Opaque pointer

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In computer programming, an opaque pointer is a datatype that hides its internal implementation using a pointer. This allows the implementation of the whole interface to change without the need to recompile the modules using it. This is important for providing binary compatibility through different versions of a shared library, for example.

Examples

Opaque pointers are present in programming languages like Ada, C/C++ or Modula-2. For example, the following Ada code sample declares an opaque pointer:

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...


end Library_Interface;

This technique is sometimes referred as "handle classes" or "Cheshire Cat", especially among the C++ community .

class Handle ;

#include "handle.hpp"


struct Handle::CheshireCat ;


Handle::Handle() 


Handle::~Handle() 

One type of opaque pointer commonly used in C++ class declarations is the d-pointer. The d-pointer is the only private data member of the class and points to an instance of a struct defined in the class's implementation file. Developed by Arnt Gulbrandsen of Trolltech, this method allows class declarations to omit private data members, except for the d-pointer itself. The result is that more of the class's implementation is hidden from view and adding new data members to the private struct does not affect binary compatibility. The d-pointer is heavily used in the Qt and KDE libraries.

See also

• Application binary interface
• Handle (computer science)

References

External links

• [Programming Tools - Opaque Pointers]

 

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