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200 lines
7.0 KiB
XML
200 lines
7.0 KiB
XML
<?xml version="1.0" encoding="utf-8"?>
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<!DOCTYPE section PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
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"http://www.boost.org/tools/boostbook/dtd/boostbook.dtd">
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<section id="variant.misc">
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<title>Miscellaneous Notes</title>
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<using-namespace name="boost"/>
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<section id="variant.versus-any">
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<title>Boost.Variant vs. Boost.Any</title>
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<para>As a discriminated union container, the Variant library shares many
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of the same features of the <libraryname>Any</libraryname> library.
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However, since neither library wholly encapsulates the features of the
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other, one library cannot be generally recommended for use over the
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other.</para>
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<para>That said, Boost.Variant has several advantages over Boost.Any,
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such as:
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<itemizedlist>
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<listitem>Boost.Variant guarantees the type of its content is one of a
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finite, user-specified set of types.</listitem>
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<listitem>Boost.Variant provides <emphasis>compile-time</emphasis>
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type-safe visitation of its content. (By contrast, the current version
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of Boost.Any provides no visitation mechanism at all.)</listitem>
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<listitem>Boost.Variant enables generic visitation of its content.
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(Even if Boost.Any did provide a visitation mechanism, it would enable
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visitation only of explicitly-specified types.)</listitem>
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<listitem>Boost.Variant offers an efficient, stack-based storage scheme
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(avoiding the overhead of dynamic allocation).</listitem>
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</itemizedlist>
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</para>
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<para>Of course, Boost.Any has several advantages over Boost.Variant,
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such as:
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<itemizedlist>
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<listitem>Boost.Any, as its name implies, allows virtually any type for
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its content, providing great flexibility.</listitem>
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<listitem>Boost.Any provides the no-throw guarantee of exception safety
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for its swap operation.</listitem>
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<listitem>Boost.Any makes little use of template metaprogramming
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techniques (avoiding potentially hard-to-read error messages and
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significant compile-time processor and memory demands).</listitem>
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</itemizedlist>
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</para>
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</section>
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<section>
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<title>Portability</title>
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<para>The library aims for 100% ANSI/ISO C++ conformance. However, this is
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strictly impossible due to the inherently non-portable nature of the
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<libraryname>Type Traits</libraryname> library's
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<code><classname>type_with_alignment</classname></code> facility. In
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practice though, no compilers or platforms have been discovered where this
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reliance on undefined behavior has been an issue.</para>
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<para>Additionally, significant effort has been expended to ensure proper
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functioning despite various compiler bugs and other conformance problems.
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To date the library <link linkend="variant.tests">testsuite</link> has
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been compiled and tested successfully on at least the following compilers
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for basic and advanced functionality:
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<informaltable>
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<tgroup cols="5">
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<thead>
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<row>
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<entry></entry>
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<entry>Basic</entry>
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<entry>
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<code>variant<T&></code>
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</entry>
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<entry>
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<link linkend="variant.tutorial.over-sequence">
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<code>make_variant_over</code>
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</link>
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</entry>
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<entry>
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<link linkend="variant.tutorial.recursive.recursive-variant">
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<code>make_recursive_variant</code>
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</link>
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</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>Borland C++ 5.5.1 and 5.6.4</entry>
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<entry>X</entry>
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<entry>X</entry>
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<entry></entry>
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<entry></entry>
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</row>
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<row>
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<entry>Comeau C++ 4.3.0</entry>
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<entry>X</entry>
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<entry>X</entry>
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<entry>X</entry>
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<entry>X</entry>
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</row>
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<row>
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<entry>GNU GCC 3.3.1</entry>
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<entry>X</entry>
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<entry>X</entry>
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<entry>X</entry>
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<entry>X</entry>
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</row>
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<row>
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<entry>GNU GCC 2.95.3</entry>
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<entry>X</entry>
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<entry>X</entry>
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<entry></entry>
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<entry>X</entry>
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</row>
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<row>
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<entry>Intel C++ 7.0</entry>
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<entry>X</entry>
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<entry></entry>
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<entry>X</entry>
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<entry>X</entry>
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</row>
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<row>
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<entry>Metrowerks CodeWarrior 8.3</entry>
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<entry>X</entry>
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<entry></entry>
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<entry>X</entry>
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<entry>X</entry>
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</row>
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<row>
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<entry>Microsoft Visual C++ 7.1</entry>
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<entry>X</entry>
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<entry>X</entry>
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<entry>X</entry>
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<entry>X</entry>
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</row>
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<row>
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<entry>Microsoft Visual C++ 6 SP5 and 7</entry>
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<entry>X</entry>
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<entry></entry>
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<entry></entry>
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<entry></entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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</para>
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<para>Finally, the current state of the testsuite in CVS may be found on the
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<ulink url="http://boost.sourceforge.net/regression-logs">Test Summary</ulink>
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page. Please note, however, that this page reports on day-to-day changes
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to inter-release code found in the Boost CVS and thus likely does not
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match the state of code found in Boost releases.</para>
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</section>
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<section id="variant.troubleshooting">
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<title>Troubleshooting</title>
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<para>Due to the heavy use of templates in the implementation of
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<code>variant</code>, it is not uncommon when compiling to encounter
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problems related to template instantiaton depth, compiler memory, etc. This
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section attempts to provide advice to common problems experienced on several
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popular compilers.</para>
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<para>(This section is still in progress, with additional advice/feedback
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welcome. Please post to the Boost-Users list with any useful experiences you
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may have.)</para>
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<section id="variant.troubleshooting.template-depth">
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<title>"Template instantiation depth exceeds maximum"</title>
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<section id="variant.troubleshooting.template-depth.gcc">
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<title>GNU GCC</title>
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<para>The compiler option
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<code>-ftemplate-depth-<emphasis>NN</emphasis></code> can increase the
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maximum allowed instantiation depth. (Try
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<code>-ftemplate-depth-50</code>.)</para>
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</section>
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</section>
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<section id="variant.troubleshooting.compiler-memory">
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<title>"Internal heap limit reached"</title>
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<section id="variant.troubleshooting.compiler-memory.msvc">
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<title>Microsoft Visual C++</title>
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<para>The compiler option <code>/Zm<emphasis>NNN</emphasis></code> can
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increase the memory allocation limit. The <code>NNN</code> is a
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scaling percentage (i.e., <code>100</code> denotes the default limit).
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(Try <code>/Zm200</code>.)</para>
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</section>
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</section>
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</section>
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</section> |