Use of enable_shared_from_this with multiple inheritance

Question

BI am using enable_shared_from_this in my code, and I am not sure if its usage is correct. This is the code:

class A: public std::enable_shared_from_this<A>
{
public:
    void foo1()
    {
        auto ptr = shared_from_this(); 
    }
};

class B:public std::enable_shared_from_this<B>
{
public:
    void foo2()
    {
        auto ptr = shared_from_this(); 
    }
};

class C:public std::enable_shared_from_this<C>
{
public:
    void foo3()
    {
        auto ptr = shared_from_this(); 
    }
};

class D: public A, public B, public C
{
public:
    void foo()
    {
        auto ptr = A::shared_from_this(); 
    }
};

Are these usage of make_shared_from_this() correct, assuming that they are always being called through shared_ptr of D?

Answer 1

Indeed you are doing it wrong. If you have simple inheritance, just inherit from enable_shared_from this in the base class, and derived class get it for free. (of course you'll need to downcast the result)

If you have multiple inheritance (like it seems), you must use the trick described here and also here :

/* Trick to allow multiple inheritance of objects
 * inheriting shared_from_this.
 * cf. https://stackoverflow.com/a/12793989/587407
 */

/* First a common base class
 * of course, one should always virtually inherit from it.
 */
class MultipleInheritableEnableSharedFromThis
: public std::enable_shared_from_this<MultipleInheritableEnableSharedFromThis> {
public:
  virtual ~MultipleInheritableEnableSharedFromThis()
  {}
};

template <class T>
class inheritable_enable_shared_from_this
: virtual public MultipleInheritableEnableSharedFromThis {
public:
  std::shared_ptr<T> shared_from_this() {
    return std::dynamic_pointer_cast<T>(
        MultipleInheritableEnableSharedFromThis::shared_from_this()
    );
  }
  /* Utility method to easily downcast.
   * Useful when a child doesn't inherit directly from enable_shared_from_this
   * but wants to use the feature.
   */
  template <class Down>
  std::shared_ptr<Down> downcasted_shared_from_this() {
    return std::dynamic_pointer_cast<Down>(
       MultipleInheritableEnableSharedFromThis::shared_from_this()
    );
  }
};

Then your code becomes :

class A: public inheritable_enable_shared_from_this<A>
{
public:
    void foo1()
    {
        auto ptr = shared_from_this(); 
    }
};

class B: public inheritable_enable_shared_from_this<B>
{
public:
    void foo2()
    {
        auto ptr = shared_from_this(); 
    }
};

class C: public inheritable_enable_shared_from_this<C>
{
public:
    void foo3()
    {
        auto ptr = shared_from_this(); 
    }
};

class D: public A, public B, public C
{
public:
    void foo()
    {
        auto ptr = A::downcasted_shared_from_this<D>(); 
    }
};