Add 'xo-expression/' from commit '5ac3c03a0c'

git-subtree-dir: xo-expression
git-subtree-mainline: d0f5ccc1ce
git-subtree-split: 5ac3c03a0c

xo-expression/include/xo/expression/ConstantInterface.hpp

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+/** @file ConstantInterface.hpp
+ *
+ *  Author: Roland Conybeare
+ **/
+
+#pragma once
+
+#include "Expression.hpp"
+#include "xo/reflect/Reflect.hpp"
+#include "xo/reflect/TypeDescr.hpp"
+#include <type_traits>
+
+namespace xo {
+    namespace ast {
+        /** @class ConstantInterface
+         *  @brief syntax for a literal constant.
+         **/
+        class ConstantInterface : public Expression {
+        public:
+            using TaggedPtr = xo::reflect::TaggedPtr;
+            using TypeDescr = xo::reflect::TypeDescr;
+
+        public:
+            /** @p extype sets expression-type;  could be constant|primitive **/
+            ConstantInterface(exprtype extype, TypeDescr valuetype) : Expression{extype, valuetype} {}
+
+            /** downcast from Expression **/
+            static ref::brw<ConstantInterface> from(ref::brw<Expression> x) {
+                return ref::brw<ConstantInterface>::from(x);
+            }
+
+            /** type description for representation of literal value **/
+            virtual TypeDescr value_td() const = 0;
+            /** reflection-tagged pointer to literal value of this constant **/
+            virtual TaggedPtr value_tp() const = 0;
+
+            // ----- Expression -----
+
+            virtual std::set<std::string> get_free_variables() const override {
+                return std::set<std::string>();
+            }
+
+            virtual void attach_envs(ref::brw<Environment> /*p*/) override {}
+
+
+        }; /*ConstantInterface*/
+
+    } /*namespace ast*/
+} /*namespace xo*/
+
+/** end ConstantInterface.hpp **/