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< pre class = "programlisting" >< span class = "keyword" > namespace</ span > < span class = "identifier" > boost</ span > < span class = "special" > {</ span >
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< span class = "keyword" > class</ span > < span class = "identifier" > none_t</ span > < span class = "special" > {/*</ span > < span class = "identifier" > see</ span > < span class = "identifier" > below</ span > < span class = "special" > */};</ span >
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< span class = "keyword" > inline</ span > < span class = "keyword" > constexpr</ span > < span class = "identifier" > none_t</ span > < span class = "identifier" > none</ span > < span class = "special" > (/*</ span > < span class = "identifier" > see</ span > < span class = "identifier" > below</ span > < span class = "special" > */);</ span >
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< span class = "special" > }</ span > < span class = "comment" > // namespace boost</ span >
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Class < code class = "computeroutput" >< span class = "identifier" > none_t</ span ></ code > is meant to
serve as a tag for selecting appropriate overloads of from < code class = "computeroutput" >< span class = "identifier" > optional</ span ></ code > 's interface. It is an empty,
trivially copyable class with disabled default constructor.
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Constant < code class = "computeroutput" >< span class = "identifier" > none</ span ></ code > is used to
indicate an optional object that does not contain a value in initialization,
assignment and relational operations of < code class = "computeroutput" >< span class = "identifier" > optional</ span ></ code > .
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< td align = "right" >< div class = "copyright-footer" > Copyright © 2003-2007 Fernando Luis Cacciola Carballal< br > Copyright © 2014-2021 Andrzej Krzemieński< p >
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Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at < a href = "http://www.boost.org/LICENSE_1_0.txt" target = "_top" > http://www.boost.org/LICENSE_1_0.txt</ a > )
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