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class="mw-redirect" title="Policy-based design">Policy-based design</a>)</span></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Book by Andrei Alexandrescu</div> <p class="mw-empty-elt"> </p> <style data-mw-deduplicate="TemplateStyles:r1257001546">.mw-parser-output .infobox-subbox{padding:0;border:none;margin:-3px;width:auto;min-width:100%;font-size:100%;clear:none;float:none;background-color:transparent}.mw-parser-output .infobox-3cols-child{margin:auto}.mw-parser-output .infobox .navbar{font-size:100%}@media screen{html.skin-theme-clientpref-night .mw-parser-output .infobox-full-data:not(.notheme)>div:not(.notheme)[style]{background:#1f1f23!important;color:#f8f9fa}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .infobox-full-data:not(.notheme) div:not(.notheme){background:#1f1f23!important;color:#f8f9fa}}@media(min-width:640px){body.skin--responsive .mw-parser-output .infobox-table{display:table!important}body.skin--responsive .mw-parser-output .infobox-table>caption{display:table-caption!important}body.skin--responsive .mw-parser-output .infobox-table>tbody{display:table-row-group}body.skin--responsive .mw-parser-output .infobox-table tr{display:table-row!important}body.skin--responsive .mw-parser-output .infobox-table th,body.skin--responsive .mw-parser-output .infobox-table td{padding-left:inherit;padding-right:inherit}}</style><table class="infobox vcard"><caption class="infobox-title" style="font-size:125%; font-style:italic; padding-bottom:0.2em;">Modern C++ Design <span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Modern+C%2B%2B+Design&rft.author=%5B%5BAndrei+Alexandrescu%5D%5D&rft.date=2001&rft.pub=%5B%5BAddison-Wesley%5D%5D&rft.pages=323+pp&rft_id=info:oclcnum/45129236"></span></caption><tbody><tr><td colspan="2" class="infobox-image"><span class="mw-default-size" typeof="mw:File/Frameless"><a href="/wiki/File:Modern_C%2B%2B_Design.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/en/thumb/c/cd/Modern_C%2B%2B_Design.jpg/220px-Modern_C%2B%2B_Design.jpg" decoding="async" width="220" height="273" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/c/cd/Modern_C%2B%2B_Design.jpg 1.5x" data-file-width="283" data-file-height="351" /></a></span></td></tr><tr><th scope="row" class="infobox-label">Author</th><td class="infobox-data"><a href="/wiki/Andrei_Alexandrescu" title="Andrei Alexandrescu">Andrei Alexandrescu</a></td></tr><tr><th scope="row" class="infobox-label">Language</th><td class="infobox-data">English</td></tr><tr><th scope="row" class="infobox-label">Subject</th><td class="infobox-data"><a href="/wiki/C%2B%2B" title="C++">C++</a></td></tr><tr><th scope="row" class="infobox-label">Publisher</th><td class="infobox-data"><a href="/wiki/Addison-Wesley" title="Addison-Wesley">Addison-Wesley</a></td></tr><tr><th scope="row" class="infobox-label"><div style="display: inline-block; line-height: 1.2em; padding: .1em 0;">Publication date</div></th><td class="infobox-data">2001</td></tr><tr><th scope="row" class="infobox-label">Pages</th><td class="infobox-data">323 pp</td></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a></th><td class="infobox-data"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation 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a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><a href="/wiki/Special:BookSources/978-0-201-70431-0" title="Special:BookSources/978-0-201-70431-0">978-0-201-70431-0</a></td></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/OCLC_(identifier)" class="mw-redirect" title="OCLC (identifier)"><abbr title="Online Computer Library Center number">OCLC</abbr></a></th><td class="infobox-data"><a rel="nofollow" class="external text" href="https://www.worldcat.org/oclc/45129236">45129236</a></td></tr><tr><th scope="row" class="infobox-label"><div style="display: inline-block; line-height: 1.2em; padding: .1em 0;"><a href="/wiki/Dewey_Decimal_Classification" title="Dewey Decimal Classification">Dewey Decimal</a></div></th><td class="infobox-data">005.13/3 21</td></tr><tr><th scope="row" class="infobox-label"><a href="/wiki/LCC_(identifier)" class="mw-redirect" title="LCC (identifier)"><abbr title="Library of Congress Classification">LC Class</abbr></a></th><td class="infobox-data">QA76.73.C153 A42 2001</td></tr></tbody></table> <p><i><b>Modern C++ Design: Generic Programming and Design Patterns Applied</b></i> is a book written by <a href="/wiki/Andrei_Alexandrescu" title="Andrei Alexandrescu">Andrei Alexandrescu</a>, published in 2001 by <a href="/wiki/Addison-Wesley" title="Addison-Wesley">Addison-Wesley</a>. It has been regarded as "one of the most important C++ books" by <a href="/wiki/Scott_Meyers" title="Scott Meyers">Scott Meyers</a>.