boost_1_45_0/libs/proto/doc/reference/extends.xml

<?xml version="1.0" encoding="utf-8"?>
<header name="boost/proto/extends.hpp">
  <para>Macros and a base class for defining end-user expression types </para>
  <namespace name="boost">
    <namespace name="proto">

      <!-- proto::is_proto_expr -->
      <struct name="is_proto_expr">
        <purpose>Empty type to be used as a dummy template parameter of POD expression wrappers. It allows
          argument-dependent lookup to find Proto's operator overloads.</purpose>
        <description>
          <para>
            <computeroutput>proto::is_proto_expr</computeroutput> allows argument-dependent lookup to find Proto's operator overloads. For example:
          </para>
          <para>
            <programlisting> template&lt;typename T, typename Dummy = <classname>proto::is_proto_expr</classname>&gt;
 struct my_terminal
 {
     <macroname>BOOST_PROTO_BASIC_EXTENDS</macroname>(
         typename <classname>proto::terminal</classname>&lt;T&gt;::type
       , my_terminal&lt;T&gt;
       , <classname>proto::default_domain</classname>
     )
 };

 // ...
 my_terminal&lt;int&gt; _1, _2;
 _1 + _2; // OK, uses proto::operator+</programlisting>
          </para>
          <para>
            Without the second <computeroutput>Dummy</computeroutput> template parameter, Proto's operator overloads
            would not be considered by name lookup.
          </para>
        </description>
      </struct>

      <!-- proto::extends -->
      <struct name="extends">
        <template>
          <template-type-parameter name="Expr"/>
          <template-type-parameter name="Derived"/>
          <template-type-parameter name="Domain">
            <default><classname>proto::default_domain</classname></default>
          </template-type-parameter>
        </template>
        <purpose>For adding behaviors to a Proto expression template.</purpose>
        <description>
          <para>
            Use <computeroutput>proto::extends&lt;&gt;</computeroutput> to give expressions in your
            domain custom data members and member functions.
          </para>
          <para>
            Conceptually, using <computeroutput>proto::extends&lt;&gt;</computeroutput> is akin
            to inheriting from <computeroutput><classname>proto::expr</classname>&lt;&gt;</computeroutput>
            and adding your own members. Using <computeroutput>proto::extends&lt;&gt;</computeroutput> is
            generally preferrable to straight inheritance because the members that would be inherited from
            <computeroutput><classname>proto::expr</classname>&lt;&gt;</computeroutput> would
            be wrong; they would incorrectly slice off your additional members when building
            larger expressions from smaller ones. <computeroutput>proto::extends&lt;&gt;</computeroutput>
            automatically gives your expression types the appropriate operator overloads that
            preserve your domain-specific members when composing expression trees.
          </para>
          <para>
            Expression extensions are typically defined as follows:
          </para>
          <para>
            <programlisting>template&lt; typename Expr &gt;
struct my_expr
  : proto::extends&lt;
        Expr            // The expression type we're extending
      , my_expr&lt; Expr &gt; // The type we're defining
      , my_domain       // The domain associated with this expression extension
    &gt;
{
    typedef proto::extends&lt; Expr, my_expr&lt; Expr &gt;, my_domain &gt; base_type;
    
    // An expression extension is constructed from the expression
    // it is extending.
    my_expr( Expr const &amp; e = Expr() )
      : base_type( e )
    {}
    
    // Unhide proto::extends::operator=
    // (This is only necessary if a lazy assignment operator
    // makes sense for your domain-specific language.)
    BOOST_PROTO_EXTENDS_USING_ASSIGN(my_expr)
    
    /*
    ... domain-specific members go here ...
    */
};</programlisting>
          </para>
          <para>
            See also:
            <itemizedlist>
              <listitem>
                <computeroutput><macroname>BOOST_PROTO_EXTENDS</macroname>()</computeroutput>
              </listitem>
              <listitem>
                <computeroutput><macroname>BOOST_PROTO_EXTENDS_USING_ASSIGN</macroname>()</computeroutput>
              </listitem>
              <listitem>
                <computeroutput><macroname>BOOST_PROTO_EXTENDS_USING_ASSIGN_NON_DEPENDENT</macroname>()</computeroutput>
              </listitem>
            </itemizedlist>
          </para>
        </description>
        <struct name="result">
          <template>
            <template-type-parameter name="Signature"/>
          </template>
          <typedef name="type">
            <type><replaceable>unspecified</replaceable></type>
          </typedef>
          <description>
            <para>So that <computeroutput>boost::result_of&lt;&gt;</computeroutput>
            can compute the return type of <computeroutput>proto::extends::operator()</computeroutput>.
          </para>
          </description>
        </struct>
        <typedef name="proto_base_expr">
          <type>typename Expr::proto_base_expr</type>
        </typedef>
        <typedef name="proto_domain">
          <type>Domain</type>
        </typedef>
        <typedef name="proto_derived_expr">
          <type>Derived</type>
        </typedef>
        <typedef name="proto_tag">
          <type>typename proto_base_expr::proto_tag</type>
        </typedef>
        <typedef name="proto_args">
          <type>typename proto_base_expr::proto_args</type>
        </typedef>
        <typedef name="proto_arity">
          <type>typename proto_base_expr::proto_arity</type>
        </typedef>
        <typedef name="proto_grammar">
          <type>typename proto_base_expr::proto_grammar</type>
        </typedef>
        <typedef name="proto_childN">
          <purpose>For each <replaceable>N</replaceable> in <replaceable>[0,max(1,proto_arity_c))</replaceable></purpose>
          <type>typename proto_base_expr::proto_child<replaceable>N</replaceable></type>
        </typedef>

