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< div class = "toc" >< dl class = "toc" >
< dt >< span class = "section" >< a href = "quick_start.html#boost_optional.quick_start.optional_return_values" > Optional
return values</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "quick_start/optional_automatic_variables.html" > Optional
automatic variables</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "quick_start/optional_data_members.html" > Optional
data members</ a ></ span ></ dt >
< dt >< span class = "section" >< a href = "quick_start/bypassing_unnecessary_default_construction.html" > Bypassing
unnecessary default construction</ a ></ span ></ dt >
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in containers</ a ></ span ></ dt >
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< a name = "boost_optional.quick_start.optional_return_values" ></ a >< a class = "link" href = "quick_start.html#boost_optional.quick_start.optional_return_values" title = "Optional return values" > Optional
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return values</ a >
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< p >
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Let's write and use a converter function that converts a < code class = "computeroutput" >< span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > string</ span ></ code >
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to an < code class = "computeroutput" >< span class = "keyword" > int</ span ></ code > . It is possible that
for a given string (e.g. < code class = "computeroutput" >< span class = "string" > "cat"</ span ></ code > )
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there exists no value of type < code class = "computeroutput" >< span class = "keyword" > int</ span ></ code >
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capable of representing the conversion result. We do not consider such situation
an error. We expect that the converter can be used only to check if the conversion
is possible. A natural signature for this function can be:
</ 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" > convert</ span >< span class = "special" > (</ span >< span class = "keyword" > const</ span > < span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > string</ span >< span class = "special" > & </ span > < span class = "identifier" > text</ span >< span class = "special" > );</ span >
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</ pre >
< p >
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All necessary functionality can be included with one header < code class = "computeroutput" >< 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 ></ code > .
The above function signature means that the function can either return a
value of type < code class = "computeroutput" >< span class = "keyword" > int</ span ></ code > or a flag
indicating that no value of < code class = "computeroutput" >< span class = "keyword" > int</ span ></ code >
is available. This does not indicate an error. It is like one additional
value of < code class = "computeroutput" >< span class = "keyword" > int</ span ></ code > . This is how we
can use our function:
</ p >
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< pre class = "programlisting" >< span class = "keyword" > const</ span > < span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > string</ span >< span class = "special" > & </ span > < span class = "identifier" > text</ span > < span class = "special" > =</ span > < span class = "comment" > /*... */</ 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" > oi</ span > < span class = "special" > =</ span > < span class = "identifier" > convert</ span >< span class = "special" > (</ span >< span class = "identifier" > text</ span >< span class = "special" > );</ span > < span class = "comment" > // move-construct</ span >
< span class = "keyword" > if</ span > < span class = "special" > (</ span >< span class = "identifier" > oi</ span >< span class = "special" > )</ span > < span class = "comment" > // contextual conversion to bool</ span >
< span class = "keyword" > int</ span > < span class = "identifier" > i</ span > < span class = "special" > =</ span > < span class = "special" > *</ span >< span class = "identifier" > oi</ span >< span class = "special" > ;</ span > < span class = "comment" > // operator*</ span >
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</ pre >
< p >
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In order to test if < code class = "computeroutput" >< span class = "identifier" > optional</ span ></ code >
contains a value, we use the contextual conversion to type < code class = "computeroutput" >< span class = "keyword" > bool</ span ></ code > . Because of this we can combine the initialization
of the optional object and the test into one instruction:
</ p >
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< pre class = "programlisting" >< 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" > convert</ span >< span class = "special" > (</ span >< span class = "identifier" > text</ span >< span class = "special" > ))</ span >
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< span class = "keyword" > int</ span > < span class = "identifier" > i</ span > < span class = "special" > =</ span > < span class = "special" > *</ span >< span class = "identifier" > oi</ span >< span class = "special" > ;</ span >
</ pre >
< p >
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We extract the contained value with < code class = "computeroutput" >< span class = "keyword" > operator</ span >< span class = "special" > *</ span ></ code > (and with < code class = "computeroutput" >< span class = "keyword" > operator</ span >< span class = "special" > -> </ span ></ code > where it makes sense). An attempt to
extract the contained value of an uninitialized optional object is an < span class = "emphasis" >< em > undefined
behaviour</ em ></ span > (UB). This implementation guards the call with < code class = "computeroutput" >< span class = "identifier" > BOOST_ASSERT</ span ></ code > . Therefore you should be sure
that the contained value is there before extracting. For instance, the following
code is reasonably UB-safe:
</ p >
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< pre class = "programlisting" >< span class = "keyword" > int</ span > < span class = "identifier" > i</ span > < span class = "special" > =</ span > < span class = "special" > *</ span >< span class = "identifier" > convert</ span >< span class = "special" > (</ span >< span class = "string" > "100"</ span >< span class = "special" > );</ span >
</ pre >
< p >
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This is because we know that string value < code class = "computeroutput" >< span class = "string" > "100"</ span ></ code >
converts to a valid value of < code class = "computeroutput" >< span class = "keyword" > int</ span ></ code > .
