util/DisposablePointer: new class

src/util/DisposablePointer.hxx

@@ -0,0 +1,127 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// Copyright CM4all GmbH
+// author: Max Kellermann <mk@cm4all.com>
+
+#pragma once
+
+#include <utility>
+
+/**
+ * A generic object which is owned by somebody who doesn't know how to
+ * dispose of it; to do this, a function pointer for disposing it is
+ * provided.  Some implementations may do "delete this", but others
+ * may be allocated from a custom allocator and may need different
+ * ways to dispose of it.
+ *
+ * Unlike std::any, this class does not require the contained object
+ * to be copyable; quite contrary, it is designed to adopt ownership
+ * of the contained value.
+ */
+class DisposablePointer {
+public:
+	using DisposeFunction = void(*)(void *ptr) noexcept;
+
+private:
+	void *ptr = nullptr;
+
+	DisposeFunction dispose;
+
+public:
+	DisposablePointer() = default;
+	DisposablePointer(std::nullptr_t) noexcept {}
+
+	DisposablePointer(void *_ptr, DisposeFunction _dispose) noexcept
+		:ptr(_ptr), dispose(_dispose) {}
+
+	DisposablePointer(DisposablePointer &&src) noexcept
+		:ptr(std::exchange(src.ptr, nullptr)), dispose(src.dispose) {}
+
+	~DisposablePointer() noexcept {
+		if (ptr != nullptr)
+			dispose(ptr);
+	}
+
+	DisposablePointer &operator=(DisposablePointer &&other) noexcept {
+		using std::swap;
+		swap(ptr, other.ptr);
+		swap(dispose, other.dispose);
+		return *this;
+	}
+
+	operator bool() const noexcept {
+		return ptr != nullptr;
+	}
+
+	void *get() const noexcept {
+		return ptr;
+	}
+
+	void reset() noexcept {
+		if (ptr != nullptr)
+			dispose(std::exchange(ptr, nullptr));
+	}
+};
+
+template<typename T>
+class TypedDisposablePointer : public DisposablePointer {
+public:
+	template<typename... Args>
+	TypedDisposablePointer(Args&&... args) noexcept
+		:DisposablePointer(std::forward<Args>(args)...) {}
+
+	TypedDisposablePointer(void *_ptr, DisposeFunction _dispose) noexcept;
+
+	TypedDisposablePointer(T *_ptr, DisposeFunction _dispose) noexcept
+		:DisposablePointer(_ptr, _dispose) {}
+
+	T *get() const noexcept {
+		return (T *)DisposablePointer::get();
+	}
+
+	T *operator->() const noexcept {
+		return get();
+	}
+
+	T &operator*() const noexcept {
+		return *get();
+	}
+};
+
+inline DisposablePointer
+ToNopPointer(const void *ptr) noexcept
+{
+	/* since the disposer is a no-op, we allow passing a const
+	   pointer here; the const_cast is necessary because
+	   DisposablePointer wants a non-const pointer */
+	return {const_cast<void *>(ptr), [](void *) noexcept {}};
+}
+
+template<typename T>
+TypedDisposablePointer<T>
+ToDeletePointer(T *ptr) noexcept
+{
+	return {ptr, [](void *p) noexcept {
+		T *t = (T *)p;
+		delete t;
+	}};
+}
+
+template<typename T>
+TypedDisposablePointer<T>
+ToDeleteArray(T *ptr) noexcept
+{
+	return {ptr, [](void *p) noexcept {
+		T *t = (T *)p;
+		delete[] t;
+	}};
+}
+
+template<typename T>
+TypedDisposablePointer<T>
+ToDestructPointer(T *ptr) noexcept
+{
+	return {ptr, [](void *p) noexcept {
+		T *t = (T *)p;
+		t->~T();
+	}};
+}