Double-chance function
Encyclopedia
In software engineering
Software engineering
Software Engineering is the application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software, and the study of these approaches; that is, the application of engineering to software...

, a double-chance function is a software design pattern with a strong application in cross-platform
Cross-platform
In computing, cross-platform, or multi-platform, is an attribute conferred to computer software or computing methods and concepts that are implemented and inter-operate on multiple computer platforms...

 and scalable development. It is easiest to explain it with an example.

Consider a graphics API
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...

 with functions to DrawPoint, DrawLine, and DrawSquare. It is easy to see that DrawLine can be implemented solely in terms of DrawPoint, and DrawSquare can in turn be implemented through four calls to DrawLine. If you were porting this API to a new architecture you would have a choice: implement three different functions natively (taking more time, but ultimately providing a better solution), or write DrawPoint natively, and implement the others as described above using common, cross-platform, code.

The double-chance function is an optimal method of creating such an implementation, whereby the first draft of the port can use the "fast to market, slow to run" version with a common DrawPoint function, while later versions can be modified as "slow to market, fast to run". Where the double-chance pattern scores high is that the base API includes the self-supporting implementation given here as part of the null driver, and all other implementations are extensions of this. Consequently the first port is, in fact, the first usable implementation.

One typical implementation in C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...

 could be:

class CBaseGfxAPI {
virtual void DrawPoint(int x, int y) = 0; /* Abstract concept for the null driver */
virtual void DrawLine(int x1, int y1, int x2, int y2) { /* DrawPoint repeated */}
virtual void DrawSquare(int x1, int y1, int x2, int y2) { /* DrawLine repeated */}
};

class COriginalGfxAPI : public CBaseGfxAPI {
virtual void DrawPoint(int x, int y) { /* The only necessary native calls */ }
virtual void DrawLine(int x1, int y1, int x2, int y2) { /* If this function exists a native DrawLine
routine will be used. Otherwise the base
implementation is run. */}
};

class CNewGfxAPI : public CBaseGfxAPI {
virtual void DrawPoint(int x, int y) { /* The only necessary for native calls */ }
};

Note that the CBaseGfxAPI::DrawPoint function is never used, per se, as any graphics call goes through one of its derived classes. So a call to CNewGfxAPI::DrawSquare would have its first chance to render a square by the CNewGfxAPI class. If no native implementation exists, then the base class is called, at which point the virtualization
Virtual function
In object-oriented programming, a virtual function or virtual method is a function or method whose behaviour can be overridden within an inheriting class by a function with the same signature...

 takes over and means that CNewGfxAPI::DrawLine is called. This gives the CNewGfxAPI class a “second chance” to use native code, if any is available.

With this method it is, theoretically, possible to build an entire 3D engine (applying software rasterizing) using only one native function in the form of DrawPoint, with other functions being implemented as and when time permits. In practise this would be hopelessly slow, but it does demonstrate the possibilities for double-chance functions.
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