1 /*
2  *          Copyright Andrey Semashev 2007 - 2015.
3  * Distributed under the Boost Software License, Version 1.0.
4  *    (See accompanying file LICENSE_1_0.txt or copy at
5  *          http://www.boost.org/LICENSE_1_0.txt)
6  */
7 /*!
8  * \file   singleton.hpp
9  * \author Andrey Semashev
10  * \date   20.04.2008
11  *
12  * \brief  This header is the Boost.Log library implementation, see the library documentation
13  *         at http://www.boost.org/doc/libs/release/libs/log/doc/html/index.html.
14  */
15 
16 #ifndef BOOST_LOG_DETAIL_SINGLETON_HPP_INCLUDED_
17 #define BOOST_LOG_DETAIL_SINGLETON_HPP_INCLUDED_
18 
19 #include <boost/log/detail/config.hpp>
20 #include <boost/log/utility/once_block.hpp>
21 #include <boost/log/detail/header.hpp>
22 
23 #ifdef BOOST_HAS_PRAGMA_ONCE
24 #pragma once
25 #endif
26 
27 namespace boost {
28 
29 BOOST_LOG_OPEN_NAMESPACE
30 
31 namespace aux {
32 
33 //! A base class for singletons, constructed on-demand
34 template< typename DerivedT, typename StorageT = DerivedT >
35 class lazy_singleton
36 {
37 public:
lazy_singleton()38     BOOST_DEFAULTED_FUNCTION(lazy_singleton(), {})
39 
40     //! Returns the singleton instance
41     static StorageT& get()
42     {
43         BOOST_LOG_ONCE_BLOCK()
44         {
45             DerivedT::init_instance();
46         }
47         return get_instance();
48     }
49 
50     //! Initializes the singleton instance
init_instance()51     static void init_instance()
52     {
53         get_instance();
54     }
55 
56     BOOST_DELETED_FUNCTION(lazy_singleton(lazy_singleton const&))
57     BOOST_DELETED_FUNCTION(lazy_singleton& operator= (lazy_singleton const&))
58 
59 protected:
60     //! Returns the singleton instance (not thread-safe)
get_instance()61     static StorageT& get_instance()
62     {
63         static StorageT instance;
64         return instance;
65     }
66 };
67 
68 //! A base class for singletons, constructed on namespace scope initialization stage
69 template< typename DerivedT, typename StorageT = DerivedT >
70 class singleton :
71     public lazy_singleton< DerivedT, StorageT >
72 {
73 public:
74     static StorageT& instance;
75 };
76 
77 template< typename DerivedT, typename StorageT >
78 StorageT& singleton< DerivedT, StorageT >::instance =
79     lazy_singleton< DerivedT, StorageT >::get();
80 
81 } // namespace aux
82 
83 BOOST_LOG_CLOSE_NAMESPACE // namespace log
84 
85 } // namespace boost
86 
87 #include <boost/log/detail/footer.hpp>
88 
89 #endif // BOOST_LOG_DETAIL_SINGLETON_HPP_INCLUDED_
90