Runtime polymorphism for two different classes - c++

Question

I have two classes B andY which I cannot change or edit by requirement. They have functions doing the same thing but with different names.
I want to a have a common interfaces with selecting the class at run time depending on the some input variable as described in the code below. I am not sure which design pattern should I use. How to create WrapperYB class which selects Y::show or B::showing depending on the object created.
class A
{
public:
A() {}
virtual ~A();
virtual void show() { cout << "show A" << endl;}
};
class B:A
{
public:
B() {}
virtual ~B();
virtual void show() { cout << "show B" << endl;}
};
class X
{
char m_i;
public:
Y() { m_i = 'X';}
virtual void showing() { cout << "showing " << m_i << endl;}
};
class Y:X
{
public:
Y() { m_i = 'Y';}
virtual void showing() { cout << "showing " << m_i << endl;}
};
class WrapperYB
{
// to be implemented
public:
explicit WrapperYB(const int& type);
void show();
};
int main(){
WrapperYB objY(1);
objY.show(); // must call Y::showing
WrapperYB objB(0);
objB.show(); // must call B::show
}

Answer 1

If your compiler supports the C++17 Standard, you could try this solution using std::variant. This is a similar idea to the solution in #Nicolas's answer, but variant will take care of the implementation details for you, won't use dynamic memory allocation, and has support for additional things like copy and assignment.
#include <variant>
#include <utility>
#include <type_traits>
class WrapperYB {
public:
using variant_type = std::variant<Y, B>;
template <typename... Args,
std::enable_if_t<std::is_constructible_v<variant_type, Args...>>* = nullptr>
WrapperYB(Args&& ... args) : m_variant(std::forward<Args>(args)...) {}
variant_type& variant() noexcept { return m_variant; }
const variant_type& variant() const noexcept { return m_variant; }
void show()
{ std::visit(ShowImpl{}, m_variant); }
private:
struct ShowImpl {
void operator() (Y& y) const { y.showing(); }
void operator() (B& b) const { b.show(); }
};
variant_type m_variant;
};
See the full working example on coliru.
You might generalize the wrapper by letting it contain a std::unique_ptr<A> or std::unique_ptr<X> instead.

Answer 2

I'm proposing this:
#include <iostream>
using namespace std;
class A
{
public:
A() {}
virtual ~A() {}
virtual void show() { cout << "show A" << endl;}
};
class B:A
{
public:
B() {}
virtual ~B() {}
virtual void show() { cout << "show B" << endl;}
};
class X
{
protected:
char m_i;
public:
X () { m_i = 'X';}
virtual void showing() { cout << "showing " << m_i << endl;}
};
class Y:X
{
public:
Y() { m_i = 'Y';}
virtual void showing() { cout << "showing " << m_i << endl;}
};
class WrapperYB
{
public:
enum class Which { B, Y };
public:
explicit WrapperYB (int n)
: which(Which(n))
{
switch (which)
{
case Which::B: ptr.b = new B; break;
case Which::Y: ptr.y = new Y; break;
}
}
~WrapperYB ()
{
switch (which)
{
case Which::B: delete ptr.b; break;
case Which::Y: delete ptr.y; break;
}
}
WrapperYB (const WrapperYB&) = delete;
WrapperYB& operator = (const WrapperYB&) = delete;
public:
void show()
{
switch (which)
{
case Which::B: ptr.b->show() ; break;
case Which::Y: ptr.y->showing(); break;
}
}
private:
Which which;
union {
Y* y;
B* b;
} ptr;
};
int main(){
WrapperYB objY(1);
objY.show(); // must call Y::showing
WrapperYB objB(0);
objB.show(); // must call B::show
}
It's not a "Vanilla" design pattern, I don't think, and more of combination of adapter and discriminated union.
Note that WrapperYB cannot be copied or assigned, as is.

Answer 3

You can use a standard virtual dispatch method with an abstract base adaptor class and subclasses for each object type needed. Create the object with a factory method.
#include <memory>
//pre-defined structures Y, B
struct Y
{
Y(){}
~Y(){}
void show(){}
};
struct B
{
B(){}
~B(){}
void showing(){}
};
// Abstract adaptor base class.
struct Adaptor
{
virtual void show() = 0;
};
// A subclass of Adaptor for each type of object to be wrapped.
struct Adaptor_Y: Adaptor
{
Adaptor_Y(): y(){}
void show() override
{
y.show();
}
private:
Y y;
};
struct Adaptor_B: Adaptor
{
Adaptor_B(): b(){}
void show() override
{
b.showing();
}
private:
B b;
};
// Factory method constructs the proper object and returns a pointer.
std::unique_ptr<Adaptor> get_adaptor(int flag)
{
if(flag == 0)
{
return std::make_unique<Adaptor_B>();
}
else if(flag == 1)
{
return std::make_unique<Adaptor_Y>();
}
else throw std::runtime_error("Invalid flag value");
}