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<h1><img src="../../boost.png" alt="C++ Boost" align="middle"
width="277" height="86">Base-from-Member Idiom</h1>

<p>The class template <code>boost::base_from_member</code> provides
a workaround for a class that needs to initialize a base class with a
member.  The class template is in <cite><a
href="../../boost/utility/base_from_member.hpp">boost/utility/base_from_member.hpp</a></cite>
which is included in <i><a href="../../boost/utility.hpp">boost/utility.hpp</a></i>.</p>

<p>There is test/example code in <cite><a
href="base_from_member_test.cpp">base_from_member_test.cpp</a></cite>.</p>

<h2><a name="contents">Contents</a></h2>

<ul>
	<li><a href="#contents">Contents</a></li>
	<li><a href="#rationale">Rationale</a></li>
	<li><a href="#synopsis">Synopsis</a></li>
	<li><a href="#usage">Usage</a></li>
	<li><a href="#example">Example</a></li>
	<li><a href="#credits">Credits</a>
		<ul>
			<li><a href="#contributors">Contributors</a></li>
		</ul></li>
</ul>

<h2><a name="rationale">Rationale</a></h2>

<p>When developing a class, sometimes a base class needs to be
initialized with a member of the current class.  As a na&iuml;ve
example:</p>

<blockquote><pre>
#include &lt;streambuf&gt;  <i>// for std::streambuf</i>
#include &lt;ostream&gt;    <i>// for std::ostream</i>

class fdoutbuf
    : public std::streambuf
{
public:
    explicit fdoutbuf( int fd );
    //...
};

class fdostream
    : public std::ostream
{
protected:
    fdoutbuf buf;
public:
    explicit fdostream( int fd )
        : buf( fd ), std::ostream( &amp;buf )
        {}
    //...
};
</pre></blockquote>

<p>This is undefined because C++'s initialization order mandates that
the base class is initialized before the member it uses.  <a
href="http://www.moocat.org">R. Samuel Klatchko</a> developed a way
around this by using the initialization order in his favor.  Base
classes are intialized in order of declaration, so moving the desired
member to another base class, that is initialized before the desired
base class, can ensure proper initialization.</p>

<p>A custom base class can be made for this idiom:</p>

<blockquote><pre>
#include &lt;streambuf&gt;  <i>// for std::streambuf</i>
#include &lt;ostream&gt;    <i>// for std::ostream</i>

class fdoutbuf
    : public std::streambuf
{
public:
    explicit fdoutbuf( int fd );
    //...
};

struct fdostream_pbase
{
    fdoutbuf sbuffer;

    explicit fdostream_pbase( int fd )
        : sbuffer( fd )
        {}
};

class fdostream
    : private fdostream_pbase
    , public std::ostream
{
    typedef fdostream_pbase  pbase_type;
    typedef std::ostream     base_type;

public:
    explicit fdostream( int fd )
        : pbase_type( fd ), base_type( &amp;sbuffer )
        {}
    //...
};
</pre></blockquote>

<p>Other projects can use similar custom base classes.  The technique
is basic enough to make a template, with a sample template class in
this library.  The main template parameter is the type of the enclosed
member.  The template class has several (explicit) constructor member
templates, which implicitly type the constructor arguments and pass them
to the member.  The template class uses implicit copy construction and
assignment, cancelling them if the enclosed member is non-copyable.</p>

<p>Manually coding a base class may be better if the construction
and/or copying needs are too complex for the supplied template class,
or if the compiler is not advanced enough to use it.</p>

<p>Since base classes are unnamed, a class cannot have multiple (direct)
base classes of the same type.  The supplied template class has an
extra template parameter, an integer, that exists solely to provide type
differentiation.  This parameter has a default value so a single use of a
particular member type does not need to concern itself with the integer.</p>

<h2><a name="synopsis">Synopsis</a></h2>

<blockquote><pre>
#ifndef BOOST_BASE_FROM_MEMBER_MAX_ARITY
#define BOOST_BASE_FROM_MEMBER_MAX_ARITY  10
#endif

template &lt; typename MemberType, int UniqueID = 0 &gt;
class boost::base_from_member
{
protected:
    MemberType  member;

    base_from_member();

    template&lt; typename T1 &gt;
    explicit  base_from_member( T1 x1 );

    template&lt; typename T1, typename T2 &gt;
    base_from_member( T1 x1, T2 x2 );

    //...

    template&lt; typename T1, typename T2, typename T3, typename T4,
     typename T5, typename T6, typename T7, typename T8, typename T9,
     typename T10 &gt;
    base_from_member( T1 x1, T2 x2, T3 x3, T4 x4, T5 x5, T6 x6, T7 x7,
     T8 x8, T9 x9, T10 x10 );
};
</pre></blockquote>

<p>The class template has a first template parameter
<var>MemberType</var> representing the type of the based-member.
It has a last template parameter <var>UniqueID</var>, that is an
<code>int</code>, to differentiate between multiple base classes that use
the same based-member type.  The last template parameter has a default
value of zero if it is omitted.  The class template has a protected
data member called <var>member</var> that the derived class can use
for later base classes (or itself).</p>

<p>There is a default constructor and several constructor member
templates.  These constructor templates can take as many arguments
(currently up to ten) as possible and pass them to a constructor of
the data member.  Since C++ does not allow any way to explicitly state
the template parameters of a templated constructor, make sure that
the arguments are already close as possible to the actual type used in
the data member's desired constructor.</p>

<p>The <var>BOOST_BASE_FROM_MEMBER_MAX_ARITY</var> macro constant specifies
the maximum argument length for the constructor templates.  The constant
may be overridden if more (or less) argument configurations are needed.  The
constant may be read for code that is expandable like the class template and
needs to maintain the same maximum size.  (Example code would be a class that
uses this class template as a base class for a member with a flexible set of
constructors.)</p>

<h2><a name="usage">Usage</a></h2>

<p>With the starting example, the <code>fdoutbuf</code> sub-object needs
to be encapsulated in a base class that is inheirited before
<code>std::ostream</code>.</p>

<blockquote><pre>
#include &lt;boost/utility/base_from_member.hpp&gt;

#include &lt;streambuf&gt;  <i>// for std::streambuf</i>
#include &lt;ostream&gt;    <i>// for std::ostream</i>

class fdoutbuf
    : public std::streambuf
{
public:
    explicit fdoutbuf( int fd );
    //...
};

class fdostream
    : private boost::base_from_member&lt;fdoutbuf&gt;
    , public std::ostream
{
    // Helper typedef's
    typedef boost::base_from_member&lt;fdoutbuf&gt;  pbase_type;
    typedef std::ostream                        base_type;

public:
    explicit fdostream( int fd )
        : pbase_type( fd ), base_type( &amp;member )
        {}
    //...
};
</pre></blockquote>

<p>The base-from-member idiom is an implementation detail, so it
should not be visible to the clients (or any derived classes) of
<code>fdostream</code>.  Due to the initialization order, the
<code>fdoutbuf</code> sub-object will get initialized before the
<code>std::ostream</code> sub-object does, making the former
sub-object safe to use in the latter sub-object's construction.  Since the
<code>fdoutbuf</code> sub-object of the final type is the only sub-object
with the name &quot;member,&quot; that name can be used
unqualified within the final class.</p>

<h2><a name="example">Example</a></h2>

<p>The base-from-member class templates should commonly involve
only one base-from-member sub-object, usually for attaching a
stream-buffer to an I/O stream.  The next example demonstrates how
to use multiple base-from-member sub-objects and the resulting
qualification issues.</p>

<blockquote><pre>
#include &lt;boost/utility/base_from_member.hpp&gt;

#include &lt;cstddef&gt;  <i>// for NULL</i>

struct an_int
{
    int  y;

    an_int( float yf );
};

class switcher
{
public:
    switcher();
    switcher( double, int * );
    //...
};

class flow_regulator
{
public:
    flow_regulator( switcher &amp;, switcher &amp; );
    //...
};

template &lt; unsigned Size &gt;
class fan
{
public:
    explicit fan( switcher );
    //...
};

class system
    : private boost::base_from_member&lt;an_int&gt;
    , private boost::base_from_member&lt;switcher&gt;
    , private boost::base_from_member&lt;switcher, 1&gt;
    , private boost::base_from_member&lt;switcher, 2&gt;
    , protected flow_regulator
    , public fan&lt;6&gt;
{
    // Helper typedef's
    typedef boost::base_from_member&lt;an_int&gt;       pbase0_type;
    typedef boost::base_from_member&lt;switcher&gt;     pbase1_type;
    typedef boost::base_from_member&lt;switcher, 1&gt;  pbase2_type;
    typedef boost::base_from_member&lt;switcher, 2&gt;  pbase3_type;

    typedef flow_regulator  base1_type;
    typedef fan&lt;6&gt;          base2_type;

public:
    system( double x );
    //...
};

system::system( double x )
    : pbase0_type( 0.2 )
    , pbase1_type()
    , pbase2_type( -16, &amp;this-&gt;pbase0_type::member )
    , pbase3_type( x, static_cast&lt;int *&gt;(NULL) )
    , base1_type( pbase3_type::member, pbase1_type::member )
    , base2_type( pbase2_type::member )
{
    //...
}
</pre></blockquote>

<p>The final class has multiple sub-objects with the name
&quot;member,&quot; so any use of that name needs qualification by
a name of the appropriate base type.  (Using <code>typedef</code>s
ease mentioning the base types.)  However, the fix introduces a new
problem when a pointer is needed.  Using the address operator with
a sub-object qualified with its class's name results in a pointer-to-member
(here, having a type of <code>an_int boost::base_from_member&lt;an_int,
0&gt; :: *</code>) instead of a pointer to the member (having a type of
<code>an_int *</code>).  The new problem is fixed by qualifying the
sub-object with &quot;<code>this-&gt;</code>,&quot; and is needed just
for pointers, and not for references or values.</p>

<p>There are some argument conversions in the initialization.  The
constructor argument for <code>pbase0_type</code> is converted from
<code>double</code> to <code>float</code>.  The first constructor
argument for <code>pbase2_type</code> is converted from <code>int</code>
to <code>double</code>.  The second constructor argument for
<code>pbase3_type</code> is a special case of necessary conversion; all
forms of the null-pointer literal in C++ also look like compile-time
integral expressions, so C++ always interprets such code as an integer
when it has overloads that can take either an integer or a pointer.  The
last conversion is necessary for the compiler to call a constructor form
with the exact pointer type used in <code>switcher</code>'s constructor.</p>

<h2><a name="credits">Credits</a></h2>

<h3><a name="contributors">Contributors</a></h3>

<dl>
	<dt><a href="http://www.boost.org/people/ed_brey.htm">Ed Brey</a>
	<dd>Suggested some interface changes.

	<dt><a href="http://www.moocat.org">R. Samuel Klatchko</a> (<a
	href="mailto:rsk@moocat.org">rsk@moocat.org</a>, <a
	href="mailto:rsk@brightmail.com">rsk@brightmail.com</a>)
	<dd>Invented the idiom of how to use a class member for initializing
		a base class.

	<dt><a href="http://www.boost.org/people/dietmar_kuehl.htm">Dietmar Kuehl</a>
	<dd>Popularized the base-from-member idiom in his
		<a href="http://www.informatik.uni-konstanz.de/~kuehl/c++/iostream/">IOStream
		example classes</a>.

	<dt>Jonathan Turkanis
	<dd>Supplied an implementation of generating the constructor templates that
		can be controlled and automated with macros.  The implementation uses
		the <a href="../preprocessor/index.html">Preprocessor library</a>.

	<dt><a href="http://www.boost.org/people/daryle_walker.html">Daryle Walker</a>
	<dd>Started the library.  Contributed the test file <cite><a
		href="base_from_member_test.cpp">base_from_member_test.cpp</a></cite>.
</dl>

<hr>

<p>Revised: 28 August 2004</p>

<p>Copyright 2001, 2003, 2004 Daryle Walker.  Use, modification, and distribution
are subject to the Boost Software License, Version 1.0.  (See accompanying
file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or a copy at &lt;<a
href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>&gt;.)</p>

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