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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,

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).


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