Re: [boost] gcc standard library experts out there?

From: Robert Ramey (ramey_at_[hidden])
Date: 2006-02-19 23:37:59

Very good.  That explains it all to me.

Of course any good answer begets another question. The problem only occurs
with GCC standard library implementation.  At first I thought that other 
libraries
included a default hash function for Free.  Now that I think about, I don't
know for a fact that any other libraries actually implement the hash_map and 
hash_set
so perhaps its never even been tested.

Thanks again

Robert Ramey

Daniel Frey wrote:
> Robert Ramey wrote:
>> http://tinyurl.com/s4ohn Shows the error message which is provoked
>> by the following test code:
>>
>> // test array of objects
>> BOOST_STD_EXTENSION_NAMESPACE::hash_set<A> ahash_set;
>> ...
>> std::vector<A> tvec, tvec1;
>> std::copy(ahash_set.begin(), ahash_set.end(),
>> std::back_inserter(tvec)); // error starts here
>> std::sort(tvec.begin(), tvec.end());
>>
>> a) I can't see anything wrong with this.
>> b) No other compilers complain
>> c) I can't follow the error message
>
> The error message means that there is no hash-function for A. The
> following example works for GCC 4.0.3, removing the
> hash<A>-specialization reveals the error you found:
>
> #include <vector>
> #include <ext/hash_set>
>
> class A {};
>
> namespace __gnu_cxx
> {
>  template<> struct hash<A>
>  {
>    size_t operator()( const A& ) const { return 0; }
>  };
> }
>
> int main()
> {
>  __gnu_cxx::hash_set<A> ahash_set;
>
>  std::vector<A> tvec, tvec1;
>  std::copy(ahash_set.begin(),
>            ahash_set.end(),
>            std::back_inserter(tvec));
> }
>
> HTH, Daniel
>
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