整理

include/boost/scope/detail/compact_storage.hpp

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+/*
+ * Distributed under the Boost Software License, Version 1.0.
+ * (See accompanying file LICENSE_1_0.txt or copy at
+ * https://www.boost.org/LICENSE_1_0.txt)
+ *
+ * Copyright (c) 2022 Andrey Semashev
+ */
+/*!
+ * \file scope/detail/compact_storage.hpp
+ *
+ * This header contains utility helpers for implementing compact storage
+ * for class members. In particular, it allows to leverage empty base optimization (EBO).
+ */
+
+#ifndef BOOST_SCOPE_DETAIL_COMPACT_STORAGE_HPP_INCLUDED_
+#define BOOST_SCOPE_DETAIL_COMPACT_STORAGE_HPP_INCLUDED_
+
+#include <type_traits>
+#include <boost/scope/detail/config.hpp>
+#include <boost/scope/detail/type_traits/is_final.hpp>
+#include <boost/scope/detail/type_traits/negation.hpp>
+#include <boost/scope/detail/type_traits/conjunction.hpp>
+#include <boost/scope/detail/header.hpp>
+
+#ifdef BOOST_HAS_PRAGMA_ONCE
+#pragma once
+#endif
+
+namespace boost {
+namespace scope {
+namespace detail {
+
+//! The class allows to place data members in the tail padding of type \a T if the user's class derives from it
+template<
+    typename T,
+    typename Tag = void,
+    bool = detail::conjunction< std::is_class< T >, detail::negation< detail::is_final< T > > >::value
+>
+class compact_storage :
+    private T
+{
+public:
+    template< typename... Args >
+    constexpr compact_storage(Args&&... args) noexcept(std::is_nothrow_constructible< T, Args... >::value) :
+        T(static_cast< Args&& >(args)...)
+    {
+    }
+
+    compact_storage(compact_storage&&) = default;
+    compact_storage& operator= (compact_storage&&) = default;
+
+    compact_storage(compact_storage const&) = default;
+    compact_storage& operator= (compact_storage const&) = default;
+
+    T& get() noexcept
+    {
+        return *static_cast< T* >(this);
+    }
+
+    T const& get() const noexcept
+    {
+        return *static_cast< const T* >(this);
+    }
+};
+
+template< typename T, typename Tag >
+class compact_storage< T, Tag, false >
+{
+private:
+    T m_data;
+
+public:
+    template< typename... Args >
+    constexpr compact_storage(Args&&... args) noexcept(std::is_nothrow_constructible< T, Args... >::value) :
+        m_data(static_cast< Args&& >(args)...)
+    {
+    }
+
+    compact_storage(compact_storage&&) = default;
+    compact_storage& operator= (compact_storage&&) = default;
+
+    compact_storage(compact_storage const&) = default;
+    compact_storage& operator= (compact_storage const&) = default;
+
+    T& get() noexcept
+    {
+        return m_data;
+    }
+
+    T const& get() const noexcept
+    {
+        return m_data;
+    }
+};
+
+} // namespace detail
+} // namespace scope
+} // namespace boost
+
+#include <boost/scope/detail/footer.hpp>
+
+#endif // BOOST_SCOPE_DETAIL_COMPACT_STORAGE_HPP_INCLUDED_