Re: [boost] [review] Review of Unordered library begins today December 7

From: David Abrahams (dave_at_[hidden])
Date: 2007-12-11 11:02:37

on Sun Dec 09 2007, Thorsten Ottosen <thorsten.ottosen-AT-dezide.com> wrote:

> Daniel James skrev:
>> Ion Gaztañaga wrote:
>>> Thorsten Ottosen wrote:
>
>>>> One thing puzzled me though:
>>>> (see http://igaztanaga.drivehq.com/unordered/unordered/comparison.html)
>>>>
>>>> "The containers hash or predicate function can throw exceptions from erase"
>>>>
>
>>>> This migth be what the standard requires, but it certainly seems 
>>>> non-intuitive. Why not just require that neither hash nor predicates can 
>>>> throw?
>>>>     
>>> The same could be said about Predicate for ordered containers like 
>>> std::set/map. I haven't seen a practical case where the comparison could 
>>> throw. But standard library experts should answer this better than me.
>
> Well, the table at
>
> http://igaztanaga.drivehq.com/unordered/unordered/comparison.html
>
> says that erase() does not throw for ordered containers (which is 
> correct). Anyway, this means any argument used to assume that predicate 
> evaluation is a no-throw should be the same for unordered containers.

No, the ordered containers don't do any predicate evaluation in
erase().

>> Another is that the 
>> data required to evaluate the hash function or check for equality is 
>> expensive and is lazily evaluated. For example, if you had a class which 
>> represents file, and based your equality test on calculating the SHA-1 
>> signature then you might only calculate it when required - which could 
>> be a call to the hash or equality function.
>
> yes, but would the lazy evaluation be done when inserting the object in 
> the first place, thus not leaving it any lazy evaluation when calling erase?

It might be cached data that is later thrown out, or stored on disk.

> Why does erase( const_iterator ) (and range version), not provide the
> no-throw guarantee?

Why should it?  Vector/deque erase don't.

> Does erase give the strong guarantee otherwise?
>
> Since erase(const Key&) for ordered containers cannot throw, so I guess 
> it is implicitly  required that Pred(x,y) must not throw? 

Nope, see above.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com