ostreams and vector<vector>s

[Russ Ford]

I'm trying to overload the << operator to output vectors, as follows:

template <class T>
ostream& operator << (ostream& o, const vector<T>& v)
{
o << "< ";
copy (v.begin(), v.end(), ostream_iterator<T>(o, " ");
return o << ">";
}

This works well, except for outputting a vector<vector> where compile errors
occur. The STL reference says that the ostream_iterator will work for any
type T where cout << T is defined. Doesn't the above define cout << for a
vector?

If I define like this instead,

template <class T>
ostream& operator << (ostream& o, const vector<T>& v)
{
o << "< ";
vector<T>::const_iterator i;
for (i=v.begin(); i<v.end(); ++i) cout << *i << " ";
return o << ">";
}

it works fine. What gives?

Russ

 

[David Hilsee]

I'm trying to overload the << operator to output vectors, as follows:

template <class T>
ostream& operator << (ostream& o, const vector<T>& v)
{
o << "< ";
copy (v.begin(), v.end(), ostream_iterator<T>(o, " ");
return o << ">";
}

This works well, except for outputting a vector<vector> where compile errors
occur. The STL reference says that the ostream_iterator will work for any
type T where cout << T is defined. Doesn't the above define cout << for a
vector?

If I define like this instead,

template <class T>
ostream& operator << (ostream& o, const vector<T>& v)
{
o << "< ";
vector<T>::const_iterator i;
for (i=v.begin(); i<v.end(); ++i) cout << *i << " ";
return o << ">";
}

it works fine. What gives?

This is a common question. A simple googling (http://groups.google.com) of
the comp.lang.c++.* groups for ostream_iterator provides many explanations
(e.g. Adrian Sandor's post and news_user's post).

 

[Jonathan Turkanis]

I'm trying to overload the << operator to output vectors, as follows:

template <class T>
ostream& operator << (ostream& o, const vector<T>& v)
{
o << "< ";
copy (v.begin(), v.end(), ostream_iterator<T>(o, " ");
return o << ">";
}

Reece Dunne's Output Formatters library, up for review for inclusion in boost
begining Sept 12, handles this type of problem nicely, with the capacity much
customization.

If you are interested, the library is available in the boost sandbox
(http://www.boost.org/more/mailing_lists.htm#sandbox) . The docs are here:
http://tinyurl.com/4mu39.

Jonathan