text: // Copyright (c) MapBox
  // All rights reserved.
  //
  // Redistribution and use in source and binary forms, with or without modification,
  // are permitted provided that the following conditions are met:
  //
  // - Redistributions of source code must retain the above copyright notice, this
  //   list of conditions and the following disclaimer.
  // - Redistributions in binary form must reproduce the above copyright notice, this
  //   list of conditions and the following disclaimer in the documentation and/or
  //   other materials provided with the distribution.
  // - Neither the name "MapBox" nor the names of its contributors may be
  //   used to endorse or promote products derived from this software without
  //   specific prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
  // ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  // WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  // DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
  // ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
  // (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  // LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
  // ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  // SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  #ifndef ARROW_UTIL_VARIANT_VISITOR_HPP
  #define ARROW_UTIL_VARIANT_VISITOR_HPP
  
  #include <utility>
  
  namespace arrow {
  namespace util {
  
  template <typename... Fns>
  struct visitor;
  
  template <typename Fn>
  struct visitor<Fn> : Fn
  {
      using Fn::operator();
  
      template<typename T>
      visitor(T&& fn) : Fn(std::forward<T>(fn)) {}
  };
  
  template <typename Fn, typename... Fns>
  struct visitor<Fn, Fns...> : Fn, visitor<Fns...>
  {
      using Fn::operator();
      using visitor<Fns...>::operator();
  
      template<typename T, typename... Ts>
      visitor(T&& fn, Ts&&... fns)
          : Fn(std::forward<T>(fn))
          , visitor<Fns...>(std::forward<Ts>(fns)...) {}
  };
  
  template <typename... Fns>
  visitor<typename std::decay<Fns>::type...> make_visitor(Fns&&... fns)
  {
      return visitor<typename std::decay<Fns>::type...>
          (std::forward<Fns>(fns)...);
  }
  
  } // namespace util
  } // namespace arrow
  
  #endif // ARROW_UTIL_VARIANT_VISITOR_HPP
  