Diamond problem

Diamond problem
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In object-oriented programming languages with multiple inheritance, 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 a method in D calls a method defined in A, which class does it inherit from, B or C?

It is called the 'diamond' problem because of the shape of the class inheritance diagram in this situation. Class A is at the top, both B and C separately beneath it, and D joins the two together at the bottom to form a diamond shape.

Different programming languages have addressed this problem in different ways:

Objective-C, PHP, C#, and Java only allow single inheritance, but as a compromise allow the multiple inheritance of interfaces (called protocols in Objective-C). Interfaces are essentially abstract base classes with all abstract methods and no data members. C# (and other .NET languages that allow interface definition) also allows property definitions to exist at the interface level.
C++ by default follows each inheritance path separately, so a D object would actually contain two separate A objects, and uses of A's members have to be properly qualified. If the inheritance from A to B and the inheritance from A to C are both marked "virtual" ("class B : virtual A"), C++ takes special care to only create one A object, and uses of A's members work correctly. If virtual inheritance and nonvirtual inheritance are mixed, there is a single virtual A and a nonvirtual A for each nonvirtual inheritance path to A.
Common Lisp attempts to provide both reasonable default behavior and the ability to override it. By default, the method with the most specific argument classes is chosen; then in the order in which parent classes are named in the subclass definition. However, the programmer can override this, by giving a specific method resolution order or stating a rule for combining methods.
Eiffel handles this situation by select and rename directives, where the ancestors' methods to use in a descendant are explicitly specified. This allows the methods of the base class to be shared between its descendants or to even give each of them a separate copy of the base class.
Perl and Io handle this by specifying the inheritance classes as an ordered list. In the above ambiguity, class B and its ancestors would be checked before class C and its ancestors, so the method in A would be inherited through B.
Python had to deal with this upon the introduction of new-style classes, all of which have a common ancestor, object. Python creates a list of the classes that would be searched in left-first depth-first order (D, B, A, C, A) and then removes all but the last occurrence of any repeated classes. Thus, the method resolution order is: D, B, C, A.
The diamond problem is not limited to inheritance. It also arises when header files A, B, C, and D "#include" one another in a diamond as above and separate precompiled headers are created from B and C. If these two precompiled headers are combined, declarations in A are duplicated and the "#ifndef" convention (see the #include guard article) is ineffective. It also is found when composing middleware stacks; for example, if A is a database and B and C are caches, D may ask both B and C to commit a transaction, resulting in duplicate commit calls to A.