Combining Predicates

Question

Is there any way that you can combine predicates?

Lets say I have something like this:

class MatchBeginning : public binary_function<CStdString, CStdString, bool>
{   public:
          bool operator()(const CStdString &inputOne, const CStdString &inputTwo) const
    {   return inputOne.substr(0, inputTwo.length()).compare(inputTwo) == 0;    }
};

int main(int argc, char* argv[])
{
    CStdString myString("foo -b ar -t az"); 

    vector<CStdString> tokens;

    // splits the string every time it encounters a "-"
    split(myString, tokens, "-", true, true);   

    vector<CStdString>::iterator searchResult = find_if(tokens.begin(), tokens.end(), not1(bind2nd(MatchBeginning(), "-")));        

    return 0;
}

This works, but now I'd like to do something like:

searchResult = find_if(tokens.begin(), tokens.end(), bind2nd(MatchBeginning(), "-b") || not1(bind2nd(MatchBeginning(), "-")));

So I'd like to find the first string that starts with "-b" or the first string that does not start with "-". However, this gives me an error (binary '||' undefined).

Is there any way to do this?

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 Left, class Right>
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<class Left, class Right>
lazy_or_impl<Left, Right> lazy_or(Left const& left, Right const& right) {
  return lazy_or_impl<Left, Right>(left, right);
}

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