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(...)) ...