// Three-state boolean logic library

 // Copyright Douglas Gregor 2002-2004. Use, modification and

 // distribution is subject to 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)

 // For more information, see http://www.boost.org

 #ifndef BOOST_LOGIC_TRIBOOL_HPP

 #define BOOST_LOGIC_TRIBOOL_HPP

 #include <boost/logic/tribool_fwd.hpp>

 #include <boost/config.hpp>

 #include <boost/detail/workaround.hpp>

 #if BOOST_WORKAROUND(_MSC_VER, >= 1200)

 #  pragma once

 #endif

 namespace boost { namespace logic {

 /// INTERNAL ONLY

 namespace detail {

 /**

  * INTERNAL ONLY

  *

  * \brief A type used only to uniquely identify the 'indeterminate'

  * function/keyword.

  */

 struct indeterminate_t

 {

 #if BOOST_WORKAROUND(__BORLANDC__, < 0x0600)

   char dummy_; // BCB would use 8 bytes by default

 #endif

 };

 } // end namespace detail

 /**

  * INTERNAL ONLY

  * The type of the 'indeterminate' keyword. This has the same type as the

  * function 'indeterminate' so that we can recognize when the keyword is

  * used.

  */

 typedef bool (*indeterminate_keyword_t)(tribool, detail::indeterminate_t);

 /**

  * \brief Keyword and test function for the indeterminate tribool value

  *

  * The \c indeterminate function has a dual role. It's first role is

  * as a unary function that tells whether the tribool value is in the

  * "indeterminate" state. It's second role is as a keyword

  * representing the indeterminate (just like "true" and "false"

  * represent the true and false states). If you do not like the name

  * "indeterminate", and would prefer to use a different name, see the

  * macro \c BOOST_TRIBOOL_THIRD_STATE.

  *

  * \returns <tt>x.value == tribool::indeterminate_value</tt>

  * \throws nothrow

  */

 inline bool

 indeterminate(tribool x,

               detail::indeterminate_t dummy = detail::indeterminate_t());

 /**

  * \brief A 3-state boolean type.

  *

  * 3-state boolean values are either true, false, or

  * indeterminate.

  */

 class tribool

 {

 private:

   /// INTERNAL ONLY

   struct dummy {

     void nonnull() {};

   };

   typedef void (dummy::*safe_bool)();

 public:

   /**

    * Construct a new 3-state boolean value with the value 'false'.

    *

    * \throws nothrow

    */

   tribool() : value(false_value) {}

   /**

    * Construct a new 3-state boolean value with the given boolean

    * value, which may be \c true or \c false.

    *

    * \throws nothrow

    */

   tribool(bool value) : value(value? true_value : false_value) {}

   /**

    * Construct a new 3-state boolean value with an indeterminate value.

    *

    * \throws nothrow

    */

   tribool(indeterminate_keyword_t) : value(indeterminate_value) {}

   /**

    * Use a 3-state boolean in a boolean context. Will evaluate true in a

    * boolean context only when the 3-state boolean is definitely true.

    *

    * \returns true if the 3-state boolean is true, false otherwise

    * \throws nothrow

    */

   operator safe_bool() const

   {

     return value == true_value? &dummy::nonnull : 0;

   }

   /**

    * The actual stored value in this 3-state boolean, which may be false, true,

    * or indeterminate.

    */

   enum value_t { false_value, true_value, indeterminate_value } value;

 };

 // Check if the given tribool has an indeterminate value. Also doubles as a

 // keyword for the 'indeterminate' value

 inline bool indeterminate(tribool x, detail::indeterminate_t)

 {

   return x.value == tribool::indeterminate_value;

 }

 /** @defgroup logical Logical operations

  */

 //@{

 /**

  * \brief Computes the logical negation of a tribool

  *

  * \returns the logical negation of the tribool, according to the

  * table:

  *  <table border=1>

  *    <tr>

  *      <th><center><code>!</code></center></th>

  *      <th/>

  *    </tr>

  *    <tr>

  *      <th><center>false</center></th>

  *      <td><center>true</center></td>

  *    </tr>

  *    <tr>

  *      <th><center>true</center></th>

  *      <td><center>false</center></td>

  *    </tr>

  *    <tr>

  *      <th><center>indeterminate</center></th>

  *      <td><center>indeterminate</center></td>

  *    </tr>

  *  </table>

  * \throws nothrow

  */

 inline tribool operator!(tribool x)

