commit 9a944cd50046badb614f1f7a89c60a3648722e15
Author: Jonathan Müller <git@foonathan.net>
Date:   Fri Jan 20 23:00:47 2017 +0100

    Add basic entity

diff --git a/include/cppast/cpp_entity.hpp b/include/cppast/cpp_entity.hpp
new file mode 100644
index 0000000..0276d62
--- /dev/null
+++ b/include/cppast/cpp_entity.hpp
@@ -0,0 +1,61 @@
+// Copyright (C) 2017 Jonathan Müller <jonathanmueller.dev@gmail.com>
+// This file is subject to the license terms in the LICENSE file
+// found in the top-level directory of this distribution.
+
+#ifndef CPPAST_CPP_ENTITY_HPP_INCLUDED
+#define CPPAST_CPP_ENTITY_HPP_INCLUDED
+
+#include <type_safe/optional_ref.hpp>
+
+#include <cppast/detail/intrusive_list.hpp>
+
+namespace cppast
+{
+    enum class cpp_entity_type;
+
+    /// The base class for all entities in the C++ AST.
+    class cpp_entity : detail::intrusive_list_node<cpp_entity>
+    {
+    public:
+        cpp_entity(const cpp_entity&) = delete;
+        cpp_entity& operator=(const cpp_entity&) = delete;
+
+        virtual ~cpp_entity() noexcept = default;
+
+        /// \returns The type of the entity.
+        cpp_entity_type type() const noexcept
+        {
+            return do_get_entity_type();
+        }
+
+        /// \returns The name of the entity.
+        /// The name is the string associated with the entity's declaration.
+        const std::string& name() const noexcept
+        {
+            return name_;
+        }
+
+        /// \returns A [ts::optional_ref]() to the parent entity in the AST.
+        type_safe::optional_ref<cpp_entity> parent() const noexcept
+        {
+            return parent_;
+        }
+
+    protected:
+        /// \effects Creates it giving it the parent entity and the name.
+        cpp_entity(type_safe::optional_ref<cpp_entity> parent, std::string name)
+        : parent_(std::move(parent)), name_(std::move(name))
+        {
+        }
+
+    private:
+        virtual cpp_entity_type do_get_entity_type() const noexcept = 0;
+
+        type_safe::optional_ref<cpp_entity> parent_;
+        std::string                         name_;
+
+        friend detail::intrusive_list_access<cpp_entity>;
+    };
+} // namespace cppast
+
+#endif // CPPAST_CPP_ENTITY_HPP_INCLUDED
diff --git a/include/cppast/cpp_entity_type.hpp b/include/cppast/cpp_entity_type.hpp
new file mode 100644
index 0000000..d79627c
--- /dev/null
+++ b/include/cppast/cpp_entity_type.hpp
@@ -0,0 +1,17 @@
+// Copyright (C) 2017 Jonathan Müller <jonathanmueller.dev@gmail.com>
+// This file is subject to the license terms in the LICENSE file
+// found in the top-level directory of this distribution.
+
+#ifndef CPPAST_CPP_ENTITY_TYPE_HPP_INCLUDED
+#define CPPAST_CPP_ENTITY_TYPE_HPP_INCLUDED
+
+namespace cppast
+{
+    /// All possible types of C++ entities.
+    enum class cpp_entity_type
+    {
+
+    };
+} // namespace cppast
+
+#endif // CPPAST_CPP_ENTITY_TYPE_HPP_INCLUDED
diff --git a/include/cppast/detail/assert.hpp b/include/cppast/detail/assert.hpp
new file mode 100644
index 0000000..e855936
--- /dev/null
+++ b/include/cppast/detail/assert.hpp
@@ -0,0 +1,20 @@
+// Copyright (C) 2017 Jonathan Müller <jonathanmueller.dev@gmail.com>
+// This file is subject to the license terms in the LICENSE file
+// found in the top-level directory of this distribution.
+
+#ifndef CPPAST_ASSERT_HPP_INCLUDED
+#define CPPAST_ASSERT_HPP_INCLUDED
+
+#include <debug_assert.hpp>
+
+namespace cppast
+{
+    namespace detail
+    {
+        struct assert_handler : debug_assert::set_level<1>, debug_assert::default_handler
+        {
+        };
+    }
+} // namespace cppast::detail
+
+#endif // CPPAST_ASSERT_HPP_INCLUDED
diff --git a/include/cppast/detail/intrusive_list.hpp b/include/cppast/detail/intrusive_list.hpp
new file mode 100644
index 0000000..4e685f8
--- /dev/null
+++ b/include/cppast/detail/intrusive_list.hpp
@@ -0,0 +1,189 @@
+// Copyright (C) 2017 Jonathan Müller <jonathanmueller.dev@gmail.com>
+// This file is subject to the license terms in the LICENSE file
+// found in the top-level directory of this distribution.
