Function template optional

boost::xpressive::optional — Make a sub-expression optional. Equivalent to !as_xpr(expr).

Synopsis

// In header: <boost/xpressive/regex_primitives.hpp>


template<typename Expr> 
  proto::result_of::make_expr< proto::tag::logical_not, proto::default_domain, Expr const & >::type const 
  optional(Expr const & expr);

Description

Parameters:

expr

The sub-expression to make optional.