        <!-- constructors -->
        <constructor/>
        <constructor>
          <parameter name="that">
            <paramtype><classname>extends</classname> const &amp;</paramtype>
          </parameter>
        </constructor>
        <constructor>
          <parameter name="expr_">
            <paramtype>Expr const &amp;</paramtype>
          </parameter>
        </constructor>

        <method-group name="public static functions">
          <method name="make" specifiers="static">
            <type>Derived const</type>
            <parameter name="expr">
              <paramtype>Expr const &amp;</paramtype>
            </parameter>
            <description>
              <para>Construct an expression extension from the base expression.</para>
            </description>
            <return>Derived(expr)</return>
          </method>
        </method-group>

        <method-group name="public member functions">

          <!-- proto_base() -->
          <method name="proto_base">
            <type>proto_base_expr &amp;</type>
            <returns><computeroutput>proto_expr_.proto_base()</computeroutput></returns>
            <throws><simpara>Will not throw.</simpara></throws>
          </method>
          <method name="proto_base" cv="const">
            <type>proto_base_expr const &amp;</type>
            <returns><computeroutput>proto_expr_.proto_base()</computeroutput></returns>
            <throws><simpara>Will not throw.</simpara></throws>
          </method>

          <!-- operator= -->
          <method name="operator=">
            <type><replaceable>unspecified</replaceable></type>
            <template>
              <template-type-parameter name="A"/>
            </template>
            <parameter name="a">
              <paramtype>A &amp;</paramtype>
            </parameter>
            <description>
              <para>Lazy assignment expression</para>
            </description>
            <returns>
              <para>A new expression node representing the assignment operation.</para>
            </returns>
          </method>

          <method name="operator=">
            <type><replaceable>unspecified</replaceable></type>
            <template>
              <template-type-parameter name="A"/>
            </template>
            <parameter name="a">
              <paramtype>A const &amp;</paramtype>
            </parameter>
            <description>
              <para>
                This is an overloaded member function, provided for convenience. It differs from
                the above function only in what argument(s) it accepts.
              </para>
            </description>
          </method>

          <method name="operator=" cv="const">
            <type><replaceable>unspecified</replaceable></type>
            <template>
              <template-type-parameter name="A"/>
            </template>
            <parameter name="a">
              <paramtype>A &amp;</paramtype>
            </parameter>
            <description>
              <para>
                This is an overloaded member function, provided for convenience. It differs from
                the above function only in what argument(s) it accepts.
              </para>
            </description>
          </method>

          <method name="operator=" cv="const">
            <type><replaceable>unspecified</replaceable></type>
            <template>
              <template-type-parameter name="A"/>
            </template>
            <parameter name="a">
              <paramtype>A const &amp;</paramtype>
            </parameter>
            <description>
              <para>
                This is an overloaded member function, provided for convenience. It differs from
                the above function only in what argument(s) it accepts.
              </para>
            </description>
          </method>

          <!-- operator[] -->
          <method name="operator[]">
            <type><replaceable>unspecified</replaceable></type>
            <template>
              <template-type-parameter name="A"/>
            </template>
            <parameter name="a">
              <paramtype>A &amp;</paramtype>
            </parameter>
            <description>
              <para>Lazy subscript expression</para>
            </description>
            <returns>
              <para>A new expression node representing the subscript operation.</para>
            </returns>
          </method>

          <method name="operator[]">
            <type><replaceable>unspecified</replaceable></type>
            <template>
              <template-type-parameter name="A"/>
            </template>
            <parameter name="a">
              <paramtype>A const &amp;</paramtype>
            </parameter>
            <description>
              <para>
                This is an overloaded member function, provided for convenience. It differs from
                the above function only in what argument(s) it accepts.
              </para>
            </description>
          </method>

