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Added func value_or_eval()
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Andrzej Krzemienski
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.acknowledgements"></a><a class="link" href="acknowledgements.html" title="Acknowledgements">Acknowledgements</a>
|
||||
</h2></div></div></div>
|
||||
<h4>
|
||||
<a name="boost_optional.acknowledgements.h0"></a>
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||||
<span class="phrase"><a name="boost_optional.acknowledgements.pre_formal_review"></a></span><a class="link" href="acknowledgements.html#boost_optional.acknowledgements.pre_formal_review">Pre-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Peter Dimov suggested the name 'optional', and was the first to point out
|
||||
the need for aligned storage.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor developed 'type_with_alignment', and later Eric Friedman
|
||||
coded 'aligned_storage', which are the core of the optional class implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Andrei Alexandrescu and Brian Parker also worked with aligned storage techniques
|
||||
and their work influenced the current implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Gennadiy Rozental made extensive and important comments which shaped the
|
||||
design.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vesa Karvonen and Douglas Gregor made quite useful comparisons between
|
||||
optional, variant and any; and made other relevant comments.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor and Peter Dimov commented on comparisons and evaluation
|
||||
in boolean contexts.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Eric Friedman helped understand the issues involved with aligned storage,
|
||||
move/copy operations and exception safety.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Many others have participated with useful comments: Aleksey Gurotov, Kevlin
|
||||
Henney, David Abrahams, and others I can't recall.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.acknowledgements.h1"></a>
|
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<span class="phrase"><a name="boost_optional.acknowledgements.post_formal_review"></a></span><a class="link" href="acknowledgements.html#boost_optional.acknowledgements.post_formal_review">Post-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
William Kempf carefully considered the originally proposed interface and
|
||||
suggested the new interface which is currently used. He also started and
|
||||
fueled the discussion about the analogy optional<>/smart pointer
|
||||
and about relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson
|
||||
focused on the relational semantics of optional (originally undefined);
|
||||
concluding with the fact that the pointer-like interface doesn't make it
|
||||
a pointer so it shall have deep relational operators.
|
||||
</li>
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||||
<li class="listitem">
|
||||
Augustus Saunders also explored the different relational semantics between
|
||||
optional<> and a pointer and developed the OptionalPointee concept
|
||||
as an aid against potential conflicts on generic code.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman noticed that optional<> can be seen as an API on top
|
||||
of variant<T,nil_t>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Dave Gomboc explained the meaning and usage of the Haskell analog to optional<>:
|
||||
the Maybe type constructor (analogy originally pointed out by David Sankel).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey,
|
||||
Rob Stewart, and others.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman made the case for the support of references and helped with
|
||||
the proper semantics.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Mat Marcus shown the virtues of a value-oriented interface, influencing
|
||||
the current design, and contributed the idea of "none".
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vladimir Batov's design of Boost.Convert library motivated the development
|
||||
of value accessors for <code class="computeroutput"><span class="identifier">optional</span></code>:
|
||||
functions <code class="computeroutput"><span class="identifier">value</span></code>, <code class="computeroutput"><span class="identifier">value_or</span></code>, <code class="computeroutput"><span class="identifier">value_or_eval</span></code>.
|
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</li>
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</ul></div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<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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</p>
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</div></td>
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</tr></table>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="dependencies_and_portability/optional_reference_binding.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a>
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@ -7,7 +7,7 @@
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<link rel="home" href="../../index.html" title="Boost.Optional">
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<link rel="up" href="../dependencies_and_portability.html" title="Dependencies and Portability">
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@ -20,7 +20,7 @@
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<div class="spirit-nav">
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<a accesskey="p" href="../dependencies_and_portability.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../dependencies_and_portability.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../acknowledgments.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<a accesskey="p" href="../dependencies_and_portability.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../dependencies_and_portability.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../acknowledgements.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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@ -94,7 +94,7 @@
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</tr></table>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="../dependencies_and_portability.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../dependencies_and_portability.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../acknowledgments.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<a accesskey="p" href="../dependencies_and_portability.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../dependencies_and_portability.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../acknowledgements.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</html>
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||||
@ -111,6 +111,20 @@
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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>.
