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417:. The main idea is to use commonality-variability analysis to divide the type into the fixed implementation and interface, the policy-based class, and the different policies. The trick is to know what goes into the main class, and what policies should one create. The article mentioned above gives the following answer: wherever we would need to make a possible limiting design decision, we should postpone that decision, we should delegate it to an appropriately named policy.
402:) base classes and implementations of interfaces by derived classes, in policy-based design the derived (host) class represents the interfaces and the base (policy) classes implement them. In the case of policies, the public inheritance does not represent an is-a relationship between the host and the policy classes. While this would traditionally be considered evidence of a design defect in OOP contexts, this doesn't apply in the context of the policy idiom.
390:. (Alternatives are for the host class to merely contain a member variable of each policy class type, or else to inherit the policy classes privately; however, inheriting the policy classes publicly has the major advantage that a policy class can add new methods, inherited by the instantiated host class and accessible to its users, which the host class itself need not even know about.) A notable feature of this aspect of the policy idiom is that, relative to
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situations requiring behaviors unforeseen by the library implementor. Even in cases where no more than one implementation of each policy will ever be used, decomposing a class into policies can aid the design process, by increasing modularity and highlighting exactly where orthogonal design decisions have been made.
1055:. Ongoing contributions by many people have improved the overall robustness and functionality of the library. Loki is not tied to the book anymore as it already has a lot of new components (e.g. StrongPtr, Printf, and Scopeguard). Loki inspired similar tools and functionality now also present in the
424:
It may be a delicate task to create a good set of policies, just the right number (e.g., the minimum necessary). The different customization points, which belong together, should go into one policy argument, such as storage policy, validation policy and so forth. Graphic designers are able to give a
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number of different behavior combinations, resolved at compile time, and selected by mixing and matching the different supplied policy classes in the instantiation of the host class template. Additionally, by writing a custom implementation of a given policy, a policy-based library can be used in
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While assembling software components out of interchangeable modules is a far-fetched concept, policy-based design represents an innovation in a way it applies that concept at the (relatively low) level of defining the behavior of an individual class. Policy classes are similar to
182:. While Alexandrescu didn't invent the technique, he has popularized it among programmers. His book contains solutions to practical problems which C++ programmers may face. Several phrases from the book are now used within the C++ community as generic terms:
420:
Policy classes can contain implementation, type definitions and so forth. Basically, the designer of the main template class will define what the policy classes should provide, what customization points they need to implement.
459:
is a host class where it takes two policies, one for specifying how a message should be shown and the other for the actual message being printed. Note that the generic implementation is in
327:(or mostly orthogonal) aspect of the behavior of the instantiated host class. By supplying a host class combined with a set of different, canned implementations for each policy, a
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A disadvantage of policies in their current incarnation is that the policy interface doesn't have a direct, explicit representation in
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metafunctions, however, the output is usually represented by the instantiated host class itself, rather than a nested output type.)
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Previous examples of this design approach, based on parameterized generic code, include parametric modules (functors) of the
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name to their policies, which represent concepts, and not those which represent operations or minor implementation details.
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can be reinterpreted for compile time, so that a main class has a
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Modern C++ Design: Generic
Programming and Design Patterns Applied
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Presented below is a simple (contrived) example of a C++
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878:// Does the same, but uses another language policy.
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386:of) each of its policy classes using (public)
284:The central idiom in policy-based design is a
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1146:Concepts: The Future of Generic Programming
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384:child class
337:exponential
136:005.13/3 21
1208:Categories
1114:artima.com
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890:HelloWorld
830:HelloWorld
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515:HelloWorld
457:HelloWorld
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325:orthogonal
315:(called a
297:parameters
279:allocators
184:modern C++
1136:cam.ac.uk
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482:#include
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441:concepts
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354:methods
329:library
319:), and
248:in the
65:Subject
60:English
791:return
776:string
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539:public
380:derive
376:policy
333:module
317:policy
264:robust
98:323 pp
47:Author
1090:Mixin
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1051:as a
1034:Clang
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881:using
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785:const
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620:class
611:Print
602:using
587:using
566:Print
557:const
512:class
465:Print
358:state
323:some
286:class
222:idiom
95:Pages
1190:Loki
1121:2023
1047:and
1036:and
1020:and
1009:and
961:Loki
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893:<
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692:endl
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548:void
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294:type
290:host
195:Loki
117:OCLC
104:ISBN
90:2001
1192:on
1038:GCC
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952:s.
929:();
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869:();
866:Run
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770:std
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331:or
212:or
176:C++
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