One pointer, two different classes in c++

Question

Suppose I have two structures a and b, each hold several variable in them (most of the variable are c++ core types but not all).

Is there a way to create a a pointer named c that can point to either one of them? Alternatively, is there a way to create a set that can hold either one of them?

Thanks

Answer 1

The usual way to create a pointer that can point to either of the two is to make them inherit from a common base-class. Any pointer of the base-class can point to any sub-class. Note that this way you can only access elements that are part of the base-class through that pointer:

class Base {
public:
    int a;
};

class Sub1 : public Base {
public:
    int b;
};

class Sub2 : public Base {
public:
    int c;
};


int main() {
    Base* p = new Sub1;
    p.a = 1; // legal
    p.b = 1; // illegal, cannot access members of sub-class
    p = new Sub2; // can point to any subclass
}

What you are trying to achieve is called polymorphism, and it is one of the fundamental concepts of object oriented programming. One way to access member of the subclass is to downcast the pointer. When you do this, you have to make sure that you cast it to the correct type:

static_cast<Sub1*>(p).b = 1; // legal, p actually points to a Sub1
static_cast<Sub2*>(p).c = 1; // illegal, p actually points to a Sub1

As for your second question, using the technique described above, you can create a set of pointers to a base-class which can then hold instance of any of the subclasses (these can also be mixed):

std::set<Base*> base_set;
base_set.insert(new Sub1);
base_set.insert(new Sub2);

Answer 2

Alternatively, is there a way to create a set that can hold either one of them?

Take a look at Boost.Any and Boost.Variant. If you have just 2 classes, then variant should suffice. If you plan other types, and don't want to recompile this 'set', then use any.

Then use any container of either any or variant.

#include <boost/any.hpp>
#include <boost/variant.hpp>

#include <vector>

class A { };
class B { };
class C { };

int main()
{
    // any

    std::vector<boost::any> anies;
    anies.push_back(A());
    anies.push_back(B());

    A a0 = boost::any_cast<A>(anies[0]);
    A b0 = boost::any_cast<A>(anies[1]); // throws boost::bad_any_cast

    // variant
    std::vector<boost::variant<A,B> > vars;
    vars.push_back(A());
    vars.push_back(B());

    A a1 = boost::get<A>(vars[0]);
    A b1 = boost::get<A>(vars[1]); // throws boost::bad_get

    // and here is the main difference:
    anies.push_back(C()); // OK
    vars.push_back(C());  // compile error
}

Edit: having more than 2 classes is of course possible for variant, too. But extending variant so it is able to hold a new unanticipated type without recompilation is not.

Answer 3

If a and b are unrelated, then you can use a void* or, better, a boost any type.

If a is superclass of b, you can use an a* instead.

Answer 4

If they both inherit from the same type you can do it. Thats how OOP frameworks work, having all classes inherit from Object.

Answer 5

Abstract Class !!!! -- simple solutions

To have a base class that can be used as a pointer to several derived sub classes. (no casting needed)

Abstract class is define when you utilize a virtual method in it. Then you implement this method in the sub-class... simple:

// abstract base class
#include <iostream>
using namespace std;

class Polygon {
  protected:
    int width, height;
  public:
    void set_values (int a, int b)
      { width=a; height=b; }
    virtual int area (void) =0;
};

class Rectangle: public Polygon {
  public:
    int area (void)
      { return (width * height); }
};

class Triangle: public Polygon {
  public:
    int area (void)
      { return (width * height / 2); }
};

int main () {
  Polygon * ppoly1 = new Rectangle (4,5);
  Polygon * ppoly2 = new Triangle (4,5);
  ppoly1->set_values (4,5);
  ppoly2->set_values (4,5);
  cout << ppoly1->area() << '\n';
  cout << ppoly2->area() << '\n';
  return 0;
}

Answer 6

Just define a common superclass C and two subclasses A, B of C. If A and B have no common structure (no common attributes), you can leave C empty.

The define:

A *a = new A();
B *b = new B();
C *c;

Then you can do both

c = a;

or

c = b;

Answer 7

Although you can do that, what would that pointer mean? If any portion of your application gets hold on the pointer to 'either a or b', it cannot do a lot with it, unless you provide extra type information.

Providing extra type information will result in client code like

if( p->type == 'a' ) {
   ... a-specific stuff
} else if( p->type == 'b' ) {
   ... b-specific stuff
} ...

Which isn't very useful.

It would be better to delegate 'type-specificness' to the object itself, which is the nature of object-oriented design, and C++ has a very good type-system for that.

class Interface {
 public:
    virtual void doClientStuff() = 0; // 
    virtual ~theInterface(){};
};

class A : public Interface {
    virtual void doClientStuff(){ ... a-specific stuff }
};

class B : public Interface {
    virtual void doClientStuff(){ ... b-specific stuff }
};

And then your client code will become more type-unaware, since the type-switching is done by C++ for you.

void clientCode( Interface* anObject ) {
   anObject->doClientStuff();
}

Interface* i = new A();
Interface* j = new B();

clientCode( i );
clientCOde( j );

Answer 8

There are several ways to do this:

1. Using the more generic base type, if there is an inheritance relationship.
2. Using void* and explicitly casting where appropriate.
3. Creating a wrapper class with the inheritance relationship needed for #1.
4. Using a discriminating container via union.

Since others have already described the first three options, I will describe the fourth. Basically, a discriminated container uses a union type to use the storage of a single object for storing one of multiple different values. Typically such a union is stored in a struct along with an enum or integral type for distinguishing which value is currently held in the union type. As an example:

// Declarations ...
class FirstType;
class SecondType;

union PointerToFirstOrSecond {
   FirstType* firstptr;
   SecondType* secondptr;
};

enum FIRST_OR_SECOND_TYPE {
   FIRST_TYPE,
   SECOND_TYPE
};

struct PointerToFirstOrSecondContainer {
   PointerToFirstOrSecond pointer;
   FIRST_OR_SECOND_TYPE which;
};

// Example usage...

void OperateOnPointer(PointerToFirstOrSecondContainer container) {
    if (container.which == FIRST_TYPE) {
       DoSomethingWith(container.pointer.firstptr);
    } else {
       DoSomethingElseWith(container.pointer.secondptr);
    }
}

Note that in the code below, "firstptr" and "secondptr" are actually two different views of the same variable (i.e. the same memory location), because unions share space for their content.

Note that even though this is a possible solution, I seriously wouldn't recommend it. This kind of thing isn't very maintainable. I strongly recommend using inheritance for this if at all possible.