 {

   return x.value == tribool::false_value? tribool(true)

         :x.value == tribool::true_value? tribool(false)

         :tribool(indeterminate);

 }

 /**

  * \brief Computes the logical conjuction of two tribools

  *

  * \returns the result of logically ANDing the two tribool values,

  * according to the following table:

  *       <table border=1>

  *           <tr>

  *             <th><center><code>&amp;&amp;</code></center></th>

  *             <th><center>false</center></th>

  *             <th><center>true</center></th>

  *             <th><center>indeterminate</center></th>

  *           </tr>

  *           <tr>

  *             <th><center>false</center></th>

  *             <td><center>false</center></td>

  *             <td><center>false</center></td>

  *             <td><center>false</center></td>

  *           </tr>

  *           <tr>

  *             <th><center>true</center></th>

  *             <td><center>false</center></td>

  *             <td><center>true</center></td>

  *             <td><center>indeterminate</center></td>

  *           </tr>

  *           <tr>

  *             <th><center>indeterminate</center></th>

  *             <td><center>false</center></td>

  *             <td><center>indeterminate</center></td>

  *             <td><center>indeterminate</center></td>

  *           </tr>

  *       </table>

  * \throws nothrow

  */

 inline tribool operator&&(tribool x, tribool y)

 {

   if (static_cast<bool>(!x) || static_cast<bool>(!y))

     return false;

   else if (static_cast<bool>(x) && static_cast<bool>(y))

     return true;

   else

     return indeterminate;

 }

 /**

  * \overload

  */

 inline tribool operator&&(tribool x, bool y)

 { return y? x : tribool(false); }

 /**

  * \overload

  */

 inline tribool operator&&(bool x, tribool y)

 { return x? y : tribool(false); }

 /**

  * \overload

  */

 inline tribool operator&&(indeterminate_keyword_t, tribool x)

 { return !x? tribool(false) : tribool(indeterminate); }

 /**

  * \overload

  */

 inline tribool operator&&(tribool x, indeterminate_keyword_t)

 { return !x? tribool(false) : tribool(indeterminate); }

 /**

  * \brief Computes the logical disjunction of two tribools

  *

  * \returns the result of logically ORing the two tribool values,

  * according to the following table:

  *       <table border=1>

  *           <tr>

  *             <th><center><code>||</code></center></th>

  *             <th><center>false</center></th>

  *             <th><center>true</center></th>

  *             <th><center>indeterminate</center></th>

  *           </tr>

  *           <tr>

  *             <th><center>false</center></th>

  *             <td><center>false</center></td>

  *             <td><center>true</center></td>

  *             <td><center>indeterminate</center></td>

  *           </tr>

  *           <tr>

  *             <th><center>true</center></th>

  *             <td><center>true</center></td>

  *             <td><center>true</center></td>

  *             <td><center>true</center></td>

  *           </tr>

  *           <tr>

  *             <th><center>indeterminate</center></th>

  *             <td><center>indeterminate</center></td>

  *             <td><center>true</center></td>

  *             <td><center>indeterminate</center></td>

  *           </tr>

  *       </table>

  *  \throws nothrow

  */

 inline tribool operator||(tribool x, tribool y)

 {

   if (static_cast<bool>(!x) && static_cast<bool>(!y))

     return false;

   else if (static_cast<bool>(x) || static_cast<bool>(y))

     return true;

   else

     return indeterminate;

 }

 /**

  * \overload

  */

 inline tribool operator||(tribool x, bool y)

 { return y? tribool(true) : x; }

 /**

  * \overload

  */

 inline tribool operator||(bool x, tribool y)

 { return x? tribool(true) : y; }

 /**

  * \overload

  */

 inline tribool operator||(indeterminate_keyword_t, tribool x)

 { return x? tribool(true) : tribool(indeterminate); }

 /**

  * \overload

  */

 inline tribool operator||(tribool x, indeterminate_keyword_t)

 { return x? tribool(true) : tribool(indeterminate); }

 //@}

 /**

  * \brief Compare tribools for equality

  *

  * \returns the result of comparing two tribool values, according to

  * the following table:

