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## Struct template expr

boost::proto::expr — Representation of a node in an expression tree.

## Synopsis

    template<typename Tag, typename Args, long Arity = Args::arity>
    struct expr {
      // types
      typedef Tag                   proto_tag;
      typedef Args                  proto_args;
      typedef mpl::long_< Arity >   proto_arity;
      typedef default_domain        proto_domain;
      typedef expr                  proto_base_expr;
      typedef expr                  proto_derived_expr;
      typedef typename Args::childN proto_childN;        // For each N in [0,max(Arity,1)).
      template<typename Signature>
      struct result {
        // types
        typedef unspecified type;
      };

      // public static functions
      template<typename... A> static expr const make(A const &...);

      // public member functions
      expr & 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;
      proto_childN childN;
    };

## Description

`proto::expr<>` is a node in an expression template tree. It is a container for its child sub-trees. It also serves as the terminal nodes of the tree.

`Tag` is type that represents the operation encoded by this expression. It is typically one of the structs in the `boost::proto::tag` namespace, but it doesn't have to be. If `Arity` is 0 then this `expr<>` type represents a leaf in the expression tree.

`Args` is a list of types representing the children of this expression. It is an instantiation of one of `proto::list1<>`, `proto::list2<>`, etc. The child types must all themselves be either `expr<>` or `proto::expr<>&`, unless `Arity` is 0, in which case `Args` must be `proto::term<T>`, where `T` can be any type.

`proto::expr<>` is a valid Fusion random-access sequence, where the elements of the sequence are the child expressions.

### `expr` public static functions

1.     template<typename... A> static expr const make(A const &... a);

   Requires: | The number of supplied arguments must be `Arity`.
   Returns: | A new `expr` object initialized with the specified arguments.

### `expr` public member functions

1.     expr & proto_base();

   Returns: | `*this`

2.     expr const & proto_base() const;

   This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

3.     template<typename A> unspecified operator=(A & a);

   Lazy assignment expression

   Returns: | A new expression node representing the assignment operation.

4.     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.

5.     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.

6.     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.

7.     template<typename A> unspecified operator[](A & a);

   Lazy subscript expression

   Returns: | A new expression node representing the subscript operation.

8.     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.

9.     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.

10.     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.

11.     template<typename... A> unspecified operator()(A const &... a);

    Lazy function call

    Returns: | A new expression node representing the function call operation.

12.     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.