xo-object2: beginning of DString + utest

include/xo/object2/DString.hpp

@@ -0,0 +1,58 @@
+/** @file DString.hpp
+*
+ *  @author Roland Conybeare, Jan 2026
+ **/
+
+#pragma once
+
+#include <xo/alloc2/Allocator.hpp>
+#include <xo/facet/obj.hpp>
+#include <cstdint>
+
+namespace xo {
+    namespace scm {
+        /** @class DString
+         *  @brief String implementation with gc hooks
+         *
+         *  String implementation for Schematika.
+         *  Size-prefixed and null-terminated.
+         *  Note however that string length != size for utf-8.
+         *
+         *  Uses flexible array for chars,
+         *  with string contents in memory immediately
+         *  following the DString itself
+         **/
+        struct DString {
+        public:
+            using size_type = std::uint32_t;
+            using AAllocator = xo::mm::AAllocator;
+
+            /** create empty string with space for @cap chars
+             *  (including null terminator).
+             *  Use memory from allocator @p mm
+             **/
+            static DString * empty(obj<AAllocator> mm,
+                                   size_type cap);
+
+            /** create string containing a copy of null-terminated @p cstr.
+             *  Use memory from allocator @p mm
+             **/
+            static DString * from_cstr(obj<AAllocator> mm,
+                                       const char * cstr);
+
+            size_type capacity() const noexcept { return capacity_; }
+            size_type size() const noexcept { return size_; }
+            const char * chars() const noexcept { return chars_; }
+
+        private:
+            /** extent of @ref chars_ array **/
+            size_type capacity_ = 0;
+            /** null terminator at @c chars_[size_] **/
+            size_type size_ = 0;
+            /** string contents **/
+            char chars_[];
+        };
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/* end DString.hpp */