polymorphic C++ references

Question

I was wondering how you can do polymorphism with references, as opposed to pointers.

To clarify, see the following minimal example:

class A;

class B {
  public:
    A& a; ///////////////// <- #1
    B();
    void doStuff();
};

class A {
  public:
    virtual void doSmth() = 0;
};
void B::doStuff() {
  a.doSmth();
}

class A1 : public A {
  public:
    void doSmth() {
    }
};

B::B() : a(
    *        ////////////// <- #2
      (new A1)  /////////// <- #3
     ) {
}

This compiles and works, but as the most important point here is that a in line #1 is a reference, so in order to be able to use it polymorphically (is that an actual word?), as shown in line #3 I have to "convert a pointer to a reference" by dereferencing it.

This strikes me as a bit odd, and I was wondering if there is a better (in the sense of cleaner) way. Is it just me?

Rationale

It would be great if I didn't need a new at all, but when declaring (!) B I have no clue how to create an instance of A1 (!) as A is a forward declaration -- A1 is implemented in the same compilation unit as B. Still, is there a real need for dynamic memory allocation in this case? How would you do this?

Sorry for the slightly twofold question.

Edit

Note: B is huge (and I cannot make a template class of it), and will go out of scope precisely when the program terminates -- a is small and makes two big modules talk to each other, it will be needed as long as the instance of B lives (there is only one).

Edit 2

I just realised, that since both A and B are effectively singletons, I can simply create a static instance of A1 in the compilation unit of B, avoiding dynamic memory allocation (even if there were two Bs they could easily use the same instance of A). To be fair, I did not post this as answer, but will accept the answer that prompted me to come up with this solution.

Answer 1

There's nothing odd. Polymorphisms works both for pointers and references:

struct Base { };
struct Derived : Base;

void foo(Base &);

int main() {
  Derived x;
  foo(x);    // fine
}

You're conflating this with another issue, namely creating a reference to a dynamic object:

T * pt = new T;
T & rt = *pt;

T & x = *new T;  // same effect

Note that it's generally very bad style to track a dynamic object only by reference, because the only way to delete it is via delete &x;, and it's very hard to see that x needs cleaning up.

There are two immediate alternatives for your design: 1) make a a member object in B, or 2) make a a shared_ptr<A> or unique_ptr<A> and change the initalizer to a(new A1). It all depends on whether you actually need the polymorphic behaviour, i.e. if you have other constructors for B which assign a different derived class to a other than A1.

Answer 2

Still, is there a real need for dynamic memory allocation in this case?

No. Just define A1 first and then make it a normal member of B.

Polymorphism works just fine with both references and pointers.

Answer 3

This is indeed a bit odd. If you want a member-variable of type A1 (rather than a reference), why not just rearrange your code so that the definition of A1 appears before the definition of B?

Answer 4

Erm, is this not sufficient?

#include <iostream>

struct A;

struct B
{
  B(A& a);

  void foo();

  A& _a;
};

struct A
{
  virtual void foo() =0;
};

struct A1 : public A
{
  virtual void foo() { std::cout << "A1::foo" << std::endl; }
};

B::B(A& a) : _a(a) {}
void B::foo() { _a.foo(); }


int main(void)
{ 
  A1 a;  // instance of A1
  B b(a); // construct B with it

  b.foo();
}

Answer 5

Still, is there a real need for dynamic memory allocation in this case?

Either the dynamic memory allocation or injecting the reference into B's ctor.

Answer 6

It's no stretch to imagine why references can work polymorphically like pointers (not to mention references are often implemented as pointers anyway). Here's a quick example:

class Base { 
public:
    virtual void something() { }
};

class Derived : public Base {
public:
    void something() { }
};

Base& foo() {
    static Derived d;
    return d;
}

foo().something(); // calls Derived's something

Also why are you allocating dynamic memory for a reference? You probably shouldn't be using a reference in this case at all. Also, writing classes with reference members effectively prevents assignment (as I heard someone say quite well).

Answer 7

I realize this is a really old post but there is another option you have for handling references for dynamically allocated objects. You can assign a reference to the dynamically allocated object. Below is some dummy code to give you an idea of how this works.

struct A
{
  int b;
  virtual void print();
  A(int val):b(val) {}
};

struct A_child:public A
{
  A_child(int val):A(val) {}
  void print();
};

void A:print()
{
cout<<"parent\n";
}

void A_child:print()
{
cout<<"child\n";
}

struct test_ref
{
A *& ref;
test_ref(A * ptr) : ref(ptr)
}

int main()
{

  test_ref parent(new A(12));
  parent.ref->print();

  test_ref child(new A_child(15));
  child.ref->print();
} 

To be honest I am not certain when this is a good idea. I just wanted to show an alternative approach where you dont have to dereference the dynamically allocated memory when initializing an object.

I am also pretty certain dynamically allocating a pointer while initializing a class where the pointer is stored as a reference pointer will probably lead to a memory leak unless you can delete the reference pointer.