Implement empty_value

2018-08-22 11:09:31 -04:00
parent e1f070b7b4
commit f504872d89
4 changed files with 252 additions and 0 deletions
@@ -0,0 +1,113 @@
+/*
+Copyright 2018 Glen Joseph Fernandes
+(glenjofe@gmail.com)
+
+Distributed under the Boost Software License, Version 1.0.
+(http://www.boost.org/LICENSE_1_0.txt)
+*/
+#ifndef BOOST_CORE_EMPTY_VALUE_HPP
+#define BOOST_CORE_EMPTY_VALUE_HPP
+
+#include <boost/config.hpp>
+#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
+#include <utility>
+#endif
+
+#if defined(BOOST_GCC_VERSION) && (BOOST_GCC_VERSION >= 40700)
+#define BOOST_DETAIL_EMPTY_VALUE_BASE
+#elif defined(BOOST_INTEL) && defined(_MSC_VER) && (_MSC_VER >= 1800)
+#define BOOST_DETAIL_EMPTY_VALUE_BASE
+#elif defined(BOOST_MSVC) && (BOOST_MSVC >= 1800)
+#define BOOST_DETAIL_EMPTY_VALUE_BASE
+#elif defined(BOOST_CLANG) && !defined(__CUDACC__)
+#if __has_feature(is_empty) && __has_feature(is_final)
+#define BOOST_DETAIL_EMPTY_VALUE_BASE
+#endif
+#endif
+
+namespace boost {
+
+template<class T>
+struct use_empty_value_base {
+    enum {
+#if defined(BOOST_DETAIL_EMPTY_VALUE_BASE)
+        value = __is_empty(T) && !__is_final(T)
+#else
+        value = false
+#endif
+    };
+};
+
+struct empty_init_t { };
+
+template<class T, unsigned = 0, bool = use_empty_value_base<T>::value>
+class empty_value {
+public:
+    empty_value()
+        : value_() { }
+
+#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
+#if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
+    template<class... Args>
+    empty_value(empty_init_t, Args&&... args)
+        : value_(std::forward<Args>(args)...) { }
+#else
+    template<class U>
+    empty_value(empty_init_t, U&& value)
+        : value_(std::forward<U>(value)) { }
+#endif
+#else
+    template<class U>
+    empty_value(empty_init_t, const U& value)
+        : value_(value) { }
+#endif
+
+    const T& get() const BOOST_NOEXCEPT {
+        return value_;
+    }
+
+    T& get() BOOST_NOEXCEPT {
+        return value_;
+    }
+
+private:
+    T value_;
+};
+
+#if !defined(BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION)
+template<class T, unsigned N>
+class empty_value<T, N, true>
+    : T {
+public:
+    empty_value()
+        : T() { }
+
+#if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
+#if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES)
+    template<class... Args>
+    empty_value(empty_init_t, Args&&... args)
+        : T(std::forward<Args>(args)...) { }
+#else
+    template<class U>
+    empty_value(empty_init_t, U&& value)
+        : T(std::forward<U>(value)) { }
+#endif
+#else
+    template<class U>
+    empty_value(empty_init_t, const U& value)
+        : T(value) { }
+#endif
+
+    const T& get() const BOOST_NOEXCEPT {
+        return *this;
+    }
+
+    T& get() BOOST_NOEXCEPT {
+        return *this;
+    }
+};
+#endif
+
+} /* boost */
+
+#endif