In computer programming, an opaque pointer is a special case of an opaque data type, a data typedeclared 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 typeT 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", "Compiler firewall idiom", "d-pointer" or "Cheshire Cat", especially among the C++ community.
Examples
Ada
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, but also limited. package body Library_Interface is type Hidden_Implementation is record ... -- The actual implementation can be anything end record; -- Definition of the operations... end Library_Interface;
These types are sometimes called "Taft types"—named after Tucker Taft, the main designer of Ada 95—because they were introduced in the so-called Taft Amendment to Ada 83.
C
/* obj.h */ struct obj; /* * The compiler considers struct obj an incomplete type. Incomplete types * can be used in declarations. */ size_t obj_size; void obj_setid; int obj_getid;
/* obj.c */
include "obj.h"
struct obj ; /* * The caller will handle allocation. * Provide the required information only */ size_t obj_size void obj_setid int obj_getid
This example demonstrates a way to achieve the information hiding aspect of object-oriented programming using the C language. If someone wanted to change the definition of struct obj, it would be unnecessary to recompile any other modules in the program that use the obj.hheader file unless the API was also changed. Note that it may be desirable for the functions to check that the passed pointer is not NULL, but such checks have been omitted above for brevity.
The d-pointer pattern is one of the implementations of the. It is commonly used in C++ classes due to its advantages. A d-pointer is a private data member of the class that points to an instance of a structure. This method allows class declarations to omit private data members, except for the d-pointer itself. As a result,
more of the class implementation is hidden
adding new data members to the private structure does not affect binary compatibility
the header file containing the class declaration only needs to include those files needed for the class interface, rather than for its implementation.
One side benefit is that compilations are faster because the header file changes less often. Note, possible disadvantage of d-pointer pattern is indirect member access through pointer, which is sometimes slower than access to plain, not-a-pointer member. The d-pointer is heavily used in the Qt and KDE libraries.
