1<?xml version="1.0" encoding="utf-8"?> 2<!-- 3 Copyright 2012 Eric Niebler 4 5 Distributed under the Boost 6 Software License, Version 1.0. (See accompanying 7 file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) 8 --> 9<header name="boost/proto/transform/pass_through.hpp"> 10 <para>Definition of the 11 <computeroutput><classname alt="boost::proto::pass_through">proto::pass_through<></classname></computeroutput> 12 transform, which is the default transform of all of the expression generator metafunctions such as 13 <computeroutput><classname alt="boost::proto::unary_plus">proto::unary_plus<></classname></computeroutput>, 14 <computeroutput><classname alt="boost::proto::plus">proto::plus<></classname></computeroutput> and 15 <computeroutput><classname alt="boost::proto::nary_expr">proto::nary_expr<></classname></computeroutput>.</para> 16 <namespace name="boost"> 17 <namespace name="proto"> 18 <struct name="pass_through"> 19 <template> 20 <template-type-parameter name="Grammar"/> 21 <template-type-parameter name="Domain"> 22 <default><classname>proto::deduce_domain</classname></default> 23 </template-type-parameter> 24 </template> 25 <inherit><type><classname>proto::transform</classname>< pass_through<Grammar, Domain> ></type></inherit> 26 <purpose>A <conceptname>PrimitiveTransform</conceptname> that transforms the child expressions of an expression 27 node according to the corresponding children of a Grammar. The resulting expression is in the specified domain.</purpose> 28 <description> 29 <para> 30 Given a Grammar such as <computeroutput><classname>proto::plus</classname><T0, T1></computeroutput>, 31 an expression type that matches the grammar such as 32 <computeroutput><classname>proto::plus</classname><E0, E1>::type</computeroutput>, a state 33 <computeroutput>S</computeroutput> and a data <computeroutput>D</computeroutput>, the result of applying 34 the <computeroutput>proto::pass_through<<classname>proto::plus</classname><T0, T1> ></computeroutput> 35 transform is: <programlisting><classname>proto::plus</classname>< 36 boost::result_of<T0(E0, S, D)>::type, 37 boost::result_of<T1(E1, S, D)>::type 38>::type</programlisting> 39 </para> 40 <para> 41 The above demonstrates how child transforms and child expressions are applied pairwise, and how the 42 results are reassembled into a new expression node with the same tag type as the original. 43 </para> 44 <para> 45 The <code>Domain</code> template parameter determines which domain the resulting expression should 46 be in. If it is <code><classname>proto::deduce_domain</classname></code>, which is the default, 47 the resulting expression is in the same domain as the expression passed in. Otherwise, the resulting 48 expression is in the specified domain. Practically, that means the specified domain's generator is 49 used to post-process the resulting expression. 50 </para> 51 <para> 52 The explicit use of <computeroutput>proto::pass_through<></computeroutput> is not usually 53 needed, since the expression generator metafunctions such as 54 <computeroutput><classname>proto::plus</classname><></computeroutput> have 55 <computeroutput>proto::pass_through<></computeroutput> as their default transform. So, 56 for instance, these are equivalent: 57 <itemizedlist> 58 <listitem> 59 <computeroutput> 60 <classname>proto::when</classname>< <classname>proto::plus</classname><X, Y>, proto::pass_through< <classname>proto::plus</classname><X, Y> > > 61 </computeroutput> 62 </listitem> 63 <listitem> 64 <computeroutput> 65 <classname>proto::when</classname>< <classname>proto::plus</classname><X, Y>, <classname>proto::plus</classname><X, Y> > 66 </computeroutput> 67 </listitem> 68 <listitem> 69 <computeroutput> 70 <classname>proto::when</classname>< <classname>proto::plus</classname><X, Y> > // because of proto::when<class X, class Y=X> 71 </computeroutput> 72 </listitem> 73 <listitem> 74 <computeroutput> 75 <classname>proto::plus</classname><X, Y> // because plus<> is both a grammar and a transform 76 </computeroutput> 77 </listitem> 78 </itemizedlist> 79 </para> 80 <para> 81 For example, consider the following transform that promotes all 82 <computeroutput>float</computeroutput> terminals in an expression to 83 <computeroutput>double</computeroutput>. 84 <programlisting>// This transform finds all float terminals in an expression and promotes 85// them to doubles. 86struct Promote : 87 <classname>proto::or_</classname>< 88 <classname>proto::when</classname><<classname>proto::terminal</classname><float>, <classname>proto::terminal</classname><double>::type(<classname>proto::_value</classname>) >, 89 // terminal<>'s default transform is a no-op: 90 <classname>proto::terminal</classname><<classname>proto::_</classname>>, 91 // nary_expr<> has a pass_through<> transform: 92 <classname>proto::nary_expr</classname><<classname>proto::_</classname>, <classname>proto::vararg</classname><Promote> > 93 > 94{};</programlisting> 95 </para> 96 </description> 97 <struct name="impl"> 98 <template> 99 <template-type-parameter name="Expr"/> 100 <template-type-parameter name="State"/> 101 <template-type-parameter name="Data"/> 102 </template> 103 <inherit><type><classname>proto::transform_impl</classname><Expr, State, Data></type></inherit> 104 <typedef name="GN"> 105 <purpose>For each N in [0,Expr arity), for exposition only</purpose> 106 <type>typename proto::result_of::child_c<Grammar, N>::type</type> 107 </typedef> 108 <typedef name="EN"> 109 <purpose>For each N in [0,Expr arity), for exposition only</purpose> 110 <type>typename proto::result_of::child_c<Expr, N>::type</type> 111 </typedef> 112 <typedef name="RN"> 113 <purpose>For each N in [0,Expr arity), for exposition only</purpose> 114 <type>typename boost::result_of<GN(EN,State,Data)>::type</type> 115 </typedef> 116 <typedef name="T"> 117 <purpose>For exposition only</purpose> 118 <type>typename Expr::proto_tag</type> 119 </typedef> 120 <typedef name="Deduce"> 121 <purpose>For exposition only</purpose> 122 <type>boost::is_same<Domain, <classname>deduce_domain</classname>></type> 123 </typedef> 124 <typedef name="DD"> 125 <purpose>For exposition only</purpose> 126 <type>typename Expr::proto_domain</type> 127 </typedef> 128 <typedef name="D"> 129 <purpose>For exposition only</purpose> 130 <type>typename mpl::if_<Deduce, DD, Domain>::type</type> 131 </typedef> 132 <typedef name="G"> 133 <purpose>For exposition only</purpose> 134 <type>typename D::proto_generator</type> 135 </typedef> 136 <typedef name="A"> 137 <purpose>For exposition only</purpose> 138 <type><classname>proto::listN</classname><R0,...RN></type> 139 </typedef> 140 <typedef name="E"> 141 <purpose>For exposition only</purpose> 142 <type><classname>proto::expr</classname><T, A></type> 143 </typedef> 144 <typedef name="BE"> 145 <purpose>For exposition only</purpose> 146 <type><classname>proto::basic_expr</classname><T, A></type> 147 </typedef> 148 <typedef name="expr_type"> 149 <purpose>For exposition only</purpose> 150 <type>typename mpl::if_<<classname>proto::wants_basic_expr</classname><G>, BE, E>::type</type> 151 </typedef> 152 <typedef name="result_type"> 153 <type>typename boost::result_of<D(expr_type)>::type</type> 154 </typedef> 155 <method-group name="public member functions"> 156 <method name="operator()" cv="const"> 157 <type>result_type</type> 158 <parameter name="expr"> 159 <paramtype>typename impl::expr_param</paramtype> 160 </parameter> 161 <parameter name="state"> 162 <paramtype>typename impl::state_param</paramtype> 163 </parameter> 164 <parameter name="data"> 165 <paramtype>typename impl::data_param</paramtype> 166 </parameter> 167 <requires> 168 <para> 169 <computeroutput> 170 <classname>proto::matches</classname><Expr, Grammar>::value 171 </computeroutput> is <computeroutput>true</computeroutput>. 172 </para> 173 </requires> 174 <returns> 175 <para> 176 <programlisting>D()(expr_type::make( 177 G0()(<functionname>proto::child_c</functionname><0>(expr), state, data), 178 ... 179 GN()(<functionname>proto::child_c</functionname><N>(expr), state, data) 180))</programlisting> 181 </para> 182 </returns> 183 </method> 184 </method-group> 185 </struct> 186 </struct> 187 </namespace> 188 </namespace> 189</header> 190