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#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;
};
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