STL Copy

[Derek Baker]

Can anyone see anything wrong with this from a standard C++ point of view:

#include <iostream>
#include <string>
#include <map>
#include <iterator>
using namespace std;

void addToMap(map<int,string>& m,int index, const char* str)
{
string strcast(str);
pair<int,string> p (index,str);
m.insert(p);
}

ostream& operator << (ostream& os, const pair<int,string>& p)
{
os << p.first << ": " << p.second << endl;
return os;
}

int main(void)
{
map<int, string> strings;
addToMap(strings,3,"Hello");
addToMap(strings,5,"World");

ostream_iterator< pair<int,string> > out_it(cout,"");
copy(strings.begin(),strings.end(),out_it); /* won't compile */

return 0;
}

This is not actually my code, but that of a poster to a forum. After
screwing up a response to that post, I'd like to help find an answer.

If you want to see the thread:

http://sourceforge.net/forum/forum.php?thread_id=973964&forum_id=48211

Thanks in advance.

 

[Victor Bazarov]

Can anyone see anything wrong with this from a standard C++ point of view:

#include <iostream>
#include <string>
#include <map>
#include <iterator>
using namespace std;

void addToMap(map<int,string>& m,int index, const char* str)
{
string strcast(str);
pair<int,string> p (index,str);
m.insert(p);
}

ostream& operator << (ostream& os, const pair<int,string>& p)
{
os << p.first << ": " << p.second << endl;
return os;
}

int main(void)
{
map<int, string> strings;
addToMap(strings,3,"Hello");
addToMap(strings,5,"World");

ostream_iterator< pair<int,string> > out_it(cout,"");
copy(strings.begin(),strings.end(),out_it); /* won't compile */

return 0;
}

This is not actually my code, but that of a poster to a forum. After
screwing up a response to that post, I'd like to help find an answer.

If you want to see the thread:

http://sourceforge.net/forum/forum.php?thread_id=973964&forum_id=48211

The problem is that the compiler is not considering the global
namespace when it's looking for a suitable operator<<. It's a hole
in the look-up rules, or so I heard. It can be cured by an illegal
(from the standard point of view) trick: place the operator<< you
defined in the namespace std. The trick is illegal because you are
not supposed to put anything in std, only the implementation may.

Of course, it's entirely possible that I just don't remember the
legal way out of that problem.

Victor

 

[Derek Baker]

The problem is that the compiler is not considering the global
namespace when it's looking for a suitable operator<<. It's a hole
in the look-up rules, or so I heard. It can be cured by an illegal
(from the standard point of view) trick: place the operator<< you
defined in the namespace std. The trick is illegal because you are
not supposed to put anything in std, only the implementation may.

Of course, it's entirely possible that I just don't remember the
legal way out of that problem.

Victor

Thanks Victor,

Someone had actually posted the answer to the forum, just after my post
here. Though without your fuller explanation.