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p><p>The book makes use of and explores a <a href="/wiki/C%2B%2B" title="C++">C++</a> programming technique called <a href="/wiki/Template_metaprogramming" title="Template metaprogramming">template metaprogramming</a>. 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: <i>modern C++</i> (as opposed to C/C++ style), policy-based design and <a href="/w/index.php?title=Typelist&action=edit&redlink=1" class="new" title="Typelist (page does not exist)">typelist</a>.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (September 2023)">citation needed</span></a></i>]</sup> </p><p>All of the code described in the book is freely available in his library <a href="/wiki/Loki_(C%2B%2B)" class="mw-redirect" title="Loki (C++)">Loki</a>. The book has been republished and translated into several languages since 2001.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (September 2023)">citation needed</span></a></i>]</sup> </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Policy-based_design">Policy-based design</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Modern_C%2B%2B_Design&action=edit&section=1" title="Edit section: Policy-based design"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><b>Policy-based design</b>, also known as <b>policy-based class design</b> or <b>policy-based programming</b>, is the term used in <i>Modern C++ Design</i> for a design approach based on an <a href="/wiki/Programming_idiom" title="Programming idiom">idiom</a> for C++ known as <b>policies</b>. It has been described as a <a href="/wiki/Compile-time" class="mw-redirect" title="Compile-time">compile-time</a> variant of the <a href="/wiki/Strategy_pattern" title="Strategy pattern">strategy pattern</a>, and has connections with C++ <a href="/wiki/Template_metaprogramming" title="Template metaprogramming">template metaprogramming</a>. It was first popularized in C++ by Andrei Alexandrescu with <i>Modern C++ Design</i> and with his column <i>Generic<Programming></i> in the <i><a href="/wiki/C/C%2B%2B_Users_Journal" title="C/C++ Users Journal">C/C++ Users Journal</a></i>, and it is currently closely associated with C++ and <a href="/wiki/D_(programming_language)" title="D (programming language)">D</a> as it requires a <a href="/wiki/Compiler" title="Compiler">compiler</a> with highly <a href="/wiki/Robustness_(computer_science)" title="Robustness (computer science)">robust</a> support for <a href="/wiki/Template_(programming)" class="mw-redirect" title="Template (programming)">templates</a>, which was not common before about 2003. </p><p>Previous examples of this design approach, based on parameterized generic code, include parametric modules (functors) of the <a href="/wiki/ML_(programming_language)" title="ML (programming language)">ML</a> languages,<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> and C++ <a href="/wiki/Allocator_(C%2B%2B)" title="Allocator (C++)">allocators</a> for memory management policy. </p><p>The central idiom in policy-based design is a <a href="/wiki/Class_(computer_science)" class="mw-redirect" title="Class (computer science)">class</a> template (called the <i>host</i> class), taking several <a href="/wiki/Data_type" title="Data type">type</a> <a href="/wiki/Parameter_(computer_science)" class="mw-redirect" title="Parameter (computer science)">parameters</a> as input, which are <a href="/wiki/Instantiation_(computer_science)" class="mw-redirect" title="Instantiation (computer science)">instantiated</a> with types selected by the user (called <i>policy classes</i>), each <a href="/wiki/Implementation" title="Implementation">implementing</a> a particular implicit <a href="/wiki/Interface_(computer_science)" class="mw-redirect" title="Interface (computer science)">interface</a> (called a <i>policy</i>), and <a href="/wiki/Separation_of_concerns" title="Separation of concerns">encapsulating</a> some <a href="/wiki/Orthogonality#Computer_science" title="Orthogonality">orthogonal</a> (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 <a href="/wiki/Library_(computing)" title="Library (computing)">library</a> or <a href="/wiki/Module_(programming)" class="mw-redirect" title="Module (programming)">module</a> can support an <a href="/wiki/Exponential_growth" title="Exponential growth">exponential</a> 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 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. </p><p>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 <a href="/wiki/Callback_(computer_science)" class="mw-redirect" title="Callback (computer science)">callbacks</a>, but differ in that, rather than consisting of a single <a href="/wiki/Subroutine" class="mw-redirect" title="Subroutine">function</a>, a policy class will typically contain several related functions (<a href="/wiki/Method_(computer_science)" class="mw-redirect" title="Method (computer science)">methods</a>), often combined with <a href="/wiki/State_(computer_science)" title="State (computer science)">state</a> <a href="/wiki/Variable_(programming)" class="mw-redirect" title="Variable (programming)">variables</a> or other facilities such as nested types. A policy-based host class can be thought of as a type of <a href="/wiki/Template_metaprogramming" title="Template metaprogramming">metafunction</a>, taking a set of behaviors represented by types as input, and returning as output a type representing the result of combining those behaviors into a functioning whole. (Unlike <a href="/wiki/Boost_C%2B%2B_Libraries" class="mw-redirect" title="Boost C++ Libraries">MPL</a> metafunctions, however, the output is usually represented by the instantiated host class itself, rather than a nested output type.) </p><p>A key feature of the <i>policy</i> idiom is that, usually (though it is not strictly necessary), the host class will <a href="/wiki/Inheritance_(object-oriented_programming)" title="Inheritance (object-oriented programming)">derive</a> from (make itself a <a href="/wiki/Subclass_(computer_science)" class="mw-redirect" title="Subclass (computer science)">child class</a> of) each of its policy classes using (public) <a href="/wiki/Multiple_inheritance" title="Multiple inheritance">multiple inheritance</a>. (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 <a href="/wiki/Object-oriented_programming" title="Object-oriented programming">object-oriented programming</a>, policies invert the relationship between <a href="/wiki/Superclass_(computer_science)" class="mw-redirect" title="Superclass (computer science)">base class</a> and derived class - whereas in OOP interfaces are traditionally represented by (<a href="/wiki/Virtual_function#Abstract_classes_and_pure_virtual_functions" title="Virtual function">abstract</a>) 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. </p><p>A disadvantage of policies in their current incarnation is that the policy interface doesn't have a direct, explicit representation in <a href="/wiki/Source_code" title="Source code">code</a>, but rather is defined implicitly, via <a href="/wiki/Duck_typing" title="Duck typing">duck typing</a>, and must be documented separately and manually, in <a href="/wiki/Comment_(computer_programming)" title="Comment (computer programming)">comments</a>. 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. </p><p>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. </p><p>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 name to their policies, which represent concepts, and not those which represent operations or minor implementation details. </p><p>Policy-based design may incorporate other useful techniques. For example, the <a href="/wiki/Template_method_pattern" title="Template method pattern">template method pattern</a> can be reinterpreted for compile time, so that a main class has a <a href="/wiki/Skeleton_algorithm" class="mw-redirect" title="Skeleton algorithm">skeleton algorithm</a>, which – at customization points – calls the appropriate functions of some of the policies. </p><p>This will be achieved dynamically by <a href="/wiki/Concepts_(C%2B%2B)" title="Concepts (C++)">concepts</a><sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> in future versions of C++. </p> <div class="mw-heading mw-heading3"><h3 id="Simple_example">Simple example</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Modern_C%2B%2B_Design&action=edit&section=2" title="Edit section: Simple example"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Presented below is a simple (contrived) example of a C++ <a href="/wiki/Hello_world_program" class="mw-redirect" title="Hello world program">hello world program</a>, where the text to be printed and the method of printing it are decomposed using policies. In this example, <i>HelloWorld</i> 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 <code>Run</code> and therefore the code is unable to be compiled unless both policies (<code>Print</code> and <code>Message</code>) are provided. </p> <div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre><span></span><span class="cp">#include</span><span class="w"> </span><span class="cpf"><iostream></span> <span class="cp">#include</span><span class="w"> </span><span class="cpf"><string></span> <span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">OutputPolicy</span><span class="p">,</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">LanguagePolicy</span><span class="o">></span> <span class="k">class</span><span class="w"> </span><span class="nc">HelloWorld</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="k">private</span><span class="w"> </span><span class="n">OutputPolicy</span><span class="p">,</span><span class="w"> </span><span class="k">private</span><span class="w"> </span><span class="n">LanguagePolicy</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">public</span><span class="o">:</span> <span class="w"> </span><span class="c1">// Behavior method.