C++ STL:map search by iterator to another map - c++

Question

I'm trying to jump through some hoops to organize data in a special way. I'm including a simplified piece of code that demonstrates my pain.
I can't use boost.
I'm using the latest version of g++ in cygwin.
#include <iostream>
#include <map>
using namespace std;
int main () {
map< int,int > genmap;
map< int,int >::iterator genmapit;
map< map<int,int>::iterator,int > itermap;
// insert something into genmap
genmap.insert (make_pair(1,500) );
// find and return iterator.
genmapit=genmap.find(1);
// insert the iterator/int into itermap. Dies on each of the following 3 versions of this line.
//itermap[genmapit] = 600; // crash
//itermap.insert ( pair< map<int,int>::iterator,int >(genmapit,600) ); // crash
itermap.insert ( make_pair(genmapit,600) ); // crash
return 0;
}
So as you can see, I have 1 simple map, an iterator to that map and another map that has the first argument as an iterator to the first map.
It's clear from this:
Why can't I put an iterator in map?
That I can have an iterator as the second argument. However, the way shown above provides this:
$ make
g++ -c -o main.o main.cpp
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_function.h: In member fun
ction `bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp =
std::_Rb_tree_iterator<std::pair<const int, int> >]':
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_tree.h:871: instantiate
d from `std::pair<typename std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _All
oc>::iterator, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::i
nsert_unique(const _Val&) [with _Key = std::_Rb_tree_iterator<std::pair<const in
t, int> >, _Val = std::pair<const std::_Rb_tree_iterator<std::pair<const int, in
t> >, int>, _KeyOfValue = std::_Select1st<std::pair<const std::_Rb_tree_iterator
<std::pair<const int, int> >, int> >, _Compare = std::less<std::_Rb_tree_iterato
r<std::pair<const int, int> > >, _Alloc = std::allocator<std::pair<const std::_R
b_tree_iterator<std::pair<const int, int> >, int> >]'
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_map.h:360: instantiated
from `std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_
Select1st<std::pair<const _Key, _Tp> >, _Compare, _Alloc>::iterator, bool> std::
map<_Key, _Tp, _Compare, _Alloc>::insert(const std::pair<const _Key, _Tp>&) [wit
h _Key = std::_Rb_tree_iterator<std::pair<const int, int> >, _Tp = int, _Compare
= std::less<std::_Rb_tree_iterator<std::pair<const int, int> > >, _Alloc = std:
:allocator<std::pair<const std::_Rb_tree_iterator<std::pair<const int, int> >, i
nt> >]'
main.cpp:23: instantiated from here
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_function.h:227: error: no
match for 'operator<' in '__x < __y'
make: *** [main.o] Error 1
"instantiated from here" tells me nothing and a web search gives me no info on this.
Does STL:map simply not allow for this? I can recode my app to work around this but it will be very inefficient and I would like to get this working. Is there another kind of pointer I can make for a map element I could use?
Thanks for your time.

Answer 1

You can't do this because std::map iterators are not random access iterators so aren't comparable with <.
Instead, you could use pointers to the value_type in the first map as a map key.

Answer 2

You have to learn to read the error messages. In particular look at the message that comes after the long-winded description where the error happened:
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_function.h:227: error: no
match for 'operator<' in '__x < __y'
Map iterators are not comparable with less-than operator which the map uses by default.
I suppose you can provide a comparison function that compares the pairs pointed to by the iterator, since the iterators themselves cannot be easily compared in a meaningful way.
struct CompareIterator
{
template <class FirstIter, class SecondIter>
bool operator()(FirstIter lhv, SecondIter rhv) const
{
return *lhv < *rhv;
}
};
//usage with map:
map< map<int,int>::iterator,int, CompareIterator > itermap;
std::pair defines operator<. I also used two iterator types, since it might be possible the types are different (iterator and const_iterator)

Answer 3

map<Key, Value>
The map iterator as key element into another map is not possible because map expects operator < to be defined by default to the key. If the Key (in this case map iterator) is not defined then you need to pass a functor as a predicate function that provides the comparison of Key (map iterator).