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4<title>Miscellaneous Notes</title>
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25<div class="section" lang="en">
26<div class="titlepage"><div><div><h3 class="title">
27<a name="variant.misc"></a>Miscellaneous Notes</h3></div></div></div>
28<div class="toc"><dl>
29<dt><span class="section"><a href="misc.html#variant.versus-any">Boost.Variant vs. Boost.Any</a></span></dt>
30<dt><span class="section"><a href="misc.html#id2841449">Portability</a></span></dt>
31<dt><span class="section"><a href="misc.html#variant.troubleshooting">Troubleshooting</a></span></dt>
32<dt><span class="section"><a href="misc.html#variant.ack">Acknowledgments</a></span></dt>
33</dl></div>
34<div class="section" lang="en">
35<div class="titlepage"><div><div><h4 class="title">
36<a name="variant.versus-any"></a>Boost.Variant vs. Boost.Any</h4></div></div></div>
37<p>As a discriminated union container, the Variant library shares many
38    of the same features of the <a href="../any.html" title="Chapter 1. Boost.Any">Any</a> library.
39    However, since neither library wholly encapsulates the features of the
40    other, one library cannot be generally recommended for use over the
41    other.</p>
42<p>That said, Boost.Variant has several advantages over Boost.Any,
43    such as:
44
45    </p>
46<div class="itemizedlist"><ul type="disc">
47<li>Boost.Variant guarantees the type of its content is one of a
48        finite, user-specified set of types.</li>
49<li>Boost.Variant provides <span class="emphasis"><em>compile-time</em></span>
50        checked visitation of its content. (By contrast, the current version
51        of Boost.Any provides no visitation mechanism at all; but even if it
52        did, it would need to be checked at run-time.)</li>
53<li>Boost.Variant enables generic visitation of its content.
54        (Even if Boost.Any did provide a visitation mechanism, it would enable
55        visitation only of explicitly-specified types.)</li>
56<li>Boost.Variant offers an efficient, stack-based storage scheme
57        (avoiding the overhead of dynamic allocation).</li>
58</ul></div>
59<p>Of course, Boost.Any has several advantages over Boost.Variant,
60    such as:
61
62    </p>
63<div class="itemizedlist"><ul type="disc">
64<li>Boost.Any, as its name implies, allows virtually any type for
65        its content, providing great flexibility.</li>
66<li>Boost.Any provides the no-throw guarantee of exception safety
67        for its swap operation.</li>
68<li>Boost.Any makes little use of template metaprogramming
69        techniques (avoiding potentially hard-to-read error messages and
70        significant compile-time processor and memory demands).</li>
71</ul></div>
72</div>
73<div class="section" lang="en">
74<div class="titlepage"><div><div><h4 class="title">
75<a name="id2841449"></a>Portability</h4></div></div></div>
76<p>The library aims for 100% ANSI/ISO C++ conformance. However, this is
77    strictly impossible due to the inherently non-portable nature of the
78    Type Traits library's
79    <code class="computeroutput">type_with_alignment</code> facility. In
80    practice though, no compilers or platforms have been discovered where this
81    reliance on undefined behavior has been an issue.</p>
82<p>Additionally, significant effort has been expended to ensure proper
83    functioning despite various compiler bugs and other conformance problems.
84    To date the library testsuite has
85    been compiled and tested successfully on at least the following compilers
86    for basic and advanced functionality:
87
88    </p>
89<div class="informaltable"><table class="table">
90<colgroup>
91<col>
92<col>
93<col>
94<col>
95<col>
96</colgroup>
97<thead><tr>
98<th> </th>
99<th>Basic</th>
100<th><code class="computeroutput">variant&lt;T&amp;&gt;</code></th>
101<th><a href="tutorial.html#variant.tutorial.over-sequence" title="Using a type sequence to specify bounded types"><code class="computeroutput">make_variant_over</code></a></th>
102<th><a href="tutorial.html#variant.tutorial.recursive.recursive-variant" title="Recursive types with make_recursive_variant"><code class="computeroutput">make_recursive_variant</code></a></th>
103</tr></thead>
104<tbody>
105<tr>
106<td>Borland C++ 5.5.1 and 5.6.4</td>
107<td>X</td>
108<td>X</td>
109<td> </td>
110<td> </td>
111</tr>
112<tr>
113<td>Comeau C++ 4.3.0</td>
114<td>X</td>
115<td>X</td>
116<td>X</td>
117<td>X</td>
118</tr>
119<tr>
120<td>GNU GCC 3.3.1</td>
121<td>X</td>
122<td>X</td>
123<td>X</td>
124<td>X</td>
125</tr>
126<tr>
127<td>GNU GCC 2.95.3</td>
128<td>X</td>
129<td>X</td>
130<td> </td>
131<td>X</td>
132</tr>
133<tr>
134<td>Intel C++ 7.0</td>
135<td>X</td>
136<td> </td>
137<td>X</td>
138<td>X</td>
139</tr>
140<tr>
141<td>Metrowerks CodeWarrior 8.3</td>
142<td>X</td>
143<td> </td>
144<td>X</td>
145<td>X</td>
146</tr>
147<tr>
148<td>Microsoft Visual C++ 7.1</td>
149<td>X</td>
150<td>X</td>
151<td>X</td>
152<td>X</td>
153</tr>
154<tr>
155<td>Microsoft Visual C++ 6 SP5 and 7</td>
156<td>X</td>
157<td> </td>
158<td> </td>
159<td> </td>
160</tr>
161</tbody>
162</table></div>
163<p>Finally, the current state of the testsuite in CVS may be found on the
164    <a href="http://boost.sourceforge.net/regression-logs" target="_top">Test Summary</a>
165    page. Please note, however, that this page reports on day-to-day changes
166    to inter-release code found in the Boost CVS and thus likely does not
167    match the state of code found in Boost releases.</p>
168</div>
169<div class="section" lang="en">
170<div class="titlepage"><div><div><h4 class="title">
171<a name="variant.troubleshooting"></a>Troubleshooting</h4></div></div></div>
172<div class="toc"><dl>
173<dt><span class="section"><a href="misc.html#variant.troubleshooting.template-depth">"Template instantiation depth exceeds maximum"</a></span></dt>
174<dt><span class="section"><a href="misc.html#variant.troubleshooting.compiler-memory">"Internal heap limit reached"</a></span></dt>
175</dl></div>
176<p>Due to the heavy use of templates in the implementation of
177  <code class="computeroutput">variant</code>, it is not uncommon when compiling to encounter
178  problems related to template instantiaton depth, compiler memory, etc. This
179  section attempts to provide advice to common problems experienced on several
180  popular compilers.</p>
181<p>(This section is still in progress, with additional advice/feedback
182  welcome. Please post to the Boost-Users list with any useful experiences you
183  may have.)</p>
184<div class="section" lang="en">
185<div class="titlepage"><div><div><h5 class="title">
186<a name="variant.troubleshooting.template-depth"></a>"Template instantiation depth exceeds maximum"</h5></div></div></div>
187<div class="toc"><dl><dt><span class="section"><a href="misc.html#variant.troubleshooting.template-depth.gcc">GNU GCC</a></span></dt></dl></div>
188<div class="section" lang="en">
189<div class="titlepage"><div><div><h6 class="title">
190<a name="variant.troubleshooting.template-depth.gcc"></a>GNU GCC</h6></div></div></div>
191<p>The compiler option
192        <code class="computeroutput">-ftemplate-depth-<span class="emphasis"><em>NN</em></span></code> can increase the
193        maximum allowed instantiation depth. (Try
194        <code class="computeroutput">-ftemplate-depth-50</code>.)</p>
195</div>
196</div>
197<div class="section" lang="en">
198<div class="titlepage"><div><div><h5 class="title">
199<a name="variant.troubleshooting.compiler-memory"></a>"Internal heap limit reached"</h5></div></div></div>
200<div class="toc"><dl><dt><span class="section"><a href="misc.html#variant.troubleshooting.compiler-memory.msvc">Microsoft Visual C++</a></span></dt></dl></div>
201<div class="section" lang="en">
202<div class="titlepage"><div><div><h6 class="title">
203<a name="variant.troubleshooting.compiler-memory.msvc"></a>Microsoft Visual C++</h6></div></div></div>
204<p>The compiler option <code class="computeroutput">/Zm<span class="emphasis"><em>NNN</em></span></code> can
205        increase the memory allocation limit. The <code class="computeroutput">NNN</code> is a
206        scaling percentage (i.e., <code class="computeroutput">100</code> denotes the default limit).
207        (Try <code class="computeroutput">/Zm200</code>.)</p>
208</div>
209</div>
210</div>
211<div class="section" lang="en">
212<div class="titlepage"><div><div><h4 class="title">
213<a name="variant.ack"></a>Acknowledgments</h4></div></div></div>
214<p>Eric Friedman and Itay Maman designed the initial submission; Eric was
215  the primary implementer.</p>
216<p>Eric is also the library maintainer and has expanded upon the initial
217  submission -- adding
218  <code class="computeroutput"><a href="../make_recursive_variant.html" title="Class template make_recursive_variant">make_recursive_variant</a></code>,
219  <code class="computeroutput"><a href="../make_variant_over.html" title="Class template make_variant_over">make_variant_over</a></code>, support for
220  reference content, etc.</p>
221<p>Andrei Alexandrescu's work in
222    [<a href="refs.html#variant.refs.ale01a">Ale01a</a>]
223and
224    [<a href="refs.html#variant.refs.ale02">Ale02</a>]
225inspired the library's design.</p>
226<p>Jeff Garland was the formal review manager.</p>
227<p>Douglas Gregor,
228Dave Abrahams,
229Anthony Williams,
230Fernando Cacciola,
231Joel de Guzman,
232Dirk Schreib,
233Brad King,
234Giovanni Bajo,
235Eugene Gladyshev,
236and others provided helpful feedback and suggestions to refine the semantics,
237interface, and implementation of the library.</p>
238</div>
239</div>
240<table width="100%"><tr>
241<td align="left"></td>
242<td align="right"><small>Copyright © 2002, 2003 Eric Friedman, Itay Maman</small></td>
243</tr></table>
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