Re: [boost] Re: shifted_ptr review

From: Gregory Colvin (gregory.colvin_at_[hidden])
Date: 2003-10-01 10:23:55

Still wondering.

On Tuesday, Sep 30, 2003, at 11:09 America/Denver, Gregory Colvin wrote:
> Given
>
>   template<typename T> shifted_ptr<T> make_shifted_ptr(cont T& t) {
>       return shifted_ptr(new(so) T(t));
>   }
>
>   ... make_shifted_ptr(Stuff()) ...
>
> are compilers allowed to optimize away the copy and construct the
> Stuff in place?
>
> If so, do they?
>
> On Tuesday, Sep 30, 2003, at 10:45 America/Denver, Powell, Gary wrote:
>>> Powell > I'm a little concerned that it appears you can create bad 
>>> pointers for
>>> Powell > this class by inadvertently not using placement new. I'd 
>>> feel better
>>> Powell > about it if there was some way to prevent this.
>>>
>>> Abrahams >Generic smart pointer factories are the right way:
>>>
>>> Abrahams >   shifted_ptr<U> p = make_shifted_ptr<T>(arg1, arg2, 
>>> arg3, ref(arg4));
>>>
>>> So you are saying remove the constructor,
>>>
>>>    template<class U>
>>>       shifted_ptr( U *);
>>>
>>> and replace it with one that takes a temporary that is created only 
>>> by make_shifted_ptr?
>>
>> Abrahams >No, I was suggesting making the constructor private, making
>> Abrahams >make_shifted_ptr a friend, and have 
>> make_shifted_ptr<T>(...) return a
>> Abrahams >shifted_ptr<T>.
>>
>> Abrahams >Your approach seems very complicated.  Is there a reason to 
>> do it
>> Abrahams >that way?
>>
>> Well depends whether you prefer and can compile.
>>
>> template<class T>
>>   class  shifted_ptr {
>> public:
>>   template<class U, class Arg1>
>>        friend shifted_ptr<T> make_shifted_ptr();
>>
>>   template<class U, class Arg1>
>>        friend shifted_ptr<T> make_shifted_ptr(Arg1 &);
>>
>>   template<class U, class Arg1, class Arg2>
>>        friend shifted_ptr<T> make_shifted_ptr(Arg1 &, Arg2 &);
>> ....
>> };
>>
>>  Or the other way. If I remember correctly not all compilers used to 
>> be able to handle template friends. On the other hand the previous 
>> approach could have been due to a lack of caffeen.
>>
>> So after looking at this for all of 5 minutes, if the friend 
>> make_shifted_ptr works, it looks easier to maintain. But in all cases 
>> taking raw pointers is to be avoided.
>
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