Re: designing classes without default c'tor; using them with STL containers and operator>>(istream&)

Hi,

I want a class whose invariant is that the contained data members are
valid - the constructor establishes this invariant, methods assume it
as their precondition and maintain it as their postcondition.
This requires the class to NOT have a default constructor.

I need to store instances of this class in STL containers, which
unfortunately requires that the class has a default constructor. The
obvious solution here is that I have to implement a default
constructor which leaves the class in an invalid state, although this
unfortunately means that every method now has to check the invariant
as a precondition to ensure that it's not vulnerable to class users
who might use the default constructor and not read the documentation.

Another requirement is that I need to implement (non-member) I/O
streaming functions. The output operator<<() is no problem, but again
with an input operator:
  istream& operator>>(istream& in, const Foo& f);
I have to use this like so:
  Foo f;
  in >> f;
This obviously requires, again, that I can construct an instance
through the default constructor thus generating an invalid object.

Is there some great guideline for designing that everyone else knows
and I don't? Is this one of those 'pain-in-the-backside' things that
we just have to put up with and end up implementing default
constructors and invariant checks just so we can do the other things
we need to do?