Combining Predicates

问题:

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?

回答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(...)) ...

回答2:

Well you have std::logical_or and std::compose2 that can do the job

find_if(tokens.begin(), tokens.end(), 
  compose2(logical_or<bool>(),
    bind2nd(MatchBeginning(), "-b"),
    bind2nd(MatchBeginning(), "-")
  ) 
);

but I think that boost::lambda and/or phoenix are more readable in the end, and are my recommended solution.

Credits should go to SGI documentation.

回答3:

If you want to compose predicates, the nicest way to write it is probably using the Boost Lambda or Boost Phoenix:

// Lambda way:
// Needs:
// #include <boost/lambda/lambda.hpp>
// #include <boost/lambda/bind.hpp>
{
    using namespace boost::lambda;
    foo_vec::const_iterator it
        = std::find_if(
                    tokens.begin(),
                    tokens.end(),
                    bind(MatchBeginning(), _1, "-b") || !bind(MatchBeginning(), _1, "-")
                    );
}
// Boost bind way:
// Needs:
// #include <boost/bind.hpp>
{
    foo_vec::const_iterator it
        = std::find_if(
                    tokens.begin(),
                    tokens.end(),
                    boost::bind(
                                std::logical_or<bool>(),
                                boost::bind(MatchBeginning(), _1, "-b"),
                                !boost::bind(MatchBeginning(), _1, "-") // ! overloaded in bind
                               )
                    );

For the Phoenix way one of the possibilities is to use phoenix lazy functions, and the solution could look similar to the one below:

// Requires:
// #include <boost/spirit/include/phoenix_core.hpp>
// #include <boost/spirit/include/phoenix_function.hpp>
// #include <boost/spirit/include/phoenix_operator.hpp>
namespace phx = boost::phoenix;

struct match_beginning_impl
{
    template <typename