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< div class = "chapter" >
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< div class = "titlepage" >< div >
< div >< h2 class = "title" >
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< a name = "optional" ></ a > Boost.Optional</ h2 ></ div >
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< div >< div class = "author" >< h3 class = "author" >
< span class = "firstname" > Fernando Luis</ span > < span class = "surname" > Cacciola Carballal</ span >
</ h3 ></ div ></ div >
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< div >< p class = "copyright" > Copyright © 2003-2007 Fernando Luis Cacciola Carballal</ p ></ div >
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< div >< p class = "copyright" > Copyright © 2014-2021 Andrzej Krzemieński</ p ></ div >
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< div >< div class = "legalnotice" >
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< a name = "optional.legal" ></ a >< 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 > )
</ p >
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< div class = "toc" >
< p >< b > Table of Contents</ b ></ p >
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< dl class = "toc" >
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< dt >< span class = "section" >< a href = "index.html#optional.introduction" > Introduction</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "boost_optional/quick_start.html" > Quick Start</ a ></ span ></ dt >
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< dd >< dl >
< dt >< span class = "section" >< a href = "boost_optional/quick_start.html#boost_optional.quick_start.optional_return_values" > Optional
return values</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "boost_optional/quick_start/optional_automatic_variables.html" > Optional
automatic variables</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "boost_optional/quick_start/optional_data_members.html" > Optional
data members</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "boost_optional/quick_start/bypassing_unnecessary_default_construction.html" > Bypassing
unnecessary default construction</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "boost_optional/quick_start/storage_in_containers.html" > Storage
in containers</ a ></ span ></ dt >
</ dl ></ dd >
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< dt >< span class = "section" >< a href = "optional/tutorial.html" > Tutorial</ a ></ span ></ dt >
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< dd >< dl >
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< dt >< span class = "section" >< a href = "optional/tutorial.html#boost_optional.tutorial.motivation" > Motivation</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "boost_optional/tutorial/design_overview.html" > Design Overview</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "boost_optional/tutorial/when_to_use_optional.html" > When to
use Optional</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "boost_optional/tutorial/relational_operators.html" > Relational
operators</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "boost_optional/tutorial/io_operators.html" > IO operators</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "boost_optional/tutorial/optional_references.html" > Optional
references</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "boost_optional/tutorial/in_place_factories.html" > In-Place
Factories</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "boost_optional/tutorial/gotchas.html" > Gotchas</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "boost_optional/tutorial/exception_safety_guarantees.html" > Exception
Safety Guarantees</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "boost_optional/tutorial/type_requirements.html" > Type requirements</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "boost_optional/tutorial/performance_considerations.html" > Performance
considerations</ a ></ span ></ dt >
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</ dl ></ dd >
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< dt >< span class = "section" >< a href = "optional/reference.html" > Reference</ a ></ span ></ dt >
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< dd >< dl >
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< dt >< span class = "section" >< a href = "optional/reference.html#boost_optional.reference.header__boost_none_hpp_" > Header
< boost/none.hpp> </ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "boost_optional/reference/header__boost_optional_bad_optional_access_hpp_.html" > Header
< boost/optional/bad_optional_access.hpp> </ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "boost_optional/reference/io_header.html" > Header < boost/optional/optional_io.hpp> </ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "boost_optional/reference/header__boost_optional_optional_fwd_hpp_.html" > Header
< boost/optional/optional_fwd.hpp> </ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "optional/reference/header__boost_optional_optional_hpp_.html" > Header
< boost/optional/optional.hpp> </ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "boost_optional/reference/header__boost_optional_hpp_.html" > Header
< boost/optional.hpp> </ a ></ span ></ dt >
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</ dl ></ dd >
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< dt >< span class = "section" >< a href = "boost_optional/dependencies_and_portability.html" > Dependencies
and Portability</ a ></ span ></ dt >
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< dd >< dl >
< dt >< span class = "section" >< a href = "boost_optional/dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies" > Dependencies</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "boost_optional/dependencies_and_portability/emplace_operations_in_older_compilers.html" > Emplace
operations in older compilers</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "boost_optional/dependencies_and_portability/optional_reference_binding.html" > Optional
Reference Binding</ a ></ span ></ dt >
</ dl ></ dd >
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< dt >< span class = "section" >< a href = "boost_optional/relnotes.html" > Release Notes</ a ></ span ></ dt >
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< dt >< span class = "section" >< a href = "boost_optional/acknowledgements.html" > Acknowledgements</ a ></ span ></ dt >
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</ dl >
</ div >
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< div class = "section" >
< div class = "titlepage" >< div >< div >< h2 class = "title" style = "clear: both" >
< a name = "optional.introduction" ></ a >< a class = "link" href = "index.html#optional.introduction" title = "Introduction" > Introduction</ a >
</ h2 ></ div ></ div ></ div >
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< p >
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Class template < code class = "computeroutput" >< span class = "identifier" > optional</ span ></ code > is
a wrapper for representing 'optional' (or 'nullable') objects who may not (yet)
contain a valid value. Optional objects offer full value semantics; they are
good for passing by value and usage inside STL containers. This is a header-only
library.
