ircd::allocator: Add simple callback allocator.

include/ircd/allocator.h

@@ -22,6 +22,7 @@ namespace ircd::allocator
 	struct state;
 	struct scope;
 	struct profile;
+	template<class T = void> struct callback;
 	template<class T = char> struct dynamic;
 	template<class T = char, size_t = 512> struct fixed;
 	template<class T = char, size_t L0_SIZE = 512> struct twolevel;
@@ -139,6 +140,101 @@ struct ircd::allocator::state
 	{}
 };
 
+/// The callback allocator is a shell around the pre-c++17/20 boilerplate
+/// jumble for allocator template creation. This is an alternative to virtual
+/// functions to accomplish the same thing here. Implement the principal
+/// allocate and deallocate functions and maintain an instance of
+/// allocator::callback with them somewhere.
+template<class T>
+struct ircd::allocator::callback
+{
+	struct allocator;
+
+  public:
+	using allocate_callback = std::function<T *(const size_t &, const T *const &)>;
+	using deallocate_callback = std::function<void (T *const &, const size_t &)>;
+
+	allocate_callback ac;
+	deallocate_callback dc;
+
+	allocator operator()();
+	operator allocator();
+
+	callback(allocate_callback ac, deallocate_callback dc)
+	:ac{std::move(ac)}
+	,dc{std::move(dc)}
+	{}
+};
+
+template<class T>
+struct ircd::allocator::callback<T>::allocator
+{
+	using value_type         = T;
+	using size_type          = std::size_t;
+	using difference_type    = std::ptrdiff_t;
+
+	using is_always_equal = std::true_type;
+	using propagate_on_container_move_assignment = std::true_type;
+
+	callback *s;
+
+  public:
+	template<class U> struct rebind
+	{
+		typedef ircd::allocator::callback<T>::allocator other;
+	};
+
+	T *
+	__attribute__((malloc, returns_nonnull, warn_unused_result))
+	allocate(const size_type n, const T *const hint = nullptr)
+	{
+		assert(s && s->ac);
+		return s->ac(n, hint);
+	}
+
+	void deallocate(T *const p, const size_type n = 1)
+	{
+		assert(s && s->dc);
+		return s->dc(p, n);
+	}
+
+	template<class U>
+	allocator(const typename ircd::allocator::callback<U>::allocator &s) noexcept
+	:s{s.s}
+	{}
+
+	allocator(callback &s) noexcept
+	:s{&s}
+	{}
+
+	allocator(allocator &&) = default;
+	allocator(const allocator &) = default;
+
+	friend bool operator==(const allocator &a, const allocator &b)
+	{
+		return &a == &b;
+	}
+
+	friend bool operator!=(const allocator &a, const allocator &b)
+	{
+		return &a == &b;
+	}
+};
+
+template<class T>
+typename ircd::allocator::callback<T>::allocator
+ircd::allocator::callback<T>::operator()()
+{
+	return ircd::allocator::callback<T>::allocator(*this);
+}
+
+template<class T>
+ircd::allocator::callback<T>::operator
+allocator()
+{
+	return ircd::allocator::callback<T>::allocator(*this);
+}
+
 /// The fixed allocator creates a block of data with a size known at compile-
 /// time. This structure itself is the state object for the actual allocator
 /// instance used in the container. Create an instance of this structure,