Re: question on map< list<T>::iterator, int>

On Sat, 03 May 2008 14:48:01 +0200, subramanian100in@[EMAIL PROTECTED]
 India  =

<subramanian100in@[EMAIL PROTECTED]
> wrote:

> Consider the program x.cpp :
>
> #include <cstdlib>
> #include <iostream>
> #include <list>
> #include <map>
>
> using namespace std;
>
> int main()
> {
>         map<list<int>::iterator, int> m;
>
>         list<int> c;
>         c.push_back(100);
>
>         // m[c.begin()] =3D 10;
>
>         return EXIT_SUCCESS;
> }
>
> This program compiles fine with the command
> g++ -std=3Dc++98 -pedantic -Wall -Wextra  x.cpp
>
> However if I remove the comment, I get compilation error because
> operator '<' is not defined for list<T>::iterator. Why doesn't the
> compiler give this error for
> 'map<list<int>::iterator, int> m;' itself. That is, in this first
> statement inside main() itself, the compiler can know that the less-
> than operator cannot be defined for the key type. So, why does the
> compiler wait for an element to be pushed on to the map to flag the
> error ?
>
> Kindly clarify
>
> Thanks
> V.Subramanian

A map is alway sorted by <.
To use an objet like a key for a std::map, these objet must have defined=
  =

operator<.