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

20 Mar 2008

      Daniel James wrote:
...
On 20/03/2008, Abir Basak  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 Mymap;
	typedef boost::unordered_multimap 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 > >' : 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,char,boost::hash<char>,std::equal_to<char>,std::allocator>>::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,char,boost::hash<char>,std::equal_to<char>,std::allocator>>::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,char,boost::hash<char>,std::equal_to<char>,std::allocator>>::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,char,boost::hash<char>,std::equal_to<char>,std::allocator>>::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,char,boost::hash<char>,std::equal_to<char>,std::allocator>>::value_allocator
         ]

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

abir basak