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## Struct template default_eval

boost::proto::context::default_eval — A BinaryFunction that accepts a Proto expression and a context, evaluates each child expression with the context, and combines the result using the standard C++ meaning for the operator represented by the current expression node.

## Synopsis

    // In header: <boost/proto/context/default.hpp>

    template<typename Expr, typename Context>
    struct default_eval {
      // types
      typedef typename Expr::tag_type Tag;          // For exposition only
      typedef see-below result_type;

      // public member functions
      result_type operator()(Expr &, Context &) const;

      // public data members
      static Expr & s_expr;  // For exposition only
      static Context & s_context;  // For exposition only
    };

## Description

Let ``OP`` be the C++ operator corresponding to `Expr::proto_tag`. (For example, if `Tag` is `proto::tag::plus`, let ``OP`` be `+`.)

The behavior of this class is specified in terms of the C++0x `decltype` keyword. In systems where this keyword is not available, Proto uses the Boost.Typeof library to approximate the behavior.

### `default_eval` public types

1. typedef see-below result_type;

   - If `Tag` corresponds to a unary prefix operator, then the result type is

         decltype(
           OP proto::eval(proto::child(s_expr), s_context)
         )

   - If `Tag` corresponds to a unary postfix operator, then the result type is

         decltype(
           proto::eval(proto::child(s_expr), s_context) OP
         )

   - If `Tag` corresponds to a binary infix operator, then the result type is

         decltype(
           proto::eval(proto::left(s_expr), s_context) OP
           proto::eval(proto::right(s_expr), s_context)
         )

   - If `Tag` is `proto::tag::subscript` , then the result type is

         decltype(
           proto::eval(proto::left(s_expr), s_context) [
           proto::eval(proto::right(s_expr), s_context) ]
         )

   - If `Tag` is `proto::tag::if_else_` , then the result type is

         decltype(
           proto::eval(proto::child_c<0>(s_expr), s_context) ?
           proto::eval(proto::child_c<1>(s_expr), s_context) :
           proto::eval(proto::child_c<2>(s_expr), s_context)
         )

   - If `Tag` is `proto::tag::function` , then the result type is

         decltype(
           proto::eval(proto::child_c<0>(s_expr), s_context) (
           proto::eval(proto::child_c<1>(s_expr), s_context),
           ...
           proto::eval(proto::child_c<N>(s_expr), s_context) )
         )

### `default_eval` public member functions

1.     result_type operator()(Expr & expr, Context & context) const;

   - If `Tag` corresponds to a unary prefix operator, then return

         OP proto::eval(proto::child(expr), context)

   - If `Tag` corresponds to a unary postfix operator, then return

         proto::eval(proto::child(expr), context) OP

   - If `Tag` corresponds to a binary infix operator, then return

         proto::eval(proto::left(expr), context) OP
         proto::eval(proto::right(expr), context)

   - If `Tag` is ``proto::tag::subscript`` , then return

         proto::eval(proto::left(expr), context) [
         proto::eval(proto::right(expr), context) ]

   - If `Tag` is ``proto::tag::if_else_`` , then return

         proto::eval(proto::child_c<0>(expr), context) ?
         proto::eval(proto::child_c<1>(expr), context) :
         proto::eval(proto::child_c<2>(expr), context)

   - If `Tag` is ``proto::tag::function`` , then return

         proto::eval(proto::child_c<0>(expr), context) (
         proto::eval(proto::child_c<1>(expr), context),
         ...
         proto::eval(proto::child_c<N>(expr), context) )

   Parameters: |
   `context` | The evaluation context
   `expr` | The current expression