|
||||
Finally, you can provide a callback to be called when trying to access the
|
||||
contained value fails:
|
||||
</p>
|
||||
<pre class="programlisting"><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="special">-></span> <span class="keyword">int</span> <span class="special">{</span>
|
||||
<span class="identifier">cout</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>
|
||||
<span class="keyword">return</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span>
|
||||
<span class="special">});</span>
|
||||
</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>
|
||||
Now, let's consider how function <code class="computeroutput"><span class="identifier">convert</span></code>
|
||||
can be implemented.
|
||||
</p>
|
||||
|
||||
@ -541,7 +541,7 @@
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<li class="listitem">
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||||
<span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
|
||||
is initialized and its value is move constructed from <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>; else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is uninitialized.
|
||||
is initialized and its value is move-constructed from <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>; else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is uninitialized.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> Whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">U</span><span class="special">&&</span> <span class="special">)</span></code>
|
||||
@ -1269,10 +1269,6 @@
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">&</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>::</span><span class="keyword">operator</span><span class="special">*()</span> <span class="special">&;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">&&</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>::</span><span class="keyword">operator</span><span class="special">*()</span> <span class="special">&&;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
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<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized
|
||||
@ -1288,8 +1284,8 @@
|
||||
<span class="bold"><strong>Notes:</strong></span> The requirement is asserted via
|
||||
<code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
|
||||
On compilers that do not support ref-qualifiers on member functions these
|
||||
three overloads are replaced with the classical two: a <code class="computeroutput"><span class="keyword">const</span></code> and non-<code class="computeroutput"><span class="keyword">const</span></code>
|
||||
member functions.
|
||||
two overloads are replaced with the classical two: a <code class="computeroutput"><span class="keyword">const</span></code>
|
||||
and non-<code class="computeroutput"><span class="keyword">const</span></code> member functions.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
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@ -1303,6 +1299,35 @@
|
||||
</pre>
|
||||
</li>
|
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</ul></div>
|
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<p>
|
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<span class="inlinemediaobject"><img src="../../images/space.png" alt="space"></span>
|
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</p>
|
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<a name="reference_optional_operator_asterisk_move"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span></code>
|
||||
<span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>::</span><span class="keyword">operator</span><span class="special">*()</span> <span class="special">&&;</span></code>
|
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</p></blockquote></div>
|
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
|
||||
and <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
|
||||
is initialized.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> A move-constructed copy the
|
||||
contained value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> Whatever the <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
constructor selected for the move throws.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> The requirement is asserted via
|
||||
<code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
|
||||
On compilers that do not support ref-qualifiers on member functions this
|
||||
overload is not present.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
@ -1357,10 +1382,6 @@
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">&</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">value</span><span class="special">()</span> <span class="special">&</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">&&</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">value</span><span class="special">()</span> <span class="special">&&</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> A reference to the contained
|
||||
@ -1373,8 +1394,8 @@
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> On compilers that do not support
|
||||
ref-qualifiers on member functions these three overloads are replaced
|
||||
with the classical two: a <code class="computeroutput"><span class="keyword">const</span></code>
|
||||
ref-qualifiers on member functions these two overloads are replaced with
|
||||
the classical two: a <code class="computeroutput"><span class="keyword">const</span></code>
|
||||
and non-<code class="computeroutput"><span class="keyword">const</span></code> member functions.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
@ -1391,9 +1412,36 @@
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="keyword">true</span> <span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<span class="inlinemediaobject"><img src="../../images/space.png" alt="space"></span>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_value_move"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">value</span><span class="special">()</span> <span class="special">&&</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> A move-constructed copy of
|
||||
the contained value, if <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> An instance of <code class="computeroutput"><span class="identifier">bad_optional_access</span></code>, if <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
|
||||
is not initialized.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> On compilers that do not support
|
||||
ref-qualifiers on member functions this overload is not present.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_value_or"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">value_or</span><span class="special">(</span><span class="identifier">U</span> <span class="special">&&</span>
|
||||
<span class="identifier">v</span><span class="special">)</span>
|
||||
@ -1407,7 +1455,7 @@
|
||||
is convertible to <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">?</span> <span class="special">**</span><span class="keyword">this</span> <span class="special">:</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">U</span><span class="special">>(</span><span class="identifier">v</span><span class="special">))</span></code>.
