Category:Smalltalk: Difference between revisions

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<lang smalltalk>a foo: b bar + c baz</lang> is equivalent to <lang smalltalk>a foo:( (b bar) + (c baz) )</lang>

As messages have no semantic meaning to the compiler (especially, the '+', '*' and other binary messages), the usual precedence rules for arithmetic expressions are not present in Smalltalk. Thus, complex expressions consisting of multiple binary messages usually need to be parenthesized.

As messages have no semantic meaning to the compiler (especially, the '+', '*' and other binary messages), the usual precedence rules for arithmetic expressions are not present in Smalltalk. Thus, complex expressions consisting of multiple binary messages usually need to be parenthesized (or are parenthesized for readability).

Message lookup is done by traversing the superclass chain, looking for a class providing an implementation (method) for the messages selector. The standard defines single inheritance, with a lookup based on the receiver's class only. However, some Smalltalk implementations allow for that lookup to be redefined and provide more sophisticated mechanisms (selector namespaces, lookup objects, lookup based on argument types etc.).

If no implementation is found (i.e. no class along the superclass chain provides a corresponding method), the original selector and arguments are packed into a container and a doesNotUnderstand: message is sent instead. This is used for error handling, but can also be used for message forwarding (proxies), delegation or dynamic creation of new code. The default implementation of doesNotUnderstand: raises an exception, which can be ~~cought~~ and handled by the program. Unhandled exceptions typically open a debugger (unless an UnhandledException-exception handler was defined).

If no implementation is found (i.e. no class along the superclass chain provides a corresponding method), the original selector and arguments are packed into a container and a doesNotUnderstand: message is sent instead. This is used for error handling, but can also be used for message forwarding (proxies), delegation or dynamic creation of new code. The default implementation of doesNotUnderstand: raises an exception, which can be caught and handled by the program. Unhandled exceptions typically open a debugger (unless an UnhandledException-exception handler was defined).

=== Metaclass Hierarchy ===