Re: [Boost-users] unordered_set insert returns const_iterator ?

From: abir basak (abirbasak_at_[hidden])
Date: 2008-03-20 10:32:32

Daniel James wrote:
> On 20/03/2008, Abir Basak <abirbasak_at_[hidden]> wrote:
>> Hi while i am using boost unordered_set , and insert a new key value pair,
>> it returns the iterator as a const one. In fact when i looked at the code,
>> the iterator is a typedef to implementation::const_iterator ... so there is
>> no way to gen an mutable iterator from the set, including begin() and end()
>>   is it a standard behavior or i am missing something ?
> 
> It's standard behaviour. The keys in unordered containers are const,
> because if you change it, it could change the position that the
> element should be in. And since the elements in unordered_sets
> unordered_multisets are just the keys, they are const as well. The
> situation is the same with std::set. There has been some debate on
> this in the past.
> 
> The typical solution is to use an unordered_map, where the key is
> constant but the mapped type is not.
> 
> Boost.Bimap, Boost.Multindex and Boost.Intrusive might offer
> alternative ways of dealing with this, but I don't know them well
> enough to say.
> 
> Daniel

Thanks for replying .... The argument is justified. So i thought to 
separate out the key and use unordered_multimap instead. But facing 
another problem while calling equal_range
The sample program shows the problem,
//typedef std::multimap<char,int> Mymap;
        typedef boost::unordered_multimap<char, int> Mymap;
        Mymap c1;

        c1.insert(Mymap::value_type('a', 1));
        c1.insert(Mymap::value_type('a', 2));
        c1.insert(Mymap::value_type('b', 3));
        c1.insert(Mymap::value_type('c', 4));

        Mymap::iterator it,end;
        boost::tie(it,end) = c1.equal_range('a');
        for( ; it!=end; ++it){
                std::cout<<(*it).second<<" ";
        } 	

Here the multimap version runs correctly, but unordered_multimap fails 
to run in the call equal_range (compiler MSVC 7.1 .NET 2003, and 
standard library from MS) ... the boost svn checkout from HEAD today)
Thanks
The error message is too complex,

e:\boost\boost\unordered\detail\hash_table_impl.hpp(301) : error C2039: 
'hash_table_data_equivalent_keys<std::allocator<std::pair<char const 
,int> > >' : is not a member of 
'boost::unordered_detail::hash_table_data_equivalent_keys<Alloc>::iterator_base'
         with
         [
 
Alloc=boost::unordered_detail::hash_types_equivalent_keys<std::pair<const 
char,int>,char,boost::hash<char>,std::equal_to<char>,std::allocator<std::pair<const 
char,int>>>::value_allocator
         ]
         e:\boost\boost\unordered\detail\hash_table_impl.hpp(255) : see 
declaration of 
'boost::unordered_detail::hash_table_data_equivalent_keys<Alloc>::iterator_base'
         with
         [
 
Alloc=boost::unordered_detail::hash_types_equivalent_keys<std::pair<const 
char,int>,char,boost::hash<char>,std::equal_to<char>,std::allocator<std::pair<const 
char,int>>>::value_allocator
         ]
         e:\boost\boost\unordered\detail\hash_table_impl.hpp(300) : 
while compiling class-template member function 'void 
boost::unordered_detail::hash_table_data_equivalent_keys<Alloc>::iterator_base::increment_group(void)'
         with
         [
 
Alloc=boost::unordered_detail::hash_types_equivalent_keys<std::pair<const 
char,int>,char,boost::hash<char>,std::equal_to<char>,std::allocator<std::pair<const 
char,int>>>::value_allocator
         ]
         e:\boost\boost\unordered\detail\hash_table_impl.hpp(1962) : see 
reference to class template instantiation 
'boost::unordered_detail::hash_table_data_equivalent_keys<Alloc>::iterator_base' 
being compiled
         with
         [
 
Alloc=boost::unordered_detail::hash_types_equivalent_keys<std::pair<const 
char,int>,char,boost::hash<char>,std::equal_to<char>,std::allocator<std::pair<const 
char,int>>>::value_allocator
         ]
 
e:\cvs_checkout\read-ink\cplusplus\ink_app\test\test_inklib.cpp(72) : 
see reference to class template instantiation 
'boost::unordered_detail::hash_iterator_equivalent_keys<Alloc>' being 
compiled
         with
         [
 
Alloc=boost::unordered_detail::hash_types_equivalent_keys<std::pair<const 
char,int>,char,boost::hash<char>,std::equal_to<char>,std::allocator<std::pair<const 
char,int>>>::value_allocator
         ]