Thread: not able to understand this tiny tiny method

I just saw a tiny method called concrete which is an one line function but I am not able to understand what it does but I am Interested.

http://cep.xor.aps.anl.gov/software/...ce.html#l00130

Look at Line 00130

Code:

static inline Node *concrete(QMapData::Node *node) {
  return reinterpret_cast<Node *>(reinterpret_cast<char *>(node) - payload());
}

payload() will return an Integer but why reinterpret_cast<char *>(node) - payload() ??
does reinterpret_cast returns an integer ??
and why cast an Integer to a pointer to an object ??

Sorry didn't understand correctly.
I can understand why it cats char* instead of void* is Qt's business logic and that doesn't makes much headech.
Oh! cause Its 1 byte.

but why it does reinterpret_cast<char *>(node) - payload()
is not understood yet.

does reinterpret_cast returns an integer ??
if it returns a pointer (I belive it does)
It could just do node--

Is it because QMapData::Node contains another data *forward[1]; ??

But If so return value of payload() is not same of the size of Node so shouldn't it crash due to orphan pointer access ??