If you do not like this potential UB, you can use an alternative way of extracting
the contained value:
</ p >
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< pre class = "programlisting" >< span class = "keyword" > try</ span > < span class = "special" > {</ span >
< span class = "keyword" > int</ span > < span class = "identifier" > j</ span > < span class = "special" > =</ span > < span class = "identifier" > convert</ span >< span class = "special" > (</ span >< span class = "identifier" > text</ span >< span class = "special" > ).</ span >< span class = "identifier" > value</ span >< span class = "special" > ();</ span >
< span class = "special" > }</ span >
< span class = "keyword" > catch</ span > < span class = "special" > (</ span >< span class = "keyword" > const</ span > < span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > bad_optional_access</ span >< span class = "special" > & )</ span > < span class = "special" > {</ span >
< span class = "comment" > // deal with it</ span >
< span class = "special" > }</ span >
</ pre >
< p >
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This version throws an exception upon an attempt to access a non-existent
contained value. If your way of dealing with the missing value is to use
some default, like < code class = "computeroutput" >< span class = "number" > 0</ span ></ code > , there exists
a yet another alternative:
</ p >
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< pre class = "programlisting" >< span class = "keyword" > int</ span > < span class = "identifier" > k</ span > < span class = "special" > =</ span > < span class = "identifier" > convert</ span >< span class = "special" > (</ span >< span class = "identifier" > text</ span >< span class = "special" > ).</ span >< span class = "identifier" > value_or</ span >< span class = "special" > (</ span >< span class = "number" > 0</ span >< span class = "special" > );</ span >
</ pre >
< p >
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This uses the < code class = "computeroutput" >< span class = "identifier" > atoi</ span ></ code > -like approach
to conversions: if < code class = "computeroutput" >< span class = "identifier" > text</ span ></ code > does
not represent an integral number just return < code class = "computeroutput" >< span class = "number" > 0</ span ></ code > .
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Finally, you can provide a callback to be called when trying to access the
contained value fails:
</ p >
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< pre class = "programlisting" >< span class = "keyword" > int</ span > < span class = "identifier" > fallback_to_default</ span >< span class = "special" > ()</ span >
< span class = "special" > {</ span >
< span class = "identifier" > cerr</ span > < span class = "special" > << </ span > < span class = "string" > "could not convert; using -1 instead"</ span > < span class = "special" > << </ span > < span class = "identifier" > endl</ span >< span class = "special" > ;</ span >
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< span class = "keyword" > return</ span > < span class = "special" > -</ span >< span class = "number" > 1</ span >< span class = "special" > ;</ span >
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< span class = "special" > }</ span >
< span class = "keyword" > int</ span > < span class = "identifier" > l</ span > < span class = "special" > =</ span > < span class = "identifier" > convert</ span >< span class = "special" > (</ span >< span class = "identifier" > text</ span >< span class = "special" > ).</ span >< span class = "identifier" > value_or_eval</ span >< span class = "special" > (</ span >< span class = "identifier" > fallback_to_default</ span >< span class = "special" > );</ span >
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</ pre >
< p >
This will call the provided callback and return whatever the callback returns.
The callback can have side effects: they will only be observed when the optional
object does not contain a value.
</ p >
< p >
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Now, let's consider how function < code class = "computeroutput" >< span class = "identifier" > convert</ span ></ code >
can be implemented.
</ p >
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< pre class = "programlisting" >< 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" > convert</ span >< span class = "special" > (</ span >< span class = "keyword" > const</ span > < span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > string</ span >< span class = "special" > & </ span > < span class = "identifier" > text</ span >< span class = "special" > )</ span >
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< span class = "special" > {</ span >
< span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > stringstream</ span > < span class = "identifier" > s</ span >< span class = "special" > (</ span >< span class = "identifier" > text</ span >< span class = "special" > );</ span >
< span class = "keyword" > int</ span > < span class = "identifier" > i</ span >< span class = "special" > ;</ span >
< span class = "keyword" > if</ span > < span class = "special" > ((</ span >< span class = "identifier" > s</ span > < span class = "special" > >> </ span > < span class = "identifier" > i</ span >< span class = "special" > )</ span > < span class = "special" > && </ span > < span class = "identifier" > s</ span >< span class = "special" > .</ span >< span class = "identifier" > get</ span >< span class = "special" > ()</ span > < span class = "special" > ==</ span > < span class = "identifier" > std</ span >< span class = "special" > ::</ span >< span class = "identifier" > char_traits</ span >< span class = "special" > < </ span >< span class = "keyword" > char</ span >< span class = "special" > > ::</ span >< span class = "identifier" > eof</ span >< span class = "special" > ())</ span >
< span class = "keyword" > return</ span > < span class = "identifier" > i</ span >< span class = "special" > ;</ span >
< span class = "keyword" > else</ span >
< span class = "keyword" > return</ span > < span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > none</ span >< span class = "special" > ;</ span >
< span class = "special" > }</ span >
</ pre >
< p >
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Observe the two return statements. < code class = "computeroutput" >< span class = "keyword" > return</ span >
< span class = "identifier" > i</ span ></ code > uses the converting constructor
that can create < code class = "computeroutput" >< span class = "identifier" > optional</ span >< span class = "special" > < </ span >< span class = "identifier" > T</ span >< span class = "special" > > </ span ></ code >
from < code class = "computeroutput" >< span class = "identifier" > T</ span ></ code > . Thus constructed
optional object is initialized and its value is a copy of < code class = "computeroutput" >< span class = "identifier" > i</ span ></ code > .
The other return statement uses another converting constructor from a special
tag < code class = "computeroutput" >< span class = "identifier" > boost</ span >< span class = "special" > ::</ span >< span class = "identifier" > none</ span ></ code > . It is used to indicate that we want
to create an uninitialized optional object.
</ p >
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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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