1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Constructor Forwarding</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../move.html" title="Chapter 25. Boost.Move"> 10<link rel="prev" href="move_and_containers.html" title="Containers and move semantics"> 11<link rel="next" href="move_return.html" title="Implicit Move when returning a local object"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="move_and_containers.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../move.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="move_return.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="move.construct_forwarding"></a><a class="link" href="construct_forwarding.html" title="Constructor Forwarding">Constructor Forwarding</a> 29</h2></div></div></div> 30<p> 31 Consider writing a generic factory function that returns an object for a newly 32 constructed generic type. Factory functions such as this are valuable for encapsulating 33 and localizing the allocation of resources. Obviously, the factory function 34 must accept exactly the same sets of arguments as the constructors of the type 35 of objects constructed: 36 </p> 37<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">factory_new</span><span class="special">()</span> 38<span class="special">{</span> <span class="keyword">return</span> <span class="keyword">new</span> <span class="identifier">T</span><span class="special">();</span> <span class="special">}</span> 39 40<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">factory_new</span><span class="special">(</span><span class="identifier">a1</span><span class="special">)</span> 41<span class="special">{</span> <span class="keyword">return</span> <span class="keyword">new</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">a1</span><span class="special">);</span> <span class="special">}</span> 42 43<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">factory_new</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">)</span> 44<span class="special">{</span> <span class="keyword">return</span> <span class="keyword">new</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">);</span> <span class="special">}</span> 45</pre> 46<p> 47 Unfortunately, in C++03 the much bigger issue with this approach is that the 48 N-argument case would require 2^N overloads, immediately discounting this as 49 a general solution. Fortunately, most constructors take arguments by value, 50 by const-reference or by rvalue reference. If these limitations are accepted, 51 the forwarding emulation of a N-argument case requires just N overloads. This 52 library makes this emulation easy with the help of <code class="computeroutput"><span class="identifier">BOOST_FWD_REF</span></code> 53 and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">forward</span></code>: 54 </p> 55<p> 56</p> 57<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">move</span><span class="special">/</span><span class="identifier">utility_core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 58<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 59 60<span class="keyword">class</span> <span class="identifier">copyable_only_tester</span> 61<span class="special">{</span> 62 <span class="keyword">public</span><span class="special">:</span> 63 <span class="identifier">copyable_only_tester</span><span class="special">()</span> 64 <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_only_tester()"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> 65 66 <span class="identifier">copyable_only_tester</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">copyable_only_tester</span><span class="special">&)</span> 67 <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_only_tester(const copyable_only_tester&)"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> 68 69 <span class="identifier">copyable_only_tester</span><span class="special">(</span><span class="keyword">int</span><span class="special">)</span> 70 <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_only_tester(int)"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> 71 72 <span class="identifier">copyable_only_tester</span><span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">)</span> 73 <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_only_tester(int, double)"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> 74<span class="special">};</span> 75 76<span class="keyword">class</span> <span class="identifier">copyable_movable_tester</span> 77<span class="special">{</span> 78 <span class="comment">// move semantics</span> 79 <span class="identifier">BOOST_COPYABLE_AND_MOVABLE</span><span class="special">(</span><span class="identifier">copyable_movable_tester</span><span class="special">)</span> 80 <span class="keyword">public</span><span class="special">:</span> 81 82 <span class="identifier">copyable_movable_tester</span><span class="special">()</span> 83 <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester()"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> 84 85 <span class="identifier">copyable_movable_tester</span><span class="special">(</span><span class="keyword">int</span><span class="special">)</span> 86 <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester(int)"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> 87 88 <span class="identifier">copyable_movable_tester</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">copyable_movable_tester</span><span class="special">))</span> 89 <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester))"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> 90 91 <span class="identifier">copyable_movable_tester</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">copyable_movable_tester</span> <span class="special">&)</span> 92 <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester(const copyable_movable_tester &)"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> 93 94 <span class="identifier">copyable_movable_tester</span><span class="special">(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">copyable_movable_tester</span><span class="special">),</span> <span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">copyable_movable_tester</span><span class="special">))</span> 95 <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester), BOOST_RV_REF(copyable_movable_tester))"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span> 96 97 <span class="identifier">copyable_movable_tester</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_RV_REF</span><span class="special">(</span><span class="identifier">copyable_movable_tester</span><span class="special">))</span> 98 <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester & operator=(BOOST_RV_REF(copyable_movable_tester))"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 99 <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span> 100 101 <span class="identifier">copyable_movable_tester</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">=(</span><span