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< dt > < span class = "section" > < a href = "quick_start.html#boost_optional.quick_start.optional_return_values" > Optional
return values< / a > < / span > < / dt >
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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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2016-02-13 15:33:07 +01:00
< td align = "right" > < div class = "copyright-footer" > Copyright © 2003-2007 Fernando Luis Cacciola Carballal< br > Copyright © 2014-2016 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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