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:

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

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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 ??

It does this so that it moves the pointer over by byte units, then casts that pointer to a node.

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 ??

Lets say we have 32-bit sized memory objects

Code:

---

[0123][4567][89AB]

---

What if I wanted to place my pointer to one of thse objects right

Code:

---

[0123][4567][89AB]
    ^

---

There. How can I possibly do that? What if I called the pointer a char instead. That way I can move over an arbitary number of bytes instead of moving in 4-byte increments. Get it now?

Thanks understood now.

Yeah no problem. Its actually a reasonably common practice to move pointers like that for alignment purposes.