subramanian100in@yahoo.com, India wrote:

> Consider the following program:
>
> #include <iostream>
> #include <map>
> #include <string>
> #include <utility>
> #include <algorithm>
>
> using namespace std;
>
> int main()
> {
> map<string, int> si;
>
> string word;
>
> while (cin >> word)
> ++si[word];
>
> multimap<int, string> is;

Note that this is a non-const object.

>
> for (map<string, int>::const_iterator i = si.begin(); i != si.end(); +
> +i)
> is.insert(make_pair(i->second, i->first));
>
> for (multimap<int, string>::const_reverse_iterator r = is.rbegin(); r !
> = is.rend(); ++r)

Note that is.rend() returns reverse_iterator and not const_reverse_iterator
since "is" is a non-const object. Thus, you compare

const_reverse_iterate != reverse_iterator


> cout << r->second << " " << r->first << endl;
>
> return 0;
> }
>
> Under g++, I get compilation error for the line
>
> for (multimap<int, string>::const_reverse_iterator r = is.rbegin(); r !
> = is.rend(); ++r)
>
> The actual error message is
>
> error: no match for 'operator!=' in 'r != std::multimap<_Key, _Tp,
> _Compare, _Alloc>::rend() [with _Key = int, _Tp = std::string,
> _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const
> int, std::string> >]()'

Yup. That's what you get.


> However this program compiles fine under VC++ 2005 Express Edition.
>
> I use the following compilation command under g++.
>
> g++ -std=c++98 -pedantic -Wall -Wextra word_count.cpp

It's a matter which STL implementation you use.

> Kindly explain why I am getting error for the above mentioned line
> under g++ only.

You hit upon a defect in the language.

std::map<>::reverse_iterator is defined to be

std::reverse_iterator< std::map<>::iterator >

and std::map<>::const_reverse_iterator is defined to be

std::reverse_iterator< std::map<>::const_iterator >

The standard only requires that std::reverse_iterator supports comparison
for reverse_iterators with identical underlying iterator types. Thus, g++
is formally correct.

HOWEVER, this has been fixed in the draft for the next revision of the C++
standard. It also has been fixed in g++. Your code compiles fine with
g++-4.1.1.


Best

Kai-Uwe Bux