class="identifier">BOOST_COPY_ASSIGN_REF</span><span class="special">(</span><span class="identifier">copyable_movable_tester</span><span class="special">))</span> 102 <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"copyable_movable_tester & operator=(BOOST_COPY_ASSIGN_REF(copyable_movable_tester))"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> 103 <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span> 104<span class="special">};</span> 105 106<span class="comment">//1 argument</span> 107<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">MaybeMovable</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">MaybeRv</span><span class="special">></span> 108<span class="keyword">void</span> <span class="identifier">function_construct</span><span class="special">(</span><span class="identifier">BOOST_FWD_REF</span><span class="special">(</span><span class="identifier">MaybeRv</span><span class="special">)</span> <span class="identifier">x</span><span class="special">)</span> 109<span class="special">{</span> <span class="identifier">MaybeMovable</span> <span class="identifier">m</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">MaybeRv</span><span class="special">>(</span><span class="identifier">x</span><span class="special">));</span> <span class="special">}</span> 110 111<span class="comment">//2 argument</span> 112<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">MaybeMovable</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">MaybeRv</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">MaybeRv2</span><span class="special">></span> 113<span class="keyword">void</span> <span class="identifier">function_construct</span><span class="special">(</span><span class="identifier">BOOST_FWD_REF</span><span class="special">(</span><span class="identifier">MaybeRv</span><span class="special">)</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">BOOST_FWD_REF</span><span class="special">(</span><span class="identifier">MaybeRv2</span><span class="special">)</span> <span class="identifier">x2</span><span class="special">)</span> 114<span class="special">{</span> <span class="identifier">MaybeMovable</span> <span class="identifier">m</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">MaybeRv</span><span class="special">>(</span><span class="identifier">x</span><span class="special">),</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">MaybeRv2</span><span class="special">>(</span><span class="identifier">x2</span><span class="special">));</span> <span class="special">}</span> 115 116<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 117<span class="special">{</span> 118 <span class="identifier">copyable_movable_tester</span> <span class="identifier">m</span><span class="special">;</span> 119 <span class="comment">//move constructor</span> 120 <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_movable_tester</span><span class="special">>(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">m</span><span class="special">));</span> 121 <span class="comment">//copy constructor</span> 122 <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_movable_tester</span><span class="special">>(</span><span class="identifier">copyable_movable_tester</span><span class="special">());</span> 123 <span class="comment">//two rvalue constructor</span> 124 <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_movable_tester</span><span class="special">>(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">m</span><span class="special">),</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">m</span><span class="special">));</span> 125 126 <span class="identifier">copyable_only_tester</span> <span class="identifier">nm</span><span class="special">;</span> 127 <span class="comment">//copy constructor (copyable_only_tester has no move ctor.)</span> 128 <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_only_tester</span><span class="special">>(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">nm</span><span class="special">));</span> 129 <span class="comment">//copy constructor</span> 130 <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_only_tester</span><span class="special">>(</span><span class="identifier">nm</span><span class="special">);</span> 131 <span class="comment">//int constructor</span> 132 <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_only_tester</span><span class="special">>(</span><span class="keyword">int</span><span class="special">(</span><span class="number">0</span><span class="special">));</span> 133 <span class="comment">//int, double constructor</span> 134 <span class="identifier">function_construct</span><span class="special"><</span><span class="identifier">copyable_only_tester</span><span class="special">>(</span><span class="keyword">int</span><span class="special">(</span><span class="number">0</span><span class="special">),</span> <span class="keyword">double</span><span class="special">(</span><span class="number">0.0</span><span class="special">));</span> 135 136 <span class="comment">//Output is:</span> 137 <span class="comment">//copyable_movable_tester()</span> 138 <span class="comment">//copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester))</span> 139 <span class="comment">//copyable_movable_tester()</span> 140 <span class="comment">//copyable_movable_tester(const copyable_movable_tester &)</span> 141 <span class="comment">//copyable_movable_tester(BOOST_RV_REF(copyable_movable_tester), BOOST_RV_REF(copyable_movable_tester))</span> 142 <span class="comment">//copyable_only_tester()</span> 143 <span class="comment">//copyable_only_tester(const copyable_only_tester&)</span> 144 <span class="comment">//copyable_only_tester(const copyable_only_tester&)</span> 145 <span class="comment">//copyable_only_tester(int)</span> 146 <span class="comment">//copyable_only_tester(int, double)</span> 147 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 148<span class="special">}</span> 149</pre> 150<p> 151 </p> 152<p> 153 Constructor forwarding comes in handy to implement placement insertion in containers 154 with just N overloads if the implementor accepts the limitations of this type 155 of forwarding for C++03 compilers. In compilers with rvalue references perfect 156 forwarding is achieved. 157 </p> 158</div> 159<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 160<td align="left"></td> 161<td align="right"><div class="copyright-footer">Copyright © 2008-2014 Ion Gaztanaga<p> 162 Distributed under the Boost Software License, Version 1.0. (See accompanying 163 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>) 164 </p> 165</div></td> 166</tr></table> 167<hr> 168<div class="spirit-nav"> 169<a accesskey="p" href="move_and_containers.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../move.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="move_return.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 170</div> 171</body> 172</html> 173
