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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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< p >
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 > .
</ p >
< p >
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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Converting assignment
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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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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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< tr >< td align = "left" valign = "top" >< p >
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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< 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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< p >
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 Andrzej Krzemień ski< p >
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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