Object-Orientation FAQ

2.3) What Is Dynamic Binding?

Dynamic binding has two forms, static and dynamic.  Statically-typed dynamic
binding is found in languages such as C++ (virtual functions) and Eiffel
(redefinition).  It is not known which function will be called for a virtual
function at run-time because a derived class may override the function, in
which case the overriding function must be called.  Statically determining all
possibilities of usage is undecidable.  When the complete program is compiled,
all such functions are resolved (statically) for actual objects. Formal object
usage must have a consistent way of accessing these functions, as achieved thru
vtables of function pointers in the actual objects (C++) or equivalent,
providing statically-typed dynamic binding (this is really just defining simple
function pointers with static typechecking in the base class, and filling them
in in the derived class, along with offsets to reset the receiver).
The run-time selection of methods is another case of dynamic binding, meaning
lookup is performed (bound) at run-time (dynamically).  This is often desired
and even required in many applications including databases, distributed
programming and user interaction (e.g. GUIs).  Examples can be found in
[Garfinkel 93, p80] and [Cox 91, pp 64-67].  To extend Garfinkels example with
multiple-polymorphism, a cut operation in an Edit submenu may pass the cut
operation (along with parameters) to any object on the desktop, each of which
handles the message in its own way (OO).  If an (application) object can cut
many kinds of objects such as text and graphical objects, multiple-polymorphism
comes into play, as many overloaded cut methods, one per type of object to be
cut, are available in the receiving object, the particular method being
selected based on the actual type of object being cut (which in the GUI case is
not available until run-time).
Again, various optimizations exist for dynamic lookup to increase efficiency
(such as found in [Agrawal 91] and [Chambers 92]).
Dynamic binding allows new objects and code to be interfaced with or added to
a system without affecting existing code and eliminates switch statements.
This removes the spread of knowledge of specific classes throughout a system,
as each object knows what operation to support.  It also allows a reduction in
program complexity by replacing a nested construct (switch statement) with a
simple call.  It also allows small packages of behavior, improving coherence
and loose coupling.  Another benefit is that code complexity increases not
linearly but exponentially with lines of code, so that packaging code into
methods reduces program complexity considerably, even further that removing
the nested switch statement!  [Martin 92] covers some of these issues.

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