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< a name = "boost_optional.tutorial.optional_references" > < / a > < a class = "link" href = "optional_references.html" title = "Optional references" > Optional
references< / a >
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This library allows the template parameter < code class = "computeroutput" > < span class = "identifier" > T< / span > < / code >
to be of reference type: < code class = "computeroutput" > < span class = "identifier" > T< / span > < span class = "special" > & < / span > < / code > , and to some extent, < code class = "computeroutput" > < span class = "identifier" > T< / span >
< span class = "keyword" > const< / span > < span class = "special" > & < / span > < / code > .
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However, since references are not real objects some restrictions apply and
some operations are not available in this case:
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Converting constructors
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< li class = "listitem" >
Converting assignment
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< li class = "listitem" >
InPlace construction
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InPlace assignment
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Value-access via pointer
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Also, even though < code class = "computeroutput" > < span class = "identifier" > optional< / span > < span class = "special" > < < / span > < span class = "identifier" > T< / span > < span class = "special" > & > < / span > < / code >
treats it wrapped pseudo-object much as a real value, a true real reference
is stored so aliasing will ocurr:
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Copies of < code class = "computeroutput" > < span class = "identifier" > optional< / span > < span class = "special" > < < / span > < span class = "identifier" > T< / span > < span class = "special" > & > < / span > < / code >
will copy the references but all these references will nonetheless refer
to the same object.
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< li class = "listitem" >
Value-access will actually provide access to the referenced object rather
than the reference itself.
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< th align = "left" > Warning< / th >
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On compilers that do not conform to Standard C++ rules of reference binding,
operations on optional references might give adverse results: rather than
binding a reference to a designated object they may create an unexpected
temporary and bind to it. For more details see < a class = "link" href = "../dependencies_and_portability/optional_reference_binding.html" title = "Optional Reference Binding" > Dependencies
and Portability section< / a > .
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< a name = "boost_optional.tutorial.optional_references.h0" > < / a >
< span class = "phrase" > < a name = "boost_optional.tutorial.optional_references.rvalue_references" > < / a > < / span > < a class = "link" href = "optional_references.html#boost_optional.tutorial.optional_references.rvalue_references" > Rvalue
references< / a >
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Rvalue references and lvalue references to const have the ability in C++
to extend the life time of a temporary they bind to. Optional references
do not have this capability, therefore to avoid surprising effects it is
not possible to initialize an optional references from a temporary. Optional
rvalue references are disabled altogether. Also, the initialization and assignment
of an optional reference to const from rvalue reference is disabled.
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< pre class = "programlisting" > < span class = "keyword" > const< / span > < span class = "keyword" > int< / span > < span class = "special" > & < / span > < span class = "identifier" > i< / span > < span class = "special" > =< / span > < span class = "number" > 1< / span > < span class = "special" > ;< / span > < span class = "comment" > // legal< / span >
< span class = "identifier" > optional< / span > < span class = "special" > < < / span > < span class = "keyword" > const< / span > < span class = "keyword" > int< / span > < span class = "special" > & > < / span > < span class = "identifier" > oi< / span > < span class = "special" > =< / span > < span class = "number" > 1< / span > < span class = "special" > ;< / span > < span class = "comment" > // illegal< / span >
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< td align = "right" > < div class = "copyright-footer" > Copyright © 2003-2007 Fernando Luis Cacciola Carballal< br > Copyright © 2014, 2015 Andrzej Krzemień ski< 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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