Hi,

In a project, I have an abstract base and several implementing classes, plus
an operator<< to append such an object to an ostream, and an ostream_iterator
on top of that:

class Abstract {
 public:
  virtual ~Abstract(void);
  virtual std::ostream &writeTo(std::ostream &) const = 0;
};

class Impl1 : public Abstract {
 public:
  virtual ~Impl1(void);
  virtual std::ostream &writeTo(std::ostream &) const;
};

std::ostream &operator<<(std::ostream &s, const Abstract &a) {
 return a.writeTo(s);
}

int main(int, char **) {
 Impl1 i;
 std::ostream_iterator<Abstract> o(std::cout);
 *o++ = i;
 return 0;
}

g++ 3.4 accepts this code, Sun CC does not, telling me that Abstract is an
abstract class (RogueWave STL obviously instantiates such an object obviously).
The C++ standard does not specify whether the first template argument to
std::ostream_iterator needs to be a concrete class.

It is obvious that it is possible to provide an implementation that accepts an
abstract class here. Should this be made a requirement, so that programmers can
use ostream_iterators with abstract base classes, or is it better to leave that
"implementation defined", effectively disallowing its use in portable code?

   Simon

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