split storage from value

also, quit inheritance in storage class

toml/storage.hpp

@@ -0,0 +1,42 @@
+#ifndef TOML11_STORAGE_HPP
+#define TOML11_STORAGE_HPP
+#include "utility.hpp"
+
+namespace toml
+{
+namespace detail
+{
+
+// this contains pointer and deep-copy the content if copied.
+// to avoid recursive pointer.
+template<typename T>
+struct storage
+{
+    using value_type = T;
+
+    storage(value_type const& v): ptr(toml::make_unique<T>(v)) {}
+    storage(value_type&&      v): ptr(toml::make_unique<T>(std::move(v))) {}
+    ~storage() = default;
+
+    storage(const storage& rhs): ptr(toml::make_unique<T>(*rhs.ptr)) {}
+    storage& operator=(const storage& rhs)
+    {
+        this->ptr = toml::make_unique<T>(*rhs.ptr);
+        return *this;
+    }
+    storage(storage&&) = default;
+    storage& operator=(storage&&) = default;
+
+    bool is_ok() const noexcept {return static_cast<bool>(ptr);}
+
+    value_type&       value() &      noexcept {return *ptr;}
+    value_type const& value() const& noexcept {return *ptr;}
+    value_type&&      value() &&     noexcept {return std::move(*ptr);}
+
+  private:
+    std::unique_ptr<value_type> ptr;
+};
+
+} // detail
+} // toml
+#endif// TOML11_STORAGE_HPP