Concept
In computer programming, an opaque pointer is a special case of an opaque data type, a data type 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++, D 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", "d-pointer" or "Cheshire Cat", especially among the C++ community. package Library_Interface is type Handle is limited private; Operations... private type Hidden_Implementation; -- Defined in the package body type Handle is access Hidden_Implementation; end Library_Interface; The type Handle is an opaque pointer to the real implementation, that is not defined in the specification. Note that the type is not only private (to forbid the clients from accessing the type directly, and only through the operations), but also limited (to avoid the copy of the data structure, and thus preventing dangling references). package body Library_Interface is type Hidden_Implementation is record -- The actual implementation can be anything end record; Definition of the operations.