</ p >
< h4 >
< a name = "optional.introduction.h0" ></ a >
< span class = "phrase" >< a name = "optional.introduction.problem" ></ a ></ span >< a class = "link" href = "index.html#optional.introduction.problem" > Problem</ a >
</ h4 >
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< p >
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Suppose we want to read a parameter form a config file which represents some
integral value, let's call it < code class = "computeroutput" >< span class = "string" > "MaxValue"</ span ></ code > .
It is possible that this parameter is not specified; such situation is no error.
It is valid to not specify the parameter and in that case the program is supposed
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to behave slightly differently. Also, suppose that any possible value of type
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< code class = "computeroutput" >< span class = "keyword" > int</ span ></ code > is a valid value for < code class = "computeroutput" >< span class = "string" > "MaxValue"</ span ></ code > , so we cannot just use
< code class = "computeroutput" >< span class = "special" > -</ span >< span class = "number" > 1</ span ></ code >
to represent the absence of the parameter in the config file.
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</ p >
< h4 >
< a name = "optional.introduction.h1" ></ a >
< span class = "phrase" >< a name = "optional.introduction.solution" ></ a ></ span >< a class = "link" href = "index.html#optional.introduction.solution" > Solution</ a >
</ h4 >
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< p >
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This is how you solve it with < code class = "computeroutput" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > optional</ span ></ code > :
</ p >
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< pre class = "programlisting" >< span class = "preprocessor" > #include</ span > < span class = "special" > < </ span >< span class = "identifier" > boost</ span >< span class = "special" > /</ span >< span class = "identifier" > optional</ span >< span class = "special" > .</ span >< span class = "identifier" > hpp</ span >< span class = "special" > > </ span >
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< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > optional</ span >< span class = "special" > < </ span >< span class = "keyword" > int</ span >< span class = "special" > > </ span > < span class = "identifier" > getConfigParam</ span >< span class = "special" > (</ span >< span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > string</ span > < span class = "identifier" > name</ span >< span class = "special" > );</ span > < span class = "comment" > // return either an int or a `not-an-int`</ span >
< span class = "keyword" > int</ span > < span class = "identifier" > main</ span >< span class = "special" > ()</ span >
< span class = "special" > {</ span >
< span class = "keyword" > if</ span > < span class = "special" > (</ span >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > optional</ span >< span class = "special" > < </ span >< span class = "keyword" > int</ span >< span class = "special" > > </ span > < span class = "identifier" > oi</ span > < span class = "special" > =</ span > < span class = "identifier" > getConfigParam</ span >< span class = "special" > (</ span >< span class = "string" > "MaxValue"</ span >< span class = "special" > ))</ span > < span class = "comment" > // did I get a real int?</ span >
< span class = "identifier" > runWithMax</ span >< span class = "special" > (*</ span >< span class = "identifier" > oi</ span >< span class = "special" > );</ span > < span class = "comment" > // use my int</ span >
< span class = "keyword" > else</ span >
< span class = "identifier" > runWithNoMax</ span >< span class = "special" > ();</ span >
< span class = "special" > }</ span >
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</ pre >
</ div >
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</ div >
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< td align = "left" >< p >< small > Last revised: November 20, 2021 at 00:57:52 GMT</ small ></ p ></ td >
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