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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 Andrzej Krzemień ski< / 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 > )
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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/optional_references.html" > Optional
references< / a > < / span > < / dt >
< dt > < span class = "section" > < a href = "boost_optional/tutorial/rebinding_semantics_for_assignment_of_optional_references.html" > Rebinding
semantics for assignment of optional references< / a > < / span > < / dt >
< dt > < span class = "section" > < a href = "boost_optional/tutorial/in_place_factories.html" > In-Place
Factories< / a > < / span > < / dt >
< dt > < span class = "section" > < a href = "boost_optional/tutorial/a_note_about_optional_bool_.html" > A
note about optional< bool> < / a > < / span > < / dt >
< 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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< / 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.synopsis" > Synopsis< / a > < / span > < / dt >
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< dt > < span class = "section" > < a href = "boost_optional/reference/detailed_semantics.html" > Detailed
Semantics< / 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 >
< 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/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 >
< 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 >
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< 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 >
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< td align = "left" > < p > < small > Last revised: November 21, 2014 at 23:32:40 GMT< / small > < / p > < / td >
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