Add method to cast ValueHolder to specific Value.

Author: heinezen

nyan/concept.h

@@ -4,17 +4,24 @@
 
 #include <concepts>
 
-#include "nyan/value/value.h"
-
 
 namespace nyan {
 
+class Value;
+class Object;
+
+/**
+ * Type that is a nyan value.
+ */
+template <typename T>
+concept ValueLike = std::derived_from<T, Value>;
+
 /**
  * Type that is either a nyan value or object.
  * Object is not a value (ObjectValue is), but want to allow an
  * overloaded conversion for direct object access.
  */
 template <typename T>
-concept ValueLike = std::derived_from<T, Value> || std::is_same_v<T, Object>;
+concept ValueOrObjectLike = std::is_same_v<T, Object> or ValueLike<T>;
 
 } // namespace nyan

nyan/object.h

@@ -96,7 +96,7 @@ class Object {
 	 *
 	 * @return Value of the member.
 	 */
-	template <ValueLike T>
+	template <ValueOrObjectLike T>
 	std::shared_ptr<T> get(const memberid_t &member, order_t t = LATEST_T) const;
 
 	/**
@@ -110,7 +110,7 @@ class Object {
 	 *
 	 * @return Value of the member.
 	 */
-	template <ValueLike T, bool may_be_none = true>
+	template <ValueOrObjectLike T, bool may_be_none = true>
 	std::optional<std::shared_ptr<T>> get_optional(const memberid_t &member, order_t t = LATEST_T) const;
 
 	/**
@@ -337,14 +337,14 @@ class Object {
 };
 
 
-template <ValueLike T>
+template <ValueOrObjectLike T>
 std::shared_ptr<T> Object::get(const memberid_t &member, order_t t) const {
 	auto ret = this->get_optional<T, false>(member, t);
 	return *ret;
 }
 
 
-template <ValueLike T, bool may_be_none>
+template <ValueOrObjectLike T, bool may_be_none>
 std::optional<std::shared_ptr<T>> Object::get_optional(const memberid_t &member, order_t t) const {
 	std::shared_ptr<Value> value = this->get_value(member, t).get_ptr();
 	if constexpr (may_be_none) {

nyan/value/value_holder.h

@@ -1,8 +1,12 @@
-// Copyright 2017-2021 the nyan authors, LGPLv3+. See copying.md for legal info.
+// Copyright 2017-2025 the nyan authors, LGPLv3+. See copying.md for legal info.
 #pragma once
 
 #include <memory>
 
+#include "../api_error.h"
+#include "../concept.h"
+#include "../util.h"
+
 
 namespace nyan {
 
@@ -26,12 +30,26 @@ class ValueHolder {
 	ValueHolder &operator=(const std::shared_ptr<Value> &value);
 
 	/**
-	 * Get the shared pointer of this ValueHolder.
+	 * Get the shared pointer to the value wrapped by this holder.
 	 *
 	 * @return Shared pointer to this holder's value.
 	 */
 	const std::shared_ptr<Value> &get_ptr() const;
 
+	/**
+	 * Get a shared pointer to the value stored by this holder.
+	 *
+	 * Auto-converts the value to type T, which must be a nyan::Value type.
+	 *
+	 * @tparam T Type of the value to retrieve.
+	 *
+	 * @return Value stored by this holder.
+	 *
+	 * @throws InternalError if the value is not of type T.
+	 */
+	template <ValueLike T>
+	const std::shared_ptr<T> get_value_ptr() const;
+
 	/**
 	 * Check if this holder points to a value.
 	 *
@@ -74,6 +92,20 @@ class ValueHolder {
 	std::shared_ptr<Value> value;
 };
 
+
+template <ValueLike T>
+const std::shared_ptr<T> ValueHolder::get_value_ptr() const {
+	auto ret = std::dynamic_pointer_cast<T>(this->value);
+
+	if (not ret) {
+		throw APIError{"ValueHolder does not contain a value of type "
+		               + util::typestring<T>() + ", but got "
+		               + util::typestring(this->value.get())};
+	}
+
+	return ret;
+}
+
 } // namespace nyan