cuberite-2a/src/FunctionRef.h

#pragma once

// Declared only so it can be partially specialized
template <class Signature>
class cFunctionRef;

/** A light-weight, type-erased reference to a function object.

### Usage

`cFunctionRef` is used whenever you call a normal function with a lambda. e.g.
one of the `cWorld::DoWith` functions,

	m_World->DoWithChunkAt(BlockPos, [](cChunk & a_Chunk) -> bool
		{
			...
		}
	);

It looks like you're calling it with a lambda but that would require
`DoWithChunkAt` to be a template. The function is really being called with
`cFunctionRef<bool(cChunk&)>` which is constructed from the lambda via a
templated constructor. This gives you a generic pointer to the lambda which
doesn't depend on the type of the function object it references.

### Notes

- This is similar to a `std::function` but doesn't copy the function object.
This means that mutable function objects will be modified for the caller but
would not be if using a `std::function` (See #3990 for implications of this).

- A `cFunctionRef` has no empty state but is non-owning and so is safe to call as
long as the referred object is still alive. */
template <class Ret, class... Args>
class cFunctionRef<Ret(Args...)>
{
public:
	/** Construct from a function object. */
	template <class FunctionObject,
		typename std::enable_if<  // Don't disable the default copy constructor
			!std::is_same<typename std::decay<FunctionObject>::type, cFunctionRef>::value,
		int>::type = 0
	>
	cFunctionRef(FunctionObject && a_FunctionObject)
	{
		// Store an opaque reference to the object.
		m_CallableData = &a_FunctionObject;

		// Along with a function that knows how to call the object.
		m_CallFunction = &ObjectFunctionCaller<FunctionObject>;
	}

	/** Call the referenced function object */
	Ret operator () (Args... a_Args)
	{
		return m_CallFunction(m_CallableData, std::forward<Args>(a_Args)...);
	}

private:

	/** Function that performs the call. */
	template <class ObjectType>
	static Ret ObjectFunctionCaller(void * a_Callable, Args...  a_Args)
	{
		// Convert opaque reference to the concrete type.
		using ObjectPtr = typename std::add_pointer<ObjectType>::type;
		auto & Object = *static_cast<ObjectPtr>(a_Callable);

		// Forward the call down to the object.
		return Object(std::forward<Args>(a_Args)...);
	}

	using cCallFunction = Ret(*)(void *, Args...);

	/** Type erased reference to a callable. */
	void * m_CallableData;

	/** Function that knows how to call the type erased reference. */
	cCallFunction m_CallFunction;
};