xo-expression: initial commit (constant + primitive)

include/xo/expression/Constant.hpp

@@ -0,0 +1,78 @@
+/** @file Constant.hpp
+ *
+ *  Author: Roland Conybeare
+ **/
+
+#pragma once
+
+#include "ConstantInterface.hpp"
+#include <type_traits>
+
+namespace xo {
+    namespace ast {
+        /** @class Constant
+         *  @brief syntax for a literal constant.
+         *
+         *  Require:
+         *  1. T must be a POD type (plain old data)
+         *     We need this to be true so that we can generate
+         *     code for constructing a T instance by memcopying
+         *     @ref value_
+         *
+         *  @tp T type of captured literal.
+         **/
+        template <typename T>
+        class Constant : public ConstantInterface {
+        public:
+            using Reflect = xo::reflect::Reflect;
+            using TaggedPtr = xo::reflect::TaggedPtr;
+            using TypeDescr = xo::reflect::TypeDescr;
+
+        public:
+            explicit Constant(const T & x)
+                : ConstantInterface(exprtype::constant),
+                  value_td_{Reflect::require<T>()},
+                  value_(x)
+                {
+                    static_assert(std::is_standard_layout_v<T> && std::is_trivial_v<T>);
+                }
+
+            const T & value() const { return value_; }
+
+            // ----- ConstantInterface -----
+
+            virtual TypeDescr value_td() const override { return value_td_; }
+            virtual TaggedPtr value_tp() const override {
+                /* note: idk why,  but need to spell this out in two steps with gcc 13.2 */
+                const void * erased_cptr = &value_;
+                void * erased_ptr = const_cast<void*>(erased_cptr);
+
+                return TaggedPtr(value_td_, erased_ptr);
+            }
+
+            // ----- Expression -----
+
+            virtual void display(std::ostream & os) const override {
+                os << "<Constant"
+                   << xtag("type", value_td_->short_name())
+                   << xtag("value", value_)
+                   << ">";
+            }
+
+        private:
+            /** type description for T **/
+            TypeDescr value_td_;
+            /** value of this constant **/
+            T value_;
+        }; /*Constant*/
+
+        template <typename T>
+        ref::rp<Constant<std::remove_reference_t<T>>>
+        make_constant(const T & x) {
+            return new Constant(x);
+        }
+
+    } /*namespace ast*/
+} /*namespace xo*/
+
+/** end Constant.hpp **/