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<?xml version="1.0" encoding="utf-8"?>
<header name="boost/proto/transform/pass_through.hpp">
<para>Definition of the
<computeroutput><classname alt="boost::proto::pass_through">proto::pass_through<></classname></computeroutput>
transform, which is the default transform of all of the expression generator metafunctions such as
<computeroutput><classname alt="boost::proto::unary_plus">proto::unary_plus<></classname></computeroutput>,
<computeroutput><classname alt="boost::proto::plus">proto::plus<></classname></computeroutput> and
<computeroutput><classname alt="boost::proto::nary_expr">proto::nary_expr<></classname></computeroutput>.</para>
<namespace name="boost">
<namespace name="proto">
<struct name="pass_through">
<template>
<template-type-parameter name="Grammar"/>
</template>
<inherit><type><classname>proto::transform</classname>< pass_through<Grammar> ></type></inherit>
<purpose>A <conceptname>PrimitiveTransform</conceptname> that transforms the child expressions of an expression
node according to the corresponding children of a Grammar.</purpose>
<description>
<para>
Given a Grammar such as <computeroutput><classname>proto::plus</classname><T0, T1></computeroutput>,
an expression type that matches the grammar such as
<computeroutput><classname>proto::plus</classname><E0, E1>::type</computeroutput>, a state
<computeroutput>S</computeroutput> and a data <computeroutput>D</computeroutput>, the result of applying
the <computeroutput>proto::pass_through<<classname>proto::plus</classname><T0, T1> ></computeroutput>
transform is: <programlisting><classname>proto::plus</classname><
boost::result_of<T0(E0, S, D)>::type,
boost::result_of<T1(E1, S, D)>::type
>::type</programlisting>
</para>
<para>
The above demonstrates how child transforms and child expressions are applied pairwise, and how the
results are reassembled into a new expression node with the same tag type as the original.
</para>
<para>
The explicit use of <computeroutput>proto::pass_through<></computeroutput> is not usually
needed, since the expression generator metafunctions such as
<computeroutput><classname>proto::plus</classname><></computeroutput> have
<computeroutput>proto::pass_through<></computeroutput> as their default transform. So,
for instance, these are equivalent:
<itemizedlist>
<listitem>
<computeroutput>
<classname>proto::when</classname>< <classname>proto::plus</classname><X, Y>, proto::pass_through< <classname>proto::plus</classname><X, Y> > >
</computeroutput>
</listitem>
<listitem>
<computeroutput>
<classname>proto::when</classname>< <classname>proto::plus</classname><X, Y>, <classname>proto::plus</classname><X, Y> >
</computeroutput>
</listitem>
<listitem>
<computeroutput>
<classname>proto::when</classname>< <classname>proto::plus</classname><X, Y> > // because of proto::when<class X, class Y=X>
</computeroutput>
</listitem>
<listitem>
<computeroutput>
<classname>proto::plus</classname><X, Y> // because plus<> is both a grammar and a transform
</computeroutput>
</listitem>
</itemizedlist>
</para>
<para>
For example, consider the following transform that promotes all
<computeroutput>float</computeroutput> terminals in an expression to
<computeroutput>double</computeroutput>.
<programlisting>// This transform finds all float terminals in an expression and promotes
// them to doubles.
struct Promote :
<classname>proto::or_</classname><
<classname>proto::when</classname><<classname>proto::terminal</classname><float>, <classname>proto::terminal</classname><double>::type(<classname>proto::_value</classname>) >,
// terminal<>'s default transform is a no-op:
<classname>proto::terminal</classname><<classname>proto::_</classname>>,
// nary_expr<> has a pass_through<> transform:
<classname>proto::nary_expr</classname><<classname>proto::_</classname>, <classname>proto::vararg</classname><Promote> >
>
{};</programlisting>
</para>
</description>
<struct name="impl">
<template>
<template-type-parameter name="Expr"/>
<template-type-parameter name="State"/>
<template-type-parameter name="Data"/>
</template>
<inherit><type><classname>proto::transform_impl</classname><Expr, State, Data></type></inherit>
<typedef name="GN">
<purpose>For each N in [0,Expr arity), for exposition only</purpose>
<type>typename proto::result_of::child_c<Grammar, N>::type</type>
</typedef>
<typedef name="EN">
<purpose>For each N in [0,Expr arity), for exposition only</purpose>
<type>typename proto::result_of::child_c<Expr, N>::type</type>
</typedef>
<typedef name="RN">
<purpose>For each N in [0,Expr arity), for exposition only</purpose>
<type>typename boost::result_of<GN(EN,State,Data)>::type</type>
</typedef>
<typedef name="T">
<purpose>For exposition only</purpose>
<type>typename Expr::proto_tag</type>
</typedef>
<typedef name="result_type">
<type><classname>proto::expr</classname><T, <classname>proto::listN</classname><R0,...RN> ></type>
</typedef>
<method-group name="public member functions">
<method name="operator()" cv="const">
<type>result_type</type>
<parameter name="expr">
<paramtype>typename impl::expr_param</paramtype>
</parameter>
<parameter name="state">
<paramtype>typename impl::state_param</paramtype>
</parameter>
<parameter name="data">
<paramtype>typename impl::data_param</paramtype>
</parameter>
<requires>
<para>
<computeroutput>
<classname>proto::matches</classname><Expr, Grammar>::value
</computeroutput> is <computeroutput>true</computeroutput>.
</para>
</requires>
<returns>
<para>
<programlisting>result_type::make(
G0()(<functionname>proto::child_c</functionname><0>(expr), state, data),
...
GN()(<functionname>proto::child_c</functionname><N>(expr), state, data)
)</programlisting>
</para>
</returns>
</method>
</method-group>
</struct>
</struct>
</namespace>
</namespace>
</header>
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