  *       <table border=1>

  *          <tr>

  *            <th><center><code>==</code></center></th>

  *            <th><center>false</center></th>

  *            <th><center>true</center></th>

  *            <th><center>indeterminate</center></th>

  *          </tr>

  *          <tr>

  *            <th><center>false</center></th>

  *            <td><center>true</center></td>

  *            <td><center>false</center></td>

  *            <td><center>indeterminate</center></td>

  *          </tr>

  *          <tr>

  *            <th><center>true</center></th>

  *            <td><center>false</center></td>

  *            <td><center>true</center></td>

  *            <td><center>indeterminate</center></td>

  *          </tr>

  *          <tr>

  *            <th><center>indeterminate</center></th>

  *            <td><center>indeterminate</center></td>

  *            <td><center>indeterminate</center></td>

  *            <td><center>indeterminate</center></td>

  *          </tr>

  *      </table>

  * \throws nothrow

  */

 inline tribool operator==(tribool x, tribool y)

 {

   if (indeterminate(x) || indeterminate(y))

     return indeterminate;

   else

     return x && y || !x && !y;

 }

 /**

  * \overload

  */

 inline tribool operator==(tribool x, bool y) { return x == tribool(y); }

 /**

  * \overload

  */

 inline tribool operator==(bool x, tribool y) { return tribool(x) == y; }

 /**

  * \overload

  */

 inline tribool operator==(indeterminate_keyword_t, tribool x)

 { return tribool(indeterminate) == x; }

 /**

  * \overload

  */

 inline tribool operator==(tribool x, indeterminate_keyword_t)

 { return tribool(indeterminate) == x; }

 /**

  * \brief Compare tribools for inequality

  *

  * \returns the result of comparing two tribool values for inequality,

  * according to the following table:

  *       <table border=1>

  *           <tr>

  *             <th><center><code>!=</code></center></th>

  *             <th><center>false</center></th>

  *             <th><center>true</center></th>

  *             <th><center>indeterminate</center></th>

  *           </tr>

  *           <tr>

  *             <th><center>false</center></th>

  *             <td><center>false</center></td>

  *             <td><center>true</center></td>

  *             <td><center>indeterminate</center></td>

  *           </tr>

  *           <tr>

  *             <th><center>true</center></th>

  *             <td><center>true</center></td>

  *             <td><center>false</center></td>

  *             <td><center>indeterminate</center></td>

  *           </tr>

  *           <tr>

  *             <th><center>indeterminate</center></th>

  *             <td><center>indeterminate</center></td>

  *             <td><center>indeterminate</center></td>

  *             <td><center>indeterminate</center></td>

  *           </tr>

  *       </table>

  * \throws nothrow

  */

 inline tribool operator!=(tribool x, tribool y)

 {

   if (indeterminate(x) || indeterminate(y))

     return indeterminate;

   else

     return !(x && y || !x && !y);

 }

 /**

  * \overload

  */

 inline tribool operator!=(tribool x, bool y) { return x != tribool(y); }

 /**

  * \overload

  */

 inline tribool operator!=(bool x, tribool y) { return tribool(x) != y; }

 /**

  * \overload

  */

 inline tribool operator!=(indeterminate_keyword_t, tribool x)

 { return tribool(indeterminate) != x; }

 /**

  * \overload

  */

 inline tribool operator!=(tribool x, indeterminate_keyword_t)

 { return x != tribool(indeterminate); }

 } } // end namespace boost::logic

 // Pull tribool and indeterminate into namespace "boost"

 namespace boost {

   using logic::tribool;

   using logic::indeterminate;

 }

 /**

  * \brief Declare a new name for the third state of a tribool

  *

  * Use this macro to declare a new name for the third state of a

  * tribool. This state can have any number of new names (in addition

  * to \c indeterminate), all of which will be equivalent. The new name will be

  * placed in the namespace in which the macro is expanded.

  *

  * Example:

  *   BOOST_TRIBOOL_THIRD_STATE(true_or_false)

  *

  *   tribool x(true_or_false);

  *   // potentially set x

  *   if (true_or_false(x)) {

  *     // don't know what x is

  *   }

  */

 #define BOOST_TRIBOOL_THIRD_STATE(Name)                                 \

 inline bool                                                             \

 Name(boost::logic::tribool x,                                           \

      boost::logic::detail::indeterminate_t dummy =                      \

        boost::logic::detail::indeterminate_t())                         \

 { return x.value == boost::logic::tribool::indeterminate_value; }

 #endif // BOOST_LOGIC_TRIBOOL_HPP