::endl; }  
};

class C  : public Base{ 
   public: 
     void printHello() { std::cout << "HELLO C" << std::endl; }  
}; 

int main () { 
    std::string key = "c"; 
    A a; 
    B b; 
    C c; 
    Base * obj; 

    if (key == "a") obj = &a; 
    else if (key == "b") obj = &b; 
    else obj = &c;
    obj->printHello(); // print Hello C.
    return 0; 
} 

Answer 2

You're looking for a variant type. There's an upcoming std::variant in C++17, and C++11-compliant versions in boost and around the web. Example with boost::variant:

struct visitor
{
    void operator()(const A&){ cout << "HELLO A" << endl; }
    void operator()(const B&){ cout << "HELLO B" << endl; }
    void operator()(const C&){ cout << "HELLO C" << endl; }
};

int main()
{
    visitor v;

    // `obj` is either an `A`, a `B` or a `C` at any given moment.
    boost::variant<A, B, C> obj{B{}};
    //                         ^^^^^
    //                   Initialize with `B`.

    boost::apply_visitor(v, obj); // prints "HELLO B"

    obj = A{};
    boost::apply_visitor(v, obj); // prints "HELLO A"
}

Answer 3

you could do something like

if (key == "a") obj=new A();
else if (key == "b")  obj=new B();