util/OptionalField: new library

src/util/OptionalField.hxx

@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// Copyright CM4all GmbH
+// author: Max Kellermann <mk@cm4all.com>
+
+#pragma once
+
+#include <utility> // for std::forward()
+
+/**
+ * Helps with declaring a field that is present only under a certain
+ * (compile-time) condition.  If `enable` is true, this struct
+ * contains a field named `value` of the specified type.  To avoid
+ * memory overhead when disabled, add the attribute
+ * [[no_unique_address]].
+ */
+template<typename T, bool enable> struct OptionalField;
+
+template<typename T>
+struct OptionalField<T, false>
+{
+	template<typename... Args>
+	constexpr OptionalField(Args&&...) {}
+};
+
+template<typename T>
+struct OptionalField<T, true>
+{
+	T value;
+
+	template<typename... Args>
+	constexpr OptionalField(Args&&... args)
+		:value(std::forward<Args>(args)...) {}
+};