Combining Predicates

Question

Is there any way that you can combine predicates?

Lets say I have something like this:

class MatchBeginning : public binary_function

        

Answer 1

I can recommend boost.lambda for combining function-objects for such tasks. Although it is a bit heavyweight for such a simple problem. (edit) See the community wiki answer started by xhantt for a good example using STL.

(old, deprecated, answer) You can write your own utility for this, similar:

// here we define the combiner...
template
class lazy_or_impl {
  Left m_left;
  Right m_right;
public:
  lazy_or_impl(Left const& left, Right const& right) : m_left(left), m_right(right) {}
  typename Left::result_type operator()(typename Left::argument_type const& a) const {
    return m_left(a) || m_right(a);
  }
};

// and a helper function which deduces the template arguments
// (thx to xtofl to point this out)
template
lazy_or_impl lazy_or(Left const& left, Right const& right) {
  return lazy_or_impl(left, right);
}

and then use it: ... lazy_or(bind1st(...), bind1st(...)) ...