Re: unexpected unique_copy constness issue
Luke Meyers wrote:
I was trying to come up with a neat STL-heavy response to this thread
about multimaps:
http://groups.google.com/group/comp.lang.c++/browse_frm/thread/5a3732e0cbaab918/35e6796296b79797?q=multimap&rnum=3#35e6796296b79797
My implementation involved using unique_copy to uniquify the set of
keys. However, the compiler (gcc 4.0.2) complains because it's trying
to (internally) use
std::pair<const int, int> & std::pair<const int,
int>::operator=(std::pair<const int,int> const&)
which it has synthesized. The code is as follows:
#include <map>
#include <algorithm>
#include <vector>
using namespace std;
typedef pair<int, int> value;
bool cmp(value const& lhs, value const& rhs)
{
return lhs.first == rhs.first;
}
int main()
{
typedef map<int, int> m_t;
m_t m;
vector<value> v;
copy(m.begin(),
m.end(),
back_inserter<vector<value> >(v));
unique_copy(m.begin(),
m.end(),
back_inserter<vector<value> >(v),
cmp);
return EXIT_SUCCESS;
}
Note that the call to copy() proceeds without problem. I can
understand why the map holds std::pair<const int, int> rather than
std::pair<int, int>, but I don't see why it would need to assign to a
value of that type to do unique_copy. Especially because copy() works
just fine... anyone got an idea here?
A look at the unique_copy implementation gives the clue. I am using g++
3.4.2 and here is how it looks
template<typename _InputIterator, typename _OutputIterator,
typename _BinaryPredicate>
_OutputIterator
__unique_copy(_InputIterator __first, _InputIterator __last,
_OutputIterator __result,
_BinaryPredicate __binary_pred,
output_iterator_tag)
{
// concept requirements -- iterators already checked
__glibcxx_function_requires(_BinaryPredicateConcept<_BinaryPredicate,
typename iterator_traits<_InputIterator>::value_type,
typename iterator_traits<_InputIterator>::value_type>)
typename iterator_traits<_InputIterator>::value_type __value =
*__first;
*__result = __value;
while (++__first != __last)
if (!__binary_pred(__value, *__first))
{
__value = *__first;
*++__result = __value;
}
return ++__result;
}
The offending line is
__value = *__first;
in the inner loop which tries to assign to a variable of type
InputIterator::value_type. That fails as its a std::pair<const int,
int> whose *first* is *const int*.
Ram