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">?</span> <span class="special">**</span><span class="keyword">this</span> <span class="special">:</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">U</span><span class="special">>(</span><span class="identifier">v</span><span class="special">))</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> Any exception thrown by the
|
||||
@ -1437,7 +1485,7 @@
|
||||
is convertible to <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">?</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(**</span><span class="keyword">this</span><span class="special">)</span> <span class="special">:</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">U</span><span class="special">>(</span><span class="identifier">v</span><span class="special">))</span></code>.
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">?</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(**</span><span class="keyword">this</span><span class="special">)</span> <span class="special">:</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">U</span><span class="special">>(</span><span class="identifier">v</span><span class="special">))</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> Any exception thrown by the
|
||||
@ -1445,10 +1493,79 @@
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> On compilers that do not support
|
||||
ref-qualifiers on member functions this overload is not present.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_value_or_call"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">value_or_eval</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a> and <code class="computeroutput"><span class="identifier">F</span></code> models a <a href="http://www.sgi.com/tech/stl/Generator.html" target="_top"><code class="computeroutput"><span class="identifier">Generator</span></code></a> whose result type
|
||||
is convertible to <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">?</span> <span class="special">**</span><span class="keyword">this</span> <span class="special">:</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">f</span><span class="special">())</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> Any exception thrown by the
|
||||
selected constructor of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
or by <code class="computeroutput"><span class="identifier">f</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> Function <code class="computeroutput"><span class="identifier">f</span></code>
|
||||
is only evaluated if <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">false</span></code>. On compilers that do not support
|
||||
ref-qualifiers on member functions this overload is replaced with the
|
||||
classical non-<code class="computeroutput"><span class="keyword">const</span></code>-qualified
|
||||
member function. On compilers without rvalue reference support the type
|
||||
of <code class="computeroutput"><span class="identifier">v</span></code> becomes <code class="computeroutput"><span class="identifier">U</span> <span class="keyword">const</span><span class="special">&</span></code>.
|
||||
<code class="computeroutput"><span class="keyword">const</span></code>-qualified member function.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">complain_and_0</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">clog</span> <span class="special"><<</span> <span class="string">"no value returned, using default"</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span>
|
||||
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</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">o1</span> <span class="special">=</span> <span class="number">1</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">oN</span> <span class="special">=</span> <span class="identifier">none</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">o1</span><span class="special">.</span><span class="identifier">value_or_eval</span><span class="special">(</span><span class="identifier">complain_and_0</span><span class="special">);</span> <span class="comment">// fun not called</span>
|
||||
<span class="identifier">assert</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="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">oN</span><span class="special">.</span><span class="identifier">value_or_eval</span><span class="special">(</span><span class="identifier">complain_and_0</span><span class="special">);</span> <span class="comment">// fun called</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_value_or_call_move"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">value_or_eval</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span> <span class="special">&&</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">F</span></code> models a <a href="http://www.sgi.com/tech/stl/Generator.html" target="_top"><code class="computeroutput"><span class="identifier">Generator</span></code></a>
|
||||
whose result type is convertible to <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">?</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(**</span><span class="keyword">this</span><span class="special">)</span> <span class="special">:</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">f</span><span class="special">())</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> Any exception thrown by the
|
||||
selected constructor of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
or by <code class="computeroutput"><span class="identifier">f</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> Function <code class="computeroutput"><span class="identifier">f</span></code>
|
||||
is only evaluated if <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">false</span></code>. On compilers that do not support
|
||||
ref-qualifiers on member functions this overload is not present.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
|
||||
Reference in New Issue
Block a user