const_iterator does not equate to value_type (or const value_type)

[Bill Buklis <boostuser@…>]

In ptr_map, const_iterator does not represent value_type. Given this map:

typedef boost::ptr_map<DWORD, int> BOOSTMAP;

BOOSTMAP::value_type equates to:

boost::ptr_container_detail::ref_pair<DWORD, int* const>

BOOSTMAP::iterator does return this, but const_iterator returns:

boost::ptr_container_detail::ref_pair<DWORD, const int* const> which is unlike std::map which represents const value_type for const_iterator. Since this is a different structure, it's incompatible with BOOSTMAP::value_type.

The following code works using std::map:

typedef std::map<DWORD, int*> STDMAP;
int			sample = 5;
STDMAP		sm;

sm.insert( STDMAP::value_type(10,&sample) );

STDMAP::iterator		sfirst = sm.begin();
STDMAP::const_iterator		scfirst = sm.begin();
STDMAP::value_type&		svt = *sfirst;
const STDMAP::value_type&	scvt = *scfirst;

But, the same fails on the last line with boost::ptr_map

typedef boost::ptr_map<DWORD, int> BOOSTMAP;
BOOSTMAP	bm;
DWORD		key = 10;

bm.insert( key, new int(5) );

BOOSTMAP::iterator		bfirst = bm.begin();
BOOSTMAP::const_iterator	bcfirst = bm.begin();
BOOSTMAP::value_type&		bvt = *bfirst;
const BOOSTMAP::value_type&	bcvt = *bcfirst;

[Thorsten Ottosen]

If you replace the last line with

BOOST_MAP::const_reference bcvt = *bcfirst;

it should work.

Since a major design goal was to propagate constness, your line can never be made to work, unfortunately.