Post-object programming
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Post-object programming is a new generation of techniques that evolved out of the object-oriented programming generation. Post-object programming emphasizes mixins, delegation, and aspects; as well as heterogeneous groups and multimethods. Post-object languages aspire to go beyond traditional object languages.
Post-object programming is related to design patterns. Many patterns can be thought of as workarounds for languages that do not support post-object techniques directly. For example, the delegation pattern is a workaround for mixins, delegation, and aspects, and the abstract factory pattern and visitor pattern are workarounds for heterogeneous groups and multimethods.
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Post-object techniques
Mixins
Mixins are classes that have inheritance defined with respect to an interface, not to a class. When instantiated, an object must define its parent class.
Delegation
Delegation is one of the original object programming techniques for a model that competes with inheritance. In delegation an object passes the responsibility of answering one function explicitly to other object (usually pointed by the original one).
Delegation is a very common technique in the Gang of Four book.
Aspects
Aspects are a new technique, epitomized by AspectJ. See also subject-oriented programming and separation of concerns.
Heterogeneous groups
Heterogeneous groups are groups where the members have different types. For example; silverware includes forks, spoons, and knives; and animals include mammals, birds, fish, reptiles, and amphibians.
What is the signature for all animals? Mammals and fish share almost no attributes. Mammals breathe air while fish breathe water. Mammals walk on land while fish swim in water. They eat different foods. Yet, it is useful to combine animals together, when feeding pets (cat, parakeet, and goldfish), or going to the zoo.
In object languages, the signature for animals would be empty. However, this leads to problems because shoes and potatoes also match the signature for animals.
Heterogeneous groups are challenging to object languages, because they link group signatures with group membership. Programmers long ago learned that removing attributes from a signature is hard to understand.
Multimethods
Multimethods implement multiple downcasting directly. The alternative is to implement the visitor pattern.
Criticism
One criticism against the techniques is that patterns already implement these concepts in Java and C++. However it takes extra work to implement patterns.
Another criticism is that the term post-object does not identify a specific technology or practice, in the way that structured programming and object-oriented programming do. This means that post-object is more ambiguous. This may simply be necessary, given that object-oriented was such a large step in impact and identity.
The name post-object was chosen to parallel post-Impressionism. Most art patrons who say they like Impressionist art really prefer post-impressionist art. Impressionism was an important but subtle change in art. Post-impressionism, for example Van Gogh, went whole hog. (In other words, Van Gogh's art made complete the change that impressionism started.)
Objects use implementation inheritance in various ways, but leave many issues unresolved. Post-object resolves some issues, specifically heterogeneous groups, mixins, and delegation.
Origins
One early language was Cecil. Another important language is AspectJ.