</span> <span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">Run</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="c1">// Two policy methods.</span> <span class="w"> </span><span class="n">Print</span><span class="p">(</span><span class="n">Message</span><span class="p">());</span> <span class="w"> </span><span class="p">}</span> <span class="w"> </span><span class="k">private</span><span class="o">:</span> <span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">LanguagePolicy</span><span class="o">::</span><span class="n">Message</span><span class="p">;</span> <span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">OutputPolicy</span><span class="o">::</span><span class="n">Print</span><span class="p">;</span> <span class="p">};</span> <span class="k">class</span><span class="w"> </span><span class="nc">OutputPolicyWriteToCout</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">protected</span><span class="o">:</span> <span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">Print</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&&</span><span class="w"> </span><span class="n">message</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">message</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span> <span class="w"> </span><span class="p">}</span> <span class="p">};</span> <span class="k">class</span><span class="w"> </span><span class="nc">LanguagePolicyEnglish</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">protected</span><span class="o">:</span> <span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="w"> </span><span class="n">Message</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="s">"Hello, World!"</span><span class="p">;</span><span class="w"> </span><span class="p">}</span> <span class="p">};</span> <span class="k">class</span><span class="w"> </span><span class="nc">LanguagePolicyGerman</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="k">protected</span><span class="o">:</span> <span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="w"> </span><span class="n">Message</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="s">"Hallo Welt!"</span><span class="p">;</span><span class="w"> </span><span class="p">}</span> <span class="p">};</span> <span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span> <span class="w"> </span><span class="c1">// Example 1</span> <span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">HelloWorldEnglish</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">HelloWorld</span><span class="o"><</span><span class="n">OutputPolicyWriteToCout</span><span class="p">,</span><span class="w"> </span><span class="n">LanguagePolicyEnglish</span><span class="o">></span><span class="p">;</span> <span class="w"> </span><span class="n">HelloWorldEnglish</span><span class="w"> </span><span class="n">hello_world</span><span class="p">;</span> <span class="w"> </span><span class="n">hello_world</span><span class="p">.</span><span class="n">Run</span><span class="p">();</span><span class="w"> </span><span class="c1">// Prints "Hello, World!".</span> <span class="w"> </span><span class="c1">// Example 2</span> <span class="w"> </span><span class="c1">// Does the same, but uses another language policy.</span> <span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">HelloWorldGerman</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">HelloWorld</span><span class="o"><</span><span class="n">OutputPolicyWriteToCout</span><span class="p">,</span><span class="w"> </span><span class="n">LanguagePolicyGerman</span><span class="o">></span><span class="p">;</span> <span class="w"> </span><span class="n">HelloWorldGerman</span><span class="w"> </span><span class="n">hello_world2</span><span class="p">;</span> <span class="w"> </span><span class="n">hello_world2</span><span class="p">.</span><span class="n">Run</span><span class="p">();</span><span class="w"> </span><span class="c1">// Prints "Hallo Welt!".