Re: [boost] [Review][bimap]

From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2007-02-26 11:58:03

Matias Capeletto wrote:
> In 2/26/07, Thorsten Ottosen <thorsten.ottosen_at_[hidden]> wrote:
> 
>>Matias Capeletto wrote:

>>>The problem is bigger in the side views because another copy
>>>construction is taking place. From the inserted pair to relation, and
>>>then to the stored relation inside Boost.MultiIndex.
>>
>>Oh, that's bad. Another consequence of not using the same underlying
>>pair type.
> 
> 
> Yes, but this problem is there independently of the using of left/right in the
> right view because in it the pair members are inverted. Do you think it will be
> better that the right view has X as first and Y as second, this will
> not allow us
> to use it like a std::map<Y,X>.

I was actually thinking that the views merely stored references.
Reading on, it seems like that is what you do.

>>>I think I can change the current implementation to avoid the first
>>>conversion if the compiler support the mutant_relation (you can read
>>>about this in the rationale), because a reinterpret_cast can be used
>>>to convert between bimap pairs and relation.
>>
>>I'm somewhat uncomfortable about this casting. Is it provable legal by
>>the standard? (What works fine for pair<int,int>, might crash for
>>pair<Foo,Bar>.)
> 
> 
> Yep... the reinterpret cast works in almost every actual compiler. It
> uses a non standard feature. It assumes that:
> 
> struct Pair_AB { TA first; TB second; };
> 
> have the same structure that
> 
> struct Pair_BA { TB first; TA second; };
> 
> In the standard this is true if TA and TB are POD types.
> But in the real world this is generally implemented this way for non
> POD types too.

I'm certainly confused here. What do you mean by "same structure"? If
you had written

   struct Pair_BA { TA first; TB second; };

I might have understood it.

> In boost/bimap/relation/relation.cpp the library checks if it can use
> the mutant_relation for the types in question and if not it uses a
> standard_relation, that is implemented using references to the types.

Right. How it sucks that std::pair did not go for reference returning 
functions instead of members!.

What if you make the members of the view pairs something like

template< class Pair& >
struct get_first
{
    Pair& p_;
    operator typename Pair::first_type&() const
    { return p_.first; }
};

?

-Thorsten