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<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis">// In header: <<a class="link" href="../../proto/reference.html#header.boost.proto.extends_hpp" title="Header <boost/proto/extends.hpp>">boost/proto/extends.hpp</a>>

template<typename <a class="link" href="../../Expr.html" title="Concept Expr">Expr</a>, typename Derived,

typename <a class="link" href="../../Domain.html" title="Concept Domain">Domain</a> = <a class="link" href="default_domain.html" title="Struct default_domain">proto::default_domain</a>>

struct <a class="link" href="extends.html" title="Struct template extends">extends</a> {

// types

typedef typename Expr::proto_base_expr <a name="boost.proto.extends.proto_base_expr"></a>proto_base_expr;

typedef Domain <a name="boost.proto.extends.proto_domain"></a>proto_domain;

typedef Derived <a name="boost.proto.extends.proto_derived_expr"></a>proto_derived_expr;

typedef extends <a name="boost.proto.extends.proto_extends"></a>proto_extends;

typedef typename proto_base_expr::proto_tag <a name="boost.proto.extends.proto_tag"></a>proto_tag;

typedef typename proto_base_expr::proto_args <a name="boost.proto.extends.proto_args"></a>proto_args;

typedef typename proto_base_expr::proto_arity <a name="boost.proto.extends.proto_arity"></a>proto_arity;

typedef typename proto_base_expr::proto_grammar <a name="boost.proto.extends.proto_grammar"></a>proto_grammar;

typedef typename proto_base_expr::proto_child<code>N</code> <a name="boost.proto.extends.proto_childN"></a>proto_childN; // For each <code>N</code> in <code>[0,max(1,proto_arity_c))</code>

// member classes/structs/unions

template<typename Signature>

struct <a class="link" href="extends/result.html" title="Struct template result">result</a> {

// types

typedef <code>unspecified</code> <a class="link" href="extends/result.html#boost.proto.extends.result.type">type</a>;

};

// <a class="link" href="extends.html#boost.proto.extendsconstruct-copy-destruct">construct/copy/destruct</a>

<a class="link" href="extends.html#id1497139-bb">extends</a>();

<a class="link" href="extends.html#id1497141-bb">extends</a>(<a class="link" href="extends.html" title="Struct template extends">extends</a> const &);

<a class="link" href="extends.html#id1497157-bb">extends</a>(Expr const &);

// <a class="link" href="extends.html#id1497170-bb">public static functions</a>

static Derived const <a class="link" href="extends.html#id1497175-bb">make</a>(Expr const &);

// <a class="link" href="extends.html#id1497210-bb">public member functions</a>

proto_base_expr & <a class="link" href="extends.html#id1497217-bb">proto_base</a>();

proto_base_expr const & <a class="link" href="extends.html#id1497237-bb">proto_base</a>() const;

template<typename A> <code>unspecified</code> <a class="link" href="extends.html#id1497262-bb">operator=</a>(A &);

template<typename A> <code>unspecified</code> <a class="link" href="extends.html#id1497306-bb">operator=</a>(A const &);

template<typename A> <code>unspecified</code> <a class="link" href="extends.html#id1497345-bb">operator=</a>(A &) const;

template<typename A> <code>unspecified</code> <a class="link" href="extends.html#id1497386-bb">operator=</a>(A const &) const;

template<typename A> <code>unspecified</code> <a class="link" href="extends.html#id1497430-bb">operator[]</a>(A &);

template<typename A> <code>unspecified</code> <a class="link" href="extends.html#id1497474-bb">operator[]</a>(A const &);

template<typename A> <code>unspecified</code> <a class="link" href="extends.html#id1497513-bb">operator[]</a>(A &) const;

template<typename A> <code>unspecified</code> <a class="link" href="extends.html#id1497554-bb">operator[]</a>(A const &) const;

template<typename... A> <code>unspecified</code> <a class="link" href="extends.html#id1497598-bb">operator()</a>(A const &...);

template<typename... A> <code>unspecified</code> <a class="link" href="extends.html#id1497647-bb">operator()</a>(A const &...) const;

// public data members

Expr proto_expr_; // For exposition only.

static const long proto_arity_c; // <code class="computeroutput">= proto_base_expr::proto_arity_c;</code>

};</pre></div>

<a name="id2725356"></a><h2>Description</h2>

Use <code class="computeroutput">proto::extends<></code> to give expressions in your

domain custom data members and member functions.

Conceptually, using <code class="computeroutput">proto::extends<></code> is akin

to inheriting from <code class="computeroutput"><a class="link" href="expr.html" title="Struct template expr">proto::expr</a><></code>

and adding your own members. Using <code class="computeroutput">proto::extends<></code> is

generally preferrable to straight inheritance because the members that would be inherited from

<code class="computeroutput"><a class="link" href="expr.html" title="Struct template expr">proto::expr</a><></code> would

be wrong; they would incorrectly slice off your additional members when building

larger expressions from smaller ones. <code class="computeroutput">proto::extends<></code>

automatically gives your expression types the appropriate operator overloads that

preserve your domain-specific members when composing expression trees.

Expression extensions are typically defined as follows:

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<pre class="programlisting">template< typename Expr >

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struct my_expr

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: proto::extends<

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Expr // The expression type we're extending

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, my_expr< Expr > // The type we're defining

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, my_domain // The domain associated with this expression extension

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>

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{

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// An expression extension is constructed from the expression

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// it is extending.

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my_expr( Expr const & e = Expr() )

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: my_expr::proto_extends( e )

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{}

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// Unhide proto::extends::operator=

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// (This is only necessary if a lazy assignment operator

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// makes sense for your domain-specific language.)

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BOOST_PROTO_EXTENDS_USING_ASSIGN(my_expr)

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/*

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... domain-specific members go here ...

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*/

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};</pre>

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