Synopsis

template <class P>
struct look_ahead;

This is a parser combinator.

Description

Parses the input using parser P. When P returns an error, look_ahead returns the error. When P returns a result, look_ahead returns the result without consuming anything from the input string.

Header

#include <boost/metaparse/look_ahead.hpp>

Expression semantics

For any p parser, s compile-time string and pos source position

look_ahead<p>::apply<s, pos>

is equivalent to

return_<get_result<p::apply<s, pos>>::type>::apply<s, pos>

when p::apply<s, pos> succeeds. It is

p::apply<s, pos>

otherwise.

Example

#include <boost/metaparse/look_ahead.hpp>
#include <boost/metaparse/int_.hpp>
#include <boost/metaparse/start.hpp>
#include <boost/metaparse/string.hpp>
#include <boost/metaparse/is_error.hpp>
#include <boost/metaparse/get_result.hpp>
#include <boost/metaparse/get_remaining.hpp>

#include <type_traits>

using namespace boost::metaparse;

static_assert(
  get_result<
    look_ahead<int_>::apply<BOOST_METAPARSE_STRING("13"), start>
  >::type::value == 13,
  "it should return the same result as the wrapped parser"
);

static_assert(
  std::is_same<
    BOOST_METAPARSE_STRING("13"),
    get_remaining<
      look_ahead<int_>::apply<BOOST_METAPARSE_STRING("13"), start>
    >::type
  >::type::value,
  "it should not consume any input"
);

static_assert(
  is_error<
    look_ahead<int_>::apply<BOOST_METAPARSE_STRING("six"), start>
  >::type::value,
  "it should fail when the wrapped parser fails"
);