Re: [boost] Smart mutex pointer?

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-02-14 20:04:35

Mark Deric wrote:
> "Peter Dimov" <pdimov_at_[hidden]> wrote:
> 
>>> struct Dummy { int a; }
>>> 
>>> class Lock {
>>> private:
>>>  Dummy &_d;
>>> public:
>>>  Lock( Dummy &d ) : _d(d) { std::cout << "Locked." << std::endl; }
>>>  ~Lock( void ) { std::cout << "Unlocked." << std::endl; }
>>> 
>>>  Dummy *operator->( void ) { return &_d; }
>>> };
>>> 
>>> class Ref {
>>> private:
>>>  Dummy &_d;
>>> public:
>>>  Ref( Dummy &d ) : _d(d) { }
>>> 
>>>   Lock operator->(void) { return Lock(_d); }
>>> };
> 
>> Your code is non-portable. You are assuming that the temporary
>> Lock(_d) will be constructed directly in the return value, but this
>> is not guaranteed.
> 
> I'm not sure I'm following "constructed directly in the return value";
> but, it will be constructed before the call to Lock's Dummy
> *operator->( 
> void ) as implied by x->a, below.  Isn't this all that is required?

No, because ~Lock will be called twice.