Opaque Pointer

In computer programming, an opaque pointer is a special case of an opaque data type, a datatype declared to be a pointer to a record or data structure of some unspecified type.

Opaque pointers are present in several programming languages including Ada, C, C++ and Modula-2.

If the language is strongly typed, programs and procedures that have no other information about an opaque pointer type T can still declare variables, arrays, and record fields of type T, assign values of that type, and compare those values for equality. However, they will not be able to de-reference such a pointer, and can only change the object's content by calling some procedure that has the missing information.

Opaque pointers are a way to hide the implementation details of an interface from ordinary clients, so that the implementation may be changed without the need to recompile the modules using it. This benefits the programmer as well since a simple interface can be created, and most details can be hidden in another file. This is important for providing binary code compatibility through different versions of a shared library, for example.

This technique is described in Design Patterns as the Bridge pattern. It is sometimes referred to as "handle classes", the "Pimpl idiom" (for "pointer to implementation idiom"), "Compiler firewall idiom" or "Cheshire Cat", especially among the C++ community.

Famous quotes containing the words opaque and/or pointer:

    ... people were so ridiculous with their illusions, carrying their fools’ caps unawares, thinking their own lies opaque while everybody else’s were transparent, making themselves exceptions to everything, as if when all the world looked yellow under a lamp they alone were rosy.
    George Eliot [Mary Ann (or Marian)

    The hardiest skeptic who has seen a horse broken, a pointer trained, or has visited a menagerie or the exhibition of the Industrious Fleas, will not deny the validity of education. “A boy,” says Plato, “is the most vicious of all beasts;” and in the same spirit the old English poet Gascoigne says, “A boy is better unborn than untaught.”
    Ralph Waldo Emerson (1803–1882)