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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.

• Inheritance semantics
• Inherited Class Ambiguity (ICA)
• Implementation inheritance (object-oriented programming)
• Method Override Ambiguity (MOA)
• Stateful Diamond Point pattern (C++ code example)
• Virtual Inheritance (object-oriented programming)