Re: [boost] [uuid] Interface

Subject: Re: [boost] [uuid] Interface
From: Michael Marcin (mike.marcin_at_[hidden])
Date: 2008-12-23 02:09:58

Vladimir Batov wrote:
>> Just to drop my 2 cents.
>>
>> I'd like to have the following use case be efficient.
>>
>> class foo
>> {
>>   foo( const char* filename )
>>   {
>>     std::ifstream file(filename);
>>     file >> m_uuid;
>>   }
>>
>> private:
>>   uuid m_uuid;
>> };
>>
>>
>> This seems like a reasonable usage pattern to me and I would be very 
>> surprised if it was more expensive than reading into a char array.
> 
> Just for clarity I'll state the obvious -- in your example 'm_uuid' *is* 
> initialized. 

I never said otherwise.

> Only built-in types allow declaration without definition 
> (as a place holder). Classes do not allow that behavior. Classes always 
> require and call constructors. Therefore, IMHO making them *look* like 
> they behave similarly to built-ins sends/enforces the wrong 
> message/impression. Therefore, I prefer the following which does exactly 
> the same -- 'm_uuid' initialized invalid -- but explicit about what it 
> actually does.

I realize invalid vs nil/null was discussed earlier but I still don't 
like using those terms interchangeably.

> 
> As for efficiency I expect this variant to be at least as efficient (if 
> not better) because uuid::nil() is likely to have and to return the same 
> initialized-once instance, i.e. no byte initialization traversal as in
> 
>    uuid() /* throw() */
>    {
>        for (data_type::iterator i=data_.begin(); i!=data_.end(); *i = 0, 
> ++i);
>    }
> 
> Then, when a copy constructor is applied, it'll likely to copy the 
> internal data array not per-byte but per-integer (4 bytes), i.e. 4 times 
> quicker (not to mention the traversal overhead).
> 

Surely that should at the very least be a std::fill which gets optimized 
down to the a memset when possible (by VC9 at least).

But is it more correct to do this initialization than applying 
data_type's default initialization?

Just to be clear this does or doesn't seem like a reasonable use case to 
you?

-- 
Michael Marcin