text: ///////////////////////////////////////////////////////////////////////////////
  // width.hpp
  //
  //  Copyright 2008 Eric Niebler. Distributed under the Boost
  //  Software License, Version 1.0. (See accompanying file
  //  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  
  #ifndef BOOST_XPRESSIVE_DETAIL_UTILITY_WIDTH_HPP_EAN_04_07_2006
  #define BOOST_XPRESSIVE_DETAIL_UTILITY_WIDTH_HPP_EAN_04_07_2006
  
  // MS compatible compilers support #pragma once
  #if defined(_MSC_VER)
  # pragma once
  #endif
  
  #include <climits> // for INT_MAX
  #include <boost/mpl/size_t.hpp>
  
  namespace boost { namespace xpressive { namespace detail
  {
  
  typedef mpl::size_t<INT_MAX / 2 - 1> unknown_width;
  struct width;
  bool is_unknown(width const &that);
  
  ///////////////////////////////////////////////////////////////////////////////
  // width
  struct width
  {
      width(std::size_t val = 0)
        : value_(val)
      {
      }
  
      bool operator !() const
      {
          return !this->value_;
      }
  
      width &operator +=(width const &that)
      {
          this->value_ =
              !is_unknown(*this) && !is_unknown(that)
            ? this->value_ + that.value_
            : unknown_width();
          return *this;
      }
  
      width &operator |=(width const &that)
      {
          this->value_ =
              this->value_ == that.value_
            ? this->value_
            : unknown_width();
          return *this;
      }
  
      std::size_t value() const
      {
          return this->value_;
      }
  
  private:
      std::size_t value_;
  };
  
  inline bool is_unknown(width const &that)
  {
      return unknown_width::value == that.value();
  }
  
  inline bool operator ==(width const &left, width const &right)
  {
      return left.value() == right.value();
  }
  
  inline bool operator !=(width const &left, width const &right)
  {
      return left.value() != right.value();
  }
  
  inline width operator +(width left, width const &right)
  {
      return left += right;
  }
  
  inline width operator |(width left, width const &right)
  {
      return left |= right;
  }
  
  }}} // namespace boost::xpressive::detail
  
  #endif
  