Re: Bjarne's exception safe sample

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Sun, 23 Dec 2007 12:35:09 GMT
On 2007-12-23 13:07, George2 wrote:

Hello everyone,

Here is Bjarne's exception safe sample,

template <class T> class Safe {

T* p ; // p points to a T allocated using new
public :
Safe () :p (new T ) { }
~Safe () { delete p ; }
Safe & operator =(const Safe & a) { *p = *a .p ; return *this; }
/ / ...
template <class T> class Unsafe { // sloppy and dangerous code
T* p ; // p points to a T
public :
Unsafe (T* pp ) :p (pp ) { }
~Unsafe () { if (!p ->destructible ()) throw E(); delete p; }
Unsafe & operator =(const Unsafe & a)
p ->~T (); // destroy old value (?10.4.11)
new (p) T (a .p ); // construct copy of a.p in *p (?10.4.11)
return *this;
/ / ...

What makes me confused is, the description about why it is not
exception safe,

The assignment operator may fail by throwing an exception from T 's
copy constructor. This would
leave a T in an undefined state because the old value of *p was
destroyed and no new value
replaced it.

In my study, I can not find a case why there is exception thrown from
Unsafe's copy constructor. Any ideas?

The critical part is "new (p) T (a .p );" since we do not know what T is
we can not guarantee that constructing an object of type T will succeed
(the most trivial example would be a failure to allocate memory for it).
If that happens the Unsafe object is left in a bad state since we have
already deleted the pointer to the old T object.

BTW: it is also appreciated if you could share some experiences about
what in your minds does invariant status mean

(in Bjarne's minds, exception safety means making the object into
invariant status). I find the word *invariant* is somethings hard to
understand. :-)

Invariants are some conditions that always have to be true for an
object. So if we have an object and performs some kind of operation on
it, then those conditions have to be true after the operations were
performed if they also were true before.

In the case above an invariant might be that p must always be a valid
pointer to an object of type T.

Erik Wikstr?m

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