          <method name="operator[]" cv="const">
            <type><replaceable>unspecified</replaceable></type>
            <template>
              <template-type-parameter name="A"/>
            </template>
            <parameter name="a">
              <paramtype>A &amp;</paramtype>
            </parameter>
            <description>
              <para>
                This is an overloaded member function, provided for convenience. It differs from
                the above function only in what argument(s) it accepts.
              </para>
            </description>
          </method>

          <method name="operator[]" cv="const">
            <type><replaceable>unspecified</replaceable></type>
            <template>
              <template-type-parameter name="A"/>
            </template>
            <parameter name="a">
              <paramtype>A const &amp;</paramtype>
            </parameter>
            <description>
              <para>
                This is an overloaded member function, provided for convenience. It differs from
                the above function only in what argument(s) it accepts.
              </para>
            </description>
          </method>

          <!-- operator() -->
          <method name="operator()">
            <type><replaceable>unspecified</replaceable></type>
            <template>
              <template-type-parameter name="A" pack="1"/>
            </template>
            <parameter name="a" pack="1">
              <paramtype>A const &amp;</paramtype>
            </parameter>
            <description>
              <para>Lazy function call</para>
            </description>
            <returns>
              <para>A new expression node representing the function call operation.</para>
            </returns>
          </method>

          <method name="operator()" cv="const">
            <type><replaceable>unspecified</replaceable></type>
            <template>
              <template-type-parameter name="A" pack="1"/>
            </template>
            <parameter name="a" pack="1">
              <paramtype>A const &amp;</paramtype>
            </parameter>
            <description>
              <para>
                This is an overloaded member function, provided for convenience. It differs from
                the above function only in what argument(s) it accepts.
              </para>
            </description>
          </method>
        </method-group>

        <data-member name="proto_expr_">
          <type>Expr</type>
          <purpose>For exposition only.</purpose>
        </data-member>

        <data-member name="proto_arity_c" specifiers="static">
          <type>const long</type>
          <purpose><computeroutput>= proto_base_expr::proto_arity_c;</computeroutput></purpose>
        </data-member>

      </struct>

    </namespace>
  </namespace>

  <macro name="BOOST_PROTO_EXTENDS" kind="functionlike">
    <macro-parameter name="Expr"/>
    <macro-parameter name="Derived"/>
    <macro-parameter name="Domain"/>
    <purpose>For creating expression wrappers that add behaviors to a Proto expression template, like
      <computeroutput><classname alt="boost::proto::extends">proto::extends&lt;&gt;</classname></computeroutput>,
      but while retaining POD-ness of the expression wrapper.</purpose>
    <description>
      <para>
        Equivalent to:
        <programlisting><macroname>BOOST_PROTO_BASIC_EXTENDS</macroname>(Expr, Derived, Domain)
<macroname>BOOST_PROTO_EXTENDS_ASSIGN</macroname>()
<macroname>BOOST_PROTO_EXTENDS_SUBSCRIPT</macroname>()
<macroname>BOOST_PROTO_EXTENDS_FUNCTION</macroname>()</programlisting>
      </para>
      <para>
        <emphasis role="bold">Example:</emphasis><programlisting>template&lt; class Expr &gt;
struct my_expr;

struct my_domain
  : <classname alt="boost::proto::domain">proto::domain</classname>&lt; <classname alt="boost::proto::pod_generator">proto::pod_generator</classname>&lt; my_expr &gt; &gt;
{};

template&lt; class Expr &gt;
struct my_expr
{
    // OK, this makes my_expr&lt;&gt; a valid Proto expression extension.
    // my_expr&lt;&gt; has overloaded assignment, subscript,
    // and function call operators that build expression templates.
    <macroname>BOOST_PROTO_EXTENDS</macroname>(Expr, my_expr, my_domain)
};

// OK, my_expr&lt;&gt; is POD, so this is statically initialized:
my_expr&lt; <classname alt="boost::proto::terminal">proto::terminal</classname>&lt;int&gt;::type &gt; const _1 = {{1}};</programlisting>
      </para>
    </description>
  </macro>