+
+#ifndef CPPAST_INTRUSIVE_LIST_HPP_INCLUDED
+#define CPPAST_INTRUSIVE_LIST_HPP_INCLUDED
+
+#include <memory>
+#include <iterator>
+
+#include <type_safe/optional_ref.hpp>
+
+#include <cppast/detail/assert.hpp>
+
+namespace cppast
+{
+    namespace detail
+    {
+        template <typename T>
+        class intrusive_list;
+
+        template <typename T>
+        class intrusive_list_node
+        {
+            std::unique_ptr<T> next_;
+
+            template <typename U>
+            friend struct intrusive_list_access;
+        };
+
+        template <typename T>
+        struct intrusive_list_access
+        {
+            template <typename U>
+            static T* get_next(const intrusive_list_node<U>& obj)
+            {
+                static_assert(std::is_base_of<U, T>::value, "must be a base");
+                return static_cast<T*>(obj.next_.get());
+            }
+
+            template <typename U>
+            static T* set_next(intrusive_list_node<U>& obj, std::unique_ptr<T> node)
+            {
+                static_assert(std::is_base_of<U, T>::value, "must be a base");
+                obj.next_ = std::move(node);
+                return obj.next_.get();
+            }
+        };
+
+        template <typename T>
+        class intrusive_list_iterator
+        {
+        public:
+            using value_type        = T;
+            using reference         = T&;
+            using pointer           = T*;
+            using difference_type   = std::ptrdiff_t;
+            using iterator_category = std::forward_iterator_tag;
+
+            intrusive_list_iterator() noexcept : cur_(nullptr)
+            {
+            }
+
+            reference operator*() const noexcept
+            {
+                return *cur_;
+            }
+
+            pointer operator->() const noexcept
+            {
+                return cur_;
+            }
+
+            intrusive_list_iterator& operator++() noexcept
+            {
+                cur_ = intrusive_list_access<T>::get_next(*cur_);
+                return *this;
+            }
+
+            intrusive_list_iterator operator++(int)noexcept
+            {
+                auto tmp = *this;
+                ++(*this);
+                return tmp;
+            }
+
+            friend bool operator==(const intrusive_list_iterator& a,
+                                   const intrusive_list_iterator& b) noexcept
+            {
+                return a.cur_ == b.cur_;
+            }
+
+            friend bool operator!=(const intrusive_list_iterator& a,
+                                   const intrusive_list_iterator& b) noexcept
+            {
+                return !(a == b);
+            }
+
+        private:
+            intrusive_list_iterator(T* ptr) : cur_(ptr)
+            {
+            }
+
+            T* cur_;
+
+            friend intrusive_list<T>;
+        };
+
+        template <typename T>
+        class intrusive_list
+        {
+        public:
+            intrusive_list() = default;
+
+            //=== modifiers ===//
+            void push_back(std::unique_ptr<T> obj) noexcept
+            {
+                DEBUG_ASSERT(obj, detail::assert_handler{});
+
+                if (last_)
+                {
+                    auto ptr = intrusive_list_access<T>::set_next(last_.value(), std::move(obj));
+                    last_    = type_safe::opt_cref(ptr);
+                }
+                else
+                {
+                    first_ = std::move(obj);
+                    last_  = type_safe::opt_ref(first_.get());
+                }
+            }
+
+            //=== accesors ===//
+            bool empty() const noexcept
+            {
+                return first_ == nullptr;
+            }
+
+            type_safe::optional_ref<T> front() noexcept
+            {
+                return type_safe::opt_ref(first_.get());
+            }
+
+            type_safe::optional_ref<const T> front() const noexcept
+            {
+                return type_safe::opt_cref(first_.get());
+            }
+
+            type_safe::optional_ref<T> back() noexcept
+            {
+                return last_;
+            }
+
+            type_safe::optional_ref<const T> back() const noexcept
+            {
+                return last_;
+            }
+
+            //=== iterators ===//
+            using iterator       = intrusive_list_iterator<T>;
+            using const_iterator = intrusive_list_iterator<const T>;
+
+            iterator begin() noexcept
+            {
+                return iterator(first_.get());
+            }
+
+            iterator end() noexcept
+            {
+                return {};
+            }
+
+            const_iterator begin() const noexcept
+            {
+                return const_iterator(first_.get());
+            }
+
+            const_iterator end() const noexcept
+            {
+                return {};
+            }
+
+        private:
+            std::unique_ptr<T>         first_;
+            type_safe::optional_ref<T> last_;
+        };
+    }
+} // namespace cppast::detail
+
+#endif // CPPAST_INTRUSIVE_LIST_HPP_INCLUDED