1 // Copyright (C) 2003, Fernando Luis Cacciola Carballal.
2 // Copyright (C) 2014, 2015 Andrzej Krzemienski.
3 //
4 // Distributed under the Boost Software License, Version 1.0.
5 // (See accompanying file LICENSE_1_0.txt or copy at
6 // http://www.boost.org/LICENSE_1_0.txt)
7 //
8 // See http://www.boost.org/libs/optional for documentation.
9 //
10 // You are welcome to contact the author at:
11 //  fernando_cacciola@hotmail.com
12 //
13 #ifndef BOOST_NONE_17SEP2003_HPP
14 #define BOOST_NONE_17SEP2003_HPP
15 
16 #include "boost/config.hpp"
17 #include "boost/none_t.hpp"
18 
19 // NOTE: Borland users have to include this header outside any precompiled headers
20 // (bcc<=5.64 cannot include instance data in a precompiled header)
21 //  -- * To be verified, now that there's no unnamed namespace
22 
23 namespace boost {
24 
25 #ifdef BOOST_OPTIONAL_USE_OLD_DEFINITION_OF_NONE
26 
27 BOOST_INLINE_VARIABLE none_t BOOST_CONSTEXPR_OR_CONST none = (static_cast<none_t>(0)) ;
28 
29 #elif defined BOOST_OPTIONAL_USE_SINGLETON_DEFINITION_OF_NONE
30 
31 namespace detail { namespace optional_detail {
32 
33   // the trick here is to make boost::none defined once as a global but in a header file
34   template <typename T>
35   struct none_instance
36   {
37     static const T instance;
38   };
39 
40   template <typename T>
41   const T none_instance<T>::instance = T(); // global, but because 'tis a template, no cpp file required
42 
43 } } // namespace detail::optional_detail
44 
45 
46 namespace {
47   // TU-local
48   const none_t& none = detail::optional_detail::none_instance<none_t>::instance;
49 }
50 
51 #else
52 
53 BOOST_INLINE_VARIABLE BOOST_CONSTEXPR_OR_CONST none_t none ((none_t::init_tag()));
54 
55 #endif // older definitions
56 
57 } // namespace boost
58 
59 #endif // header guard
60