🚀 go-pugleaf

akv@cacs.usl.edu (Anil Vijendran) writes:
	...
>I have a question in relation to this... I've seen persistence
>systems/libraries where the allocator or the heap itself is passed
>using the placement syntax... for example,

>	ptr = new (pstore) shape;

>I have always been wondering how the constructor of the class could
>allocate its data members on the same heap the top level object was
>placed in.

>I think the placement syntax when extended to persistent stores and
>such, whatever goes as the placement-expression, should be passed to
>the constructor too.. Or can this be done sans such an extension? 

Of course it can!  Just declare your "class shape" constructor to
(perhaps optionally) take a reference-to-allocator, eg:

class allocator {
public: virtual void* givemem(size_t bytes) = 0;
	virtual void freemem(void* ptr) = 0;
// etc
};

class default_allocator: public allocator {
public: virtual void* givemem(size_t bytes) {return ::operator new(bytes); }
  virtual void freemem(void *ptr) {::operator delete(ptr); }
// etc
};

void* operator new(size_t bytes, allocator& al) {
    return al.givemem(bytes);
}

class shape {
    int *amember;
    allocator &_al;
public: shape(allocator &al=default_allocator): _al(al) {
	    amember = new(al) int;
	}
        ~shape() {
	    _al.freemem(amember); // I just can't remember the placement
	                          // delete syntax...:-)
	}
	// etc
};

class pstore: public allocator { // etc
};
// ...
	shape *s = new(pstore) shape(pstore);

Basically, while it _might_ perhaps be nice to have syntax sugar to
avoid having to write pstore twice in this way, I do not think there
is really any burning need for such sugar...


Alex
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