</span> <span class="p">}</span> </pre></div> <p>Designers can easily write more <code>OutputPolicy</code>s by adding new classes with the member function <code>Print</code> and take those as new <code>OutputPolicy</code>s. </p> <div class="mw-heading mw-heading2"><h2 id="Loki_library">Loki library</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Modern_C%2B%2B_Design&action=edit&section=3" title="Edit section: Loki library"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><b>Loki</b> is the name of a <a href="/wiki/C%2B%2B" title="C++">C++</a> <a href="/wiki/Library_(computer_science)" class="mw-redirect" title="Library (computer science)">software library</a> written by <a href="/wiki/Andrei_Alexandrescu" title="Andrei Alexandrescu">Andrei Alexandrescu</a> as part of his book <i>Modern C++ Design</i>. </p><p>The library makes extensive use of C++ <a href="/wiki/Template_metaprogramming" title="Template metaprogramming">template metaprogramming</a> and implements several commonly used tools: <a href="/w/index.php?title=Typelist&action=edit&redlink=1" class="new" title="Typelist (page does not exist)">typelist</a>, <a href="/wiki/Function_object" title="Function object">functor</a>, <a href="/wiki/Singleton_pattern" title="Singleton pattern">singleton</a>, <a href="/wiki/Smart_pointer" title="Smart pointer">smart pointer</a>, <a href="/wiki/Factory_object" class="mw-redirect" title="Factory object">object factory</a>, <a href="/wiki/Visitor_pattern" title="Visitor pattern">visitor</a> and <a href="/wiki/Multiple_dispatch" title="Multiple dispatch">multimethods</a>. </p><p>Originally the library was only compatible with two of the most standard conforming C++ compilers (<a href="/wiki/CodeWarrior" title="CodeWarrior">CodeWarrior</a> and <a href="/wiki/Comeau_C/C%2B%2B" title="Comeau C/C++">Comeau C/C++</a>): later efforts have made it usable with a wide array of compilers (including older <a href="/wiki/Visual_C%2B%2B" class="mw-redirect" title="Visual C++">Visual C++ 6.0</a>, <a href="/wiki/C%2B%2B_Builder" class="mw-redirect" title="C++ Builder">Borland C++ Builder 6.0</a>, <a href="/wiki/Clang" title="Clang">Clang</a> and <a href="/wiki/GNU_Compiler_Collection" title="GNU Compiler Collection">GCC</a>). Compiler vendors used Loki as a compatibility benchmark, further increasing the number of compliant compilers.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> </p><p>Maintenance and further development of Loki has been continued through an open-source community led by <a href="/w/index.php?title=Peter_K%C3%BCmmel&action=edit&redlink=1" class="new" title="Peter Kümmel (page does not exist)">Peter Kümmel</a> and <a href="/w/index.php?title=Richard_Sposato&action=edit&redlink=1" class="new" title="Richard Sposato (page does not exist)">Richard Sposato</a> as a SourceForge project. 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 <a href="/wiki/Boost_(C%2B%2B_libraries)" title="Boost (C++ libraries)">Boost</a> library collection.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (November 2010)">citation needed</span></a></i>]</sup> </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Modern_C%2B%2B_Design&action=edit&section=4" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1259569809">.mw-parser-output .portalbox{padding:0;margin:0.5em 0;display:table;box-sizing:border-box;max-width:175px;list-style:none}.mw-parser-output .portalborder{border:1px solid var(--border-color-base,#a2a9b1);padding:0.1em;background:var(--background-color-neutral-subtle,#f8f9fa)}.mw-parser-output .portalbox-entry{display:table-row;font-size:85%;line-height:110%;height:1.9em;font-style:italic;font-weight:bold}.mw-parser-output .portalbox-image{display:table-cell;padding:0.2em;vertical-align:middle;text-align:center}.mw-parser-output .portalbox-link{display:table-cell;padding:0.2em 0.2em 0.2em 0.3em;vertical-align:middle}@media(min-width:720px){.mw-parser-output .portalleft{clear:left;float:left;margin:0.5em 1em 0.5em 0}.mw-parser-output .portalright{clear:right;float:right;margin:0.5em 0 0.5em 1em}}</style><ul role="navigation" aria-label="Portals" class="noprint portalbox portalborder portalright"> <li class="portalbox-entry"><span class="portalbox-image"><span class="noviewer" typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/3/31/Free_and_open-source_software_logo_%282009%29.svg/28px-Free_and_open-source_software_logo_%282009%29.svg.png" decoding="async" width="28" height="28" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/3/31/Free_and_open-source_software_logo_%282009%29.svg/42px-Free_and_open-source_software_logo_%282009%29.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/3/31/Free_and_open-source_software_logo_%282009%29.svg/56px-Free_and_open-source_software_logo_%282009%29.svg.png 2x" data-file-width="512" data-file-height="512" /></span></span></span><span class="portalbox-link"><a href="/wiki/Portal:Free_and_open-source_software" title="Portal:Free and open-source software">Free and open-source software portal</a></span></li></ul> <ul><li><a href="/wiki/Boost_(C%2B%2B_libraries)" title="Boost (C++ libraries)">Boost (C++ libraries)</a></li> <li><a href="/wiki/Mixin" title="Mixin">Mixin</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Modern_C%2B%2B_Design&action=edit&section=5" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.artima.com/articles/the-most-important-c-booksemeverem">"The Most Important C++ Books...Ever"</a>. <i>artima.com</i><span class="reference-accessdate">. Retrieved <span class="nowrap">1 September</span> 2023</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=artima.com&rft.atitle=The+Most+Important+C%2B%2B+Books...Ever&rft_id=https%3A%2F%2Fwww.artima.com%2Farticles%2Fthe-most-important-c-booksemeverem&rfr_id=info%3Asid%2Fen.wikipedia.org%3AModern+C%2B%2B+Design" class="Z3988"></span></span> </li> <li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="https://www.cl.cam.ac.uk/~lp15/MLbook/PDF/chapter7.pdf">Chapter 7. Abstract Types and Functors</a> cam.ac.uk</span> </li> <li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="http://www.stroustrup.com/good_concepts.pdf">Concepts: The Future of Generic Programming</a> stroustrup.com</span> </li> <li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text">C++ and Beyond 2011: "Ask Us Anything" session, <a rel="nofollow" class="external free" href="http://channel9.msdn.com/Shows/Going+Deep/C-and-Beyond-2011-Scott-Andrei-and-Herb-Ask-Us-Anything">http://channel9.msdn.com/Shows/Going+Deep/C-and-Beyond-2011-Scott-Andrei-and-Herb-Ask-Us-Anything</a> at 51:40-51:51</span> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Modern_C%2B%2B_Design&action=edit&section=6" title="Edit section: External links"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a rel="nofollow" class="external text" href="http://erdani.org">Alexandrescu's website</a> (with book errata <a rel="nofollow" class="external autonumber" href="https://web.archive.org/web/20061010054554/http://erdani.org/errata/">[1]</a>)</li> <li><a rel="nofollow" class="external text" href="http://www.informit.com/articles/article.aspx?p=25264">Smart Pointers</a> (sample chapter from the book)</li> <li><a rel="nofollow" class="external text" href="https://sourceforge.net/projects/loki-lib/">Loki</a> on <a href="/wiki/SourceForge" title="SourceForge">SourceForge</a></li> <li><a rel="nofollow" class="external text" href="http://www.awprofessional.com/content/images/0201704315/sourcecode/loki.zip">Original source code from the book publisher</a></li></ul>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?type=1x1" alt="" width="1" height="1" style="border: none; position: absolute;"></noscript> <div class="printfooter" data-nosnippet="">Retrieved from "<a dir="ltr" href="https://en.wikipedia.org/w/index.php?title=Modern_C%2B%2B_Design&oldid=1258408111#Policy-based_design">https://en.wikipedia.org/w/index.php?title=Modern_C%2B%2B_Design&oldid=1258408111#Policy-based_design</a>"</div></div> <div id="catlinks" class="catlinks" data-mw="interface"><div id="mw-normal-catlinks" class="mw-normal-catlinks"><a href="/wiki/Help:Category" title="Help:Category">Categories</a>: <ul><li><a href="/wiki/Category:2001_non-fiction_books" title="Category:2001 non-fiction books">2001 non-fiction books</a></li><li><a href="/wiki/Category:Books_about_C%2B%2B" title="Category:Books about C++">Books about C++</a></li></ul></div><div id="mw-hidden-catlinks" class="mw-hidden-catlinks mw-hidden-cats-hidden">Hidden categories: <ul><li><a href="/wiki/Category:Articles_with_short_description" title="Category:Articles with short description">Articles with short description</a></li><li><a href="/wiki/Category:Short_description_matches_Wikidata" title="Category:Short description matches Wikidata">Short description matches Wikidata</a></li><li><a href="/wiki/Category:Use_dmy_dates_from_September_2023" title="Category:Use dmy dates from September 2023">Use dmy dates from September 2023</a></li><li><a href="/wiki/Category:All_articles_with_unsourced_statements" title="Category:All articles with unsourced statements">All articles with unsourced statements</a></li><li><a href="/wiki/Category:Articles_with_unsourced_statements_from_September_2023" title="Category:Articles with unsourced statements from September 2023">Articles with unsourced statements from September 2023</a></li><li><a href="/wiki/Category:Articles_with_unsourced_statements_from_November_2010" title="Category:Articles with unsourced statements from November 2010">Articles with unsourced statements from November 2010</a></li></ul></div></div> </div> </main> </div> <div class="mw-footer-container"> <footer id="footer" class="mw-footer" > <ul id="footer-info"> <li id="footer-info-lastmod"> This page was last edited on 19 November 2024, at 15:26<span class="anonymous-show"> (UTC)</span>.</li> <li id="footer-info-copyright">Text is available under the <a href="/wiki/Wikipedia:Text_of_the_Creative_Commons_Attribution-ShareAlike_4.0_International_License" title="Wikipedia:Text of the Creative Commons Attribution-ShareAlike 4.0 International License">Creative Commons Attribution-ShareAlike 4.0 License</a>; additional terms may apply. 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