Struct template extends

boost::proto::extends — For adding behaviors to a Proto expression template.

Synopsis

// In header: <boost/proto/extends.hpp>

template<typename Expr, typename Derived, 
         typename Domain = proto::default_domain> 
struct extends {
  // types
  typedef typename Expr::proto_base_expr          proto_base_expr;   
  typedef Domain                                  proto_domain;      
  typedef Derived                                 proto_derived_expr;
  typedef extends                                 proto_extends;     
  typedef typename proto_base_expr::proto_tag     proto_tag;         
  typedef typename proto_base_expr::proto_args    proto_args;        
  typedef typename proto_base_expr::proto_arity   proto_arity;       
  typedef typename proto_base_expr::proto_grammar proto_grammar;     
  typedef typename proto_base_expr::proto_childN  proto_childN;        // For each N in [0,max(1,proto_arity_c))

  // member classes/structs/unions
  template<typename Signature> 
  struct result {
    // types
    typedef unspecified type;
  };

  // construct/copy/destruct
  extends();
  extends(extends const &);
  extends(Expr const &);

  // public static functions
  static Derived const make(Expr const &);

  // public member functions
  proto_base_expr & proto_base();
  proto_base_expr const & proto_base() const;
  template<typename A> unspecified operator=(A &);
  template<typename A> unspecified operator=(A const &);
  template<typename A> unspecified operator=(A &) const;
  template<typename A> unspecified operator=(A const &) const;
  template<typename A> unspecified operator[](A &);
  template<typename A> unspecified operator[](A const &);
  template<typename A> unspecified operator[](A &) const;
  template<typename A> unspecified operator[](A const &) const;
  template<typename... A> unspecified operator()(A const &...);
  template<typename... A> unspecified operator()(A const &...) const;

  // public data members
  Expr proto_expr_;  // For exposition only.
  static const long proto_arity_c;  // = proto_base_expr::proto_arity_c;
};

Description

Use proto::extends<> to give expressions in your domain custom data members and member functions.

Conceptually, using proto::extends<> is akin to inheriting from proto::expr<> and adding your own members. Using proto::extends<> is generally preferrable to straight inheritance because the members that would be inherited from proto::expr<> would be wrong; they would incorrectly slice off your additional members when building larger expressions from smaller ones. proto::extends<> 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:

template< typename Expr >
struct my_expr
  : proto::extends<
        Expr            // The expression type we're extending
      , my_expr< Expr > // The type we're defining
      , my_domain       // The domain associated with this expression extension
    >
{
    // An expression extension is constructed from the expression
    // it is extending.
    my_expr( Expr const & e = Expr() )
      : my_expr::proto_extends( 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 ...
    */
};

`extends` public construct/copy/destruct

```
extends();
```
```
extends(extends const & that);
```
```
extends(Expr const & expr_);
```

`extends` public static functions

```
static Derived const make(Expr const & expr);
```
Construct an expression extension from the base expression.

`extends` public member functions

```
proto_base_expr & proto_base();
```
Returns:
proto_expr_.proto_base()

Throws:
Will not throw.
```
proto_base_expr const & proto_base() const;
```
Returns:
proto_expr_.proto_base()

Throws:
Will not throw.
```
template<typename A> unspecified operator=(A & a);
```
Lazy assignment expression

Returns:

A new expression node representing the assignment operation.
```
template<typename A> unspecified operator=(A const & a);
```
This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
```
template<typename A> unspecified operator=(A & a) const;
```
This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
```
template<typename A> unspecified operator=(A const & a) const;
```
This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
```
template<typename A> unspecified operator[](A & a);
```
Lazy subscript expression

Returns:

A new expression node representing the subscript operation.
```
template<typename A> unspecified operator[](A const & a);
```
This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
```
template<typename A> unspecified operator[](A & a) const;
```
This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
```
template<typename A> unspecified operator[](A const & a) const;
```
This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
```
template<typename... A> unspecified operator()(A const &... a);
```
Lazy function call

Returns:

A new expression node representing the function call operation.
```
template<typename... A> unspecified operator()(A const &... a) const;
```
This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Returns:	`proto_expr_.proto_base()`
Throws:	Will not throw.

Returns:	`proto_expr_.proto_base()`
Throws:	Will not throw.

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