How can I avoid the Diamond of Death when using multiple inheritance?

http://en.wikipedia.org/wiki/Diamond_problem

I know what it means, but what steps can I take to avoid it?

A practical example:

class A {};
class B : public A {};
class C : public A {};
class D : public B, public C {};

Notice how class D inherits from both B & C. But both B & C inherit from A. That will result in 2 copies of the class A being included in the vtable.

To solve this, we need virtual inheritance. It's class A that needs to be virtually inherited. So, this will fix the issue:

class A {};
class B : virtual public A {};
class C : virtual public A {};
class D : public B, public C {};

virtual inheritance. That's what it's there for.

I'd stick to using multiple inheritance of interfaces only. While multiple inheritance of classes is attractive sometimes, it can also be confusing and painful if you rely on it regularly.

Inheritance is a strong, strong weapon. Use it only when you really need it. In the past, diamond inheritance was a sign that I was going to far with classification, saying that a user is an "employee" but they are also a "widget listener", but also a ...

In these cases, it's easy to hit multiple inheritance issues.

I solved them by using composition and pointers back to the owner:

Before:

class Employee : public WidgetListener, public LectureAttendee
{
public:
     Employee(int x, int y)
         WidgetListener(x), LectureAttendee(y)
     {}
};

After:

class Employee
{
public:
     Employee(int x, int y)
         : listener(this, x), attendee(this, y)
     {}
     
     WidgetListener listener;
     LectureAttendee attendee;
};

Yes, access rights are different, but if you can get away with such an approach, without duplicating code, it's better because it's less powerful. (You can save the power for when you have no alternative.)

class A {}; 
class B : public A {}; 
class C : public A {}; 
class D : public B, public C {};

In this the attributes of Class A repeated twice in Class D which makes more memory usage... So to save memory we make a virtual attribute for all inherited attributes of class A which are stored in a Vtable.

Well, the great thing about the Dreaded Diamond is that it's an error when it occurs. The best way to avoid is to figure out your inheritance structure beforehand. For instance, one project I work on has Viewers and Editors. Editors are logical subclasses of Viewers, but since all Viewers are subclasses - TextViewer, ImageViewer, etc., Editor does not derive from Viewer, thus allowing the final TextEditor, ImageEditor classes to avoid the diamond.

In cases where the diamond is not avoidable, using virtual inheritance. The biggest caveat, however, with virtual bases, is that the constructor for the virtual base must be called by the most derived class, meaning that a class that derives virtually has no control over the constructor parameters. Also, the presence of a virtual base tends to incur a performance/space penalty on casting through the chain, though I don't believe there is much of a penalty for more beyond the first.

Plus, you can always use the diamond if you are explicit about which base you want to use. Sometimes it's the only way.

I would suggest a better class design. I'm sure there are some problems that are solved best through multiple inheritance, but check to see if there is another way first.

If not, use virtual functions/interfaces.

Use inheritance by delegation. Then both classes will point to a base A, but have to implement methods that redirect to A. It has the side effect of turning protected members of A into "private" members in B,C, and D, but now you don't need virtual, and you don't have a diamond.

This is all I have in my notes about this topic. I think this would help you.

The diamond problem is an ambiguity that arises when two classes B and C inherit from A, and class D inherits from both B and C. If there is a member in A that B and C, and D does not override it, then which member does D inherit: that of B, or that of C?

struct A { int a; };
struct B : A { int b; };
struct C : A { int c; };
struct D : B, C {};

D d;
d.a = 10;		//error: ambiguous request for 'a'

In the above example, both B & C inherit A, and they both have a single copy of A. However D inherits both B & C, therefore D has two copies of A, one from B and another from C. If we need to access the data member an of A through the object of D, we must specify the path from which the member will be accessed: whether it is from B or C because most compilers can’t differentiate between two copies of A in D.

There are 4 ways to avoid this ambiguity:

1- Using the scope resolution operator we can manually specify the path from which a data member will be accessed, but note that, still there are two copies (two separate subjects) of A in D, so there is still a problem.

d.B::a = 10; // OK
d.C::a = 100; // OK
d.A::a = 20; // ambiguous: which path the compiler has to take D::B::A or D::C::A to initialize A::a

2- Using static_cast we can specify which path the compiler can take to reach to data member, but note that, still there are two copies (two separate suobjects) of A in D, so there is still a problem.

static_cast<B&>(static_cast<D&>(d)).a = 10;
static_cast<C&>(static_cast<D&>(d)).a = 100;
d.A::a = 20; // ambiguous: which path the compiler has to take D::B::A or D::C::A to initialize A::a

3- Using overridden, the ambiguous class can overriden the member, but note that, still there are two copies (two separate suobjects) of A in D, so there is still a problem.

struct A { int a; };
struct B : A { int b; };
struct C : A { int c; };
struct D : B, C { int a; };
 
D d;
d.a = 10;    // OK: D::a = 10
d.A::a = 20; // ambiguous: which path the compiler has to take D::B::A or D::C::A to initialize A::a

3- Using virtual inheritance, the problem is completely solved: If the inheritance from A to B and the inheritance from A to C are both marked "virtual", C++ takes special care to create only one A subobject,

struct A { int a; };
struct B : virtual A { int b; };
struct C : virtual A { int c; };
struct D : B, C {};
 
D d;
d.a = 10;    // OK: D has only one copy of A - D::a = 10
d.A::a = 20; // OK: D::a = 20

Note that "both" B and C have to be virtual, otherwise if one of them is non-virtual, D would have a virtual A subobject and another non-virtual A subobject, and ambiguity will be still taken place even if class D itself is virtual. For example, class D is ambiguous in all of the following:

struct A { int a; };
struct B : A { int b; };
struct C : virtual A { int c; };
struct D : B, C {};

Or

struct A { int a; };
struct B : virtual A { int b; };
struct C : A { int c; };
struct D : B, C {};

Or

struct A { int a; };
struct B : A { int b; };
struct C : virtual A { int c; };
struct D : virtual B, C {};

Or

struct A { int a; };
struct B : virtual A { int b; };
struct C : A { int c; };
struct D : virtual B, C {};