  <macro name="BOOST_PROTO_BASIC_EXTENDS" kind="functionlike">
    <macro-parameter name="Expr"/>
    <macro-parameter name="Derived"/>
    <macro-parameter name="Domain"/>
    <purpose>For creating expression wrappers that add members to a Proto expression template, like
      <computeroutput><classname alt="boost::proto::extends">proto::extends&lt;&gt;</classname></computeroutput>,
      but while retaining POD-ness of the expression wrapper.</purpose>
    <description>
      <para>
        <computeroutput>BOOST_PROTO_BASIC_EXTENDS()</computeroutput> adds the basic typedefs, member functions, and
        data members necessary to make a struct a valid Proto expression extension. It does <emphasis>not</emphasis>
        add any constructors, virtual functions or access control blocks that would render the containing
        struct non-POD.
      </para>
      <para>
        <computeroutput>Expr</computeroutput> is the Proto expression that the enclosing struct extends.
        <computeroutput>Derived</computeroutput> is the type of the enclosing struct.
        <computeroutput>Domain</computeroutput> is the Proto domain to which this expression extension belongs.
        (See <computeroutput><classname alt="boost::proto::domain">proto::domain&lt;&gt;</classname></computeroutput>.)
      </para>
      <para><computeroutput>BOOST_PROTO_BASIC_EXTENDS()</computeroutput> adds to its enclosing struct
        exactly one data member of type <computeroutput>Expr</computeroutput>. 
      </para>
      <para>
        <emphasis role="bold">Example:</emphasis><programlisting>template&lt; class Expr &gt;
struct my_expr;

struct my_domain
  : <classname alt="boost::proto::domain">proto::domain</classname>&lt; <classname alt="boost::proto::pod_generator">proto::pod_generator</classname>&lt; my_expr &gt; &gt;
{};

template&lt; class Expr &gt;
struct my_expr
{
    // OK, this makes my_expr&lt;&gt; a valid Proto expression extension.
    // my_expr&lt;&gt; does /not/ have overloaded assignment, subscript,
    // and function call operators that build expression templates, however.
    <macroname>BOOST_PROTO_BASIC_EXTENDS</macroname>(Expr, my_expr, my_domain)
};

// OK, my_expr&lt;&gt; is POD, so this is statically initialized:
my_expr&lt; <classname alt="boost::proto::terminal">proto::terminal</classname>&lt;int&gt;::type &gt; const _1 = {{1}};</programlisting>
      </para>
      <para>
        See also:
        <itemizedlist>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_EXTENDS_ASSIGN</macroname>()</computeroutput>
          </listitem>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_EXTENDS_SUBSCRIPT</macroname>()</computeroutput>
          </listitem>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_EXTENDS_FUNCTION</macroname>()</computeroutput>
          </listitem>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_EXTENDS</macroname>()</computeroutput>
          </listitem>
        </itemizedlist>
      </para>
    </description>
  </macro>

  <macro name="BOOST_PROTO_EXTENDS_ASSIGN" kind="functionlike">
    <purpose>For adding to an expression extension class an overloaded assignment operator that
      builds an expression template.</purpose>
    <description>
      <para>
        Use <computeroutput>BOOST_PROTO_EXTENDS_ASSIGN()</computeroutput> after <computeroutput>
        <macroname>BOOST_PROTO_BASIC_EXTENDS</macroname>()</computeroutput> to give an expression
        extension class an overloaded assignment operator that builds an expression template.
      </para>
      <para>
        See also:
        <itemizedlist>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_BASIC_EXTENDS</macroname>()</computeroutput>
          </listitem>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_EXTENDS_SUBSCRIPT</macroname>()</computeroutput>
          </listitem>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_EXTENDS_FUNCTION</macroname>()</computeroutput>
          </listitem>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_EXTENDS</macroname>()</computeroutput>
          </listitem>
        </itemizedlist>
      </para>
    </description>
  </macro>

  <macro name="BOOST_PROTO_EXTENDS_FUNCTION" kind="functionlike">
    <purpose>For adding to an expression extension class a set of overloaded function call operators
      that build expression templates.</purpose>
    <description>
      <para>
        Use <computeroutput>BOOST_PROTO_EXTENDS_FUNCTION()</computeroutput> after <computeroutput>
        <macroname>BOOST_PROTO_BASIC_EXTENDS</macroname>()</computeroutput> to give an expression
        extension class a set of overloaded function call operators that build expression templates.
        In addition, <computeroutput>BOOST_PROTO_EXTENDS_FUNCTION()</computeroutput> adds a nested
        <computeroutput>result&lt;&gt;</computeroutput> class template that is a metafunction for
        calculating the return type of the overloaded function call operators.
      </para>
      <para>
        See also:
        <itemizedlist>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_BASIC_EXTENDS</macroname>()</computeroutput>
          </listitem>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_EXTENDS_ASSIGN</macroname>()</computeroutput>
          </listitem>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_EXTENDS_SUBSCRIPT</macroname>()</computeroutput>
          </listitem>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_EXTENDS</macroname>()</computeroutput>
          </listitem>
        </itemizedlist>
      </para>
    </description>
  </macro>

  <macro name="BOOST_PROTO_EXTENDS_SUBSCRIPT" kind="functionlike">
    <purpose>For adding to an expression extension class an overloaded subscript operator that
      builds an expression template.</purpose>
    <description>
      <para>
        Use <computeroutput>BOOST_PROTO_EXTENDS_SUBSCRIPT()</computeroutput> after <computeroutput>
        <macroname>BOOST_PROTO_BASIC_EXTENDS</macroname>()</computeroutput> to give an expression
        extension class an overloaded subscript operator that builds an expression template.
      </para>
      <para>
        See also:
        <itemizedlist>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_BASIC_EXTENDS</macroname>()</computeroutput>
          </listitem>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_EXTENDS_ASSIGN</macroname>()</computeroutput>
          </listitem>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_EXTENDS_FUNCTION</macroname>()</computeroutput>
          </listitem>
          <listitem>
            <computeroutput><macroname>BOOST_PROTO_EXTENDS</macroname>()</computeroutput>
          </listitem>
        </itemizedlist>
      </para>
    </description>
  </macro>

  <macro name="BOOST_PROTO_EXTENDS_USING_ASSIGN" kind="functionlike">
    <macro-parameter name="Derived"/>
    <purpose>For exposing in classes that inherit from
      <computeroutput><classname alt="boost::proto::extends">proto::extends&lt;&gt;</classname></computeroutput>
      the overloaded assignment operators defined therein.</purpose>
    <description>
      <para>
        The standard usage of
        <computeroutput><classname alt="boost::proto::extends">proto::extends&lt;&gt;</classname></computeroutput>
        is to inherit from it. However, the derived class automatically gets a compiler-generated assignment
        operator that will hide the ones defined in
        <computeroutput><classname alt="boost::proto::extends">proto::extends&lt;&gt;</classname></computeroutput>.
        Use <code>BOOST_PROTO_EXTENDS_USING_ASSIGN()</code> in the derived class to unhide the assignment
        operators defined in
        <computeroutput><classname alt="boost::proto::extends">proto::extends&lt;&gt;</classname></computeroutput>.
      </para>
      <para>
        See <computeroutput><classname alt="boost::proto::extends">proto::extends&lt;&gt;</classname></computeroutput>
        for an example that demonstrates usage of <code>BOOST_PROTO_EXTENDS_USING_ASSIGN()</code>.
      </para>
    </description>
  </macro>

  <macro name="BOOST_PROTO_EXTENDS_USING_ASSIGN_NON_DEPENDENT" kind="functionlike">
    <macro-parameter name="Derived"/>
    <purpose>For exposing in classes that inherit from
      <computeroutput><classname alt="boost::proto::extends">proto::extends&lt;&gt;</classname></computeroutput>
      the overloaded assignment operators defined therein. Unlike the
      <computeroutput><macroname>BOOST_PROTO_EXTENDS_USING_ASSIGN</macroname>()</computeroutput> macro,
      <code>BOOST_PROTO_EXTENDS_USING_ASSIGN_NON_DEPENDENT()</code> is for use in non-dependent
      contexts.
      </purpose>
    <description>
      <para>
        The standard usage of
        <computeroutput><classname alt="boost::proto::extends">proto::extends&lt;&gt;</classname></computeroutput>
        is to define a class template that inherits from it. The derived class template automatically gets a
        compiler-generated assignment operator that hides the ones defined in
        <computeroutput><classname alt="boost::proto::extends">proto::extends&lt;&gt;</classname></computeroutput>.
        Using <code>BOOST_PROTO_EXTENDS_USING_ASSIGN()</code> in the derived class solves this problem.
      </para>
      <para>
        However, if the expression extension is an ordinary class and not a class template, the usage of
        <code>BOOST_PROTO_EXTENDS_USING_ASSIGN()</code> is in a so-called non-dependent context. In plain English,
        it means it is illegal to use <code>typename</code> in some places where it is required in a class template. 
        In those cases, you should use <code>BOOST_PROTO_EXTENDS_USING_ASSIGN_NON_DEPENDENT()</code> instead.
      </para>
      <para>
		See also:
		<itemizedlist>
		  <listitem>
			<computeroutput><classname alt="boost::proto::extends">proto::extends&lt;&gt;</classname></computeroutput>
		  </listitem>
		  <listitem>
			<computeroutput><macroname>BOOST_PROTO_EXTENDS_USING_ASSIGN</macroname>()</computeroutput>
		  </listitem>
		</itemizedlist>
      </para>
    </description>
  </macro>

</header>