Re: overloading << for map and multimap simultaneously

 James Kanze <>
Tue, 31 Jul 2007 07:56:31 -0000
On Jul 30, 12:39 pm, ozizus <> wrote:

On Jul 29, 10:34 pm, James Kanze <> wrote:

On Jul 29, 3:00 pm, ozizus <> wrote:

I overloaded operator << for STL map successfully:
template <typename T1, typename T2> ostream & operator << (ostream &
o, map <T1,T2> & m)
the code works like a charm.

Are you sure? In what namespace did you put it?

I put it in no namespace. I use "using namespace std;". This may be
bad practice but it works.

No it doesn't. (I mean putting your operator in no namespace.)
Try something like:

        c.begin(), c.end(),
        std::ostream_iterator< std::map< X, Y > >( std::cout,
"\n" ) ) ;

Your operator won't be found, unless either X or Y are user
defined types in the same namespace as your operator.

Since I overloaded << for other containers,
I can output any object that is a combination of these containers in
one line. Same is true for >> too.

map<vector<int>, set<string>> m;

very powerfull.

Very unmaintainable, you mean. Not something you really want to
do; it's a definite recepe for undefined behavior.

The answer to your question is simple: there's no way to define
an overload of operator<< for a standard type in a way that is
guaranteed to work in all cases, and you don't want to, even if
you could.

James Kanze (GABI Software)
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