libcamera: yaml_parser: Delegate YamlObject::get() to helper structure

The YamlObject::get() function is a function template that gets fully
specialized for various types. This works fine for non-template types,
but specializing it for template types (e.g. a std::vector<U>) would
require partial template specialization, which C++ allows for classes
and variables but not functions.

To work around this problem, delegate the implementation to a new
YamlObject::Getter structure template, which will support partial
specialization.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/internal/yaml_parser.h

@@ -162,7 +162,10 @@ public:
 	std::size_t size() const;
 
 	template<typename T>
-	std::optional<T> get() const;
+	std::optional<T> get() const
+	{
+		return Getter<T>{}.get(*this);
+	}
 
 	template<typename T, typename U>
 	T get(U &&defaultValue) const
@@ -199,6 +202,8 @@ public:
 private:
 	LIBCAMERA_DISABLE_COPY_AND_MOVE(YamlObject)
 
+	template<typename T>
+	friend struct Getter;
 	friend class YamlParserContext;
 
 	enum class Type {
@@ -207,6 +212,11 @@ private:
 		Value,
 	};
 
+	template<typename T>
+	struct Getter {
+		std::optional<T> get(const YamlObject &obj) const;
+	};
+
 	Type type_;
 
 	std::string value_;