Re: [boost] [uuid] Interface

Subject: Re: [boost] [uuid] Interface
From: Vladimir Batov (batov_at_[hidden])
Date: 2008-12-23 01:24:47

> 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. 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.

class foo
{
  foo( const char* filename ) : m_uuid(uuid::nil())
  {
    std::ifstream file(filename);
    file >> m_uuid;
  }
  private: uuid m_uuid;
};

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).

Best,
V.