question abot Exceptional C++ (47. eng puzzles) Iby Sutter

[puzzlecracker]

Item 30 page 115:

template <size_t S>
class FixedAllocator
{
public:
void* Allocate (/*requested size is always S*/);
void Deallocate (void *);
private:
/*implemented using statics*/

};
"... Because the private details are likely to use statics,however,
there could be problems if Deallocate is ever called from a static
object's destructor. Probably safer is a singleton that manages a
sepearate free list..."

First of all how is it possible to ahave a static object?

Why would there be a problem, where (could someone please provide an
example)?


Thanks.

 

[Victor Bazarov]

Item 30 page 115:

template <size_t S>
class FixedAllocator
{
public:
void* Allocate (/*requested size is always S*/);
void Deallocate (void *);
private:
/*implemented using statics*/

};
"... Because the private details are likely to use statics,however,
there could be problems if Deallocate is ever called from a static
object's destructor. Probably safer is a singleton that manages a
sepearate free list..."

First of all how is it possible to ahave a static object?

Uh... What do you mean? Any object *defined* at a namespace scope has
static storage duration. Any object defined in other scopes but declared
'static' also has static storage duration. Those are static objects.

Why would there be a problem, where (could someone please provide an
example)?

The problem is the (unspecified) order of initialisation (and destruction)
of static objects. If whatever storage 'FixedAllocator' uses is created
_later_ than the object whose destructor is going to use 'FixedAllocator',
then 'FixedAllocator's storage is going to be destroyed before the d-tor
of the other static object is invoked, so the d-tor is going to try to use
the part of 'FixedAllocator' that doesn't exist any longer.

V