boost_1_45_0/libs/variant/doc/misc.xml

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<!DOCTYPE section PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
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<section id="variant.misc">
  <title>Miscellaneous Notes</title>

<using-namespace name="boost"/>

<section id="variant.versus-any">
  <title>Boost.Variant vs. Boost.Any</title>

  <para>As a discriminated union container, the Variant library shares many
    of the same features of the <libraryname>Any</libraryname> library.
    However, since neither library wholly encapsulates the features of the
    other, one library cannot be generally recommended for use over the
    other.</para>

  <para>That said, Boost.Variant has several advantages over Boost.Any,
    such as:

    <itemizedlist>
      <listitem>Boost.Variant guarantees the type of its content is one of a
        finite, user-specified set of types.</listitem>
      <listitem>Boost.Variant provides <emphasis>compile-time</emphasis>
        checked visitation of its content. (By contrast, the current version
        of Boost.Any provides no visitation mechanism at all; but even if it
        did, it would need to be checked at run-time.)</listitem>
      <listitem>Boost.Variant enables generic visitation of its content.
        (Even if Boost.Any did provide a visitation mechanism, it would enable
        visitation only of explicitly-specified types.)</listitem>
      <listitem>Boost.Variant offers an efficient, stack-based storage scheme
        (avoiding the overhead of dynamic allocation).</listitem>
    </itemizedlist>

  </para>

  <para>Of course, Boost.Any has several advantages over Boost.Variant,
    such as:

    <itemizedlist>
      <listitem>Boost.Any, as its name implies, allows virtually any type for
        its content, providing great flexibility.</listitem>
      <listitem>Boost.Any provides the no-throw guarantee of exception safety
        for its swap operation.</listitem>
      <listitem>Boost.Any makes little use of template metaprogramming
        techniques (avoiding potentially hard-to-read error messages and
        significant compile-time processor and memory demands).</listitem>
    </itemizedlist>

  </para>

</section>

<section>
  <title>Portability</title>

  <para>The library aims for 100% ANSI/ISO C++ conformance. However, this is
    strictly impossible due to the inherently non-portable nature of the
    <libraryname>Type Traits</libraryname> library's
    <code><classname>type_with_alignment</classname></code> facility. In
    practice though, no compilers or platforms have been discovered where this
    reliance on undefined behavior has been an issue.</para>

  <para>Additionally, significant effort has been expended to ensure proper
    functioning despite various compiler bugs and other conformance problems.
    To date the library testsuite has
    been compiled and tested successfully on at least the following compilers
    for basic and advanced functionality:

    <informaltable>
      <tgroup cols="5">
        <thead>
          <row>
            <entry></entry>
            <entry>Basic</entry>
            <entry>
              <code>variant&lt;T&amp;&gt;</code>
            </entry>
            <entry>
              <link linkend="variant.tutorial.over-sequence">
                <code>make_variant_over</code>
              </link>
            </entry>
            <entry>
              <link linkend="variant.tutorial.recursive.recursive-variant">
                <code>make_recursive_variant</code>
              </link>
            </entry>
          </row>
        </thead>
        <tbody>
          <row>
            <entry>Borland C++ 5.5.1 and 5.6.4</entry>
            <entry>X</entry>
            <entry>X</entry>
            <entry></entry>
            <entry></entry>
          </row>
          <row>
            <entry>Comeau C++ 4.3.0</entry>
            <entry>X</entry>
            <entry>X</entry>
            <entry>X</entry>
            <entry>X</entry>
          </row>
          <row>
            <entry>GNU GCC 3.3.1</entry>
            <entry>X</entry>
            <entry>X</entry>
            <entry>X</entry>
            <entry>X</entry>
          </row>
          <row>
            <entry>GNU GCC 2.95.3</entry>
            <entry>X</entry>
            <entry>X</entry>
            <entry></entry>
            <entry>X</entry>
          </row>
          <row>
            <entry>Intel C++ 7.0</entry>
            <entry>X</entry>
            <entry></entry>
            <entry>X</entry>
            <entry>X</entry>
          </row>
          <row>
            <entry>Metrowerks CodeWarrior 8.3</entry>
            <entry>X</entry>
            <entry></entry>
            <entry>X</entry>
            <entry>X</entry>
          </row>
          <row>
            <entry>Microsoft Visual C++ 7.1</entry>
            <entry>X</entry>
            <entry>X</entry>
            <entry>X</entry>
            <entry>X</entry>
          </row>
          <row>
            <entry>Microsoft Visual C++ 6 SP5 and 7</entry>
            <entry>X</entry>
            <entry></entry>
            <entry></entry>
            <entry></entry>
          </row>
        </tbody>
      </tgroup>
    </informaltable>

  </para>

  <para>Finally, the current state of the testsuite in CVS may be found on the
    <ulink url="http://boost.sourceforge.net/regression-logs">Test Summary</ulink>
    page. Please note, however, that this page reports on day-to-day changes
    to inter-release code found in the Boost CVS and thus likely does not
    match the state of code found in Boost releases.</para>

</section>

<section id="variant.troubleshooting">
  <title>Troubleshooting</title>

<para>Due to the heavy use of templates in the implementation of
  <code>variant</code>, it is not uncommon when compiling to encounter
  problems related to template instantiaton depth, compiler memory, etc. This
  section attempts to provide advice to common problems experienced on several
  popular compilers.</para>
  
<para>(This section is still in progress, with additional advice/feedback
  welcome. Please post to the Boost-Users list with any useful experiences you
  may have.)</para>

  <section id="variant.troubleshooting.template-depth">
    <title>&quot;Template instantiation depth exceeds maximum&quot;</title>
    
    <section id="variant.troubleshooting.template-depth.gcc">
      <title>GNU GCC</title>
      <para>The compiler option
        <code>-ftemplate-depth-<emphasis>NN</emphasis></code> can increase the
        maximum allowed instantiation depth. (Try
        <code>-ftemplate-depth-50</code>.)</para>
    </section>
  </section>
  
  <section id="variant.troubleshooting.compiler-memory">
    <title>&quot;Internal heap limit reached&quot;</title>
    
    <section id="variant.troubleshooting.compiler-memory.msvc">
      <title>Microsoft Visual C++</title>
      <para>The compiler option <code>/Zm<emphasis>NNN</emphasis></code> can
        increase the memory allocation limit. The <code>NNN</code> is a
        scaling percentage (i.e., <code>100</code> denotes the default limit).
        (Try <code>/Zm200</code>.)</para>
    </section>
  </section>

</section>

<section id="variant.ack">
  <title>Acknowledgments</title>

<para>Eric Friedman and Itay Maman designed the initial submission; Eric was
  the primary implementer.</para>

<para>Eric is also the library maintainer and has expanded upon the initial
  submission -- adding
  <code><classname>make_recursive_variant</classname></code>,
  <code><classname>make_variant_over</classname></code>, support for
  reference content, etc.</para>

<para>Andrei Alexandrescu's work in
    [<link linkend="variant.refs.ale01a">Ale01a</link>]
and
    [<link linkend="variant.refs.ale02">Ale02</link>]
inspired the library's design.</para>

<para>Jeff Garland was the formal review manager.</para>

<para>Douglas Gregor,
Dave Abrahams,
Anthony Williams,
Fernando Cacciola,
Joel de Guzman,
Dirk Schreib,
Brad King,
Giovanni Bajo,
Eugene Gladyshev,
and others provided helpful feedback and suggestions to refine the semantics,
interface, and implementation of the library.</para>

</section>

</section>