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;
  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

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

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

Boost C++ Libraries