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This technique is sometimes referred as "handle classes", the "'''Pimpl idiom'''" (for "private implementation idiom") or "''' Cheshire Cat '''", especially among the C++ community . EXAMPLES Opaque pointers are present in several Programming Language s, for example Ada , C / C++ or Modula-2 . 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 (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; These types are sometimes called "Taft types" —named after Tucker Taft , the main designer of Ada 95— because were introduced in the so-called Taft Amendment to Ada 83. C++ class Handle { private: struct CheshireCat; // ''Not defined here''
public: Handle(); // ''Constructor'' ~Handle(); // ''Destructor'' // ''Other operations...'' }; #include "handle.h" struct Handle::CheshireCat { ... // ''The actual implementation can be anything'' }; Handle::Handle() { smile = new CheshireCat; } Handle::~Handle() { delete smile; } 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, that adding new data members to the private struct does not affect binary compatibility, and that the header file containing the class declaration only has to #include those other files that are needed for the class interface, rather than for its implementation. As a side benefit, compiles are faster because the header file changes less often. The d-pointer is heavily used in the Qt and KDE libraries. C
cat_handle cat_Create(int smile);
void cat_Destroy(cat_handle cat);
int cat_Smile(cat_handle cat, int newsmile);
struct cat_t { int smile; };
cat_handle cat_Create(int smile) { cat_handle result = (cat_handle)malloc(sizeof('''struct''' cat_t)); if (result) { result->smile = smile; } return result; }
void cat_Destroy(cat_handle cat) { free(cat); }
int cat_Smile(cat_handle cat, int newsmile) { int result = -1; if (cat) { result = cat->smile; cat->smile = newsmile; } return result; } This example demonstrates a way to achieve the Information Hiding ( Encapsulation ) aspect of Object-Oriented Programming using the C language. If someone would want to change the declaration of struct cat_t, it would be unnecessary to recompile any other modules in the program that use the cat.h header file unless the API was also changed. SEE ALSO REFERENCES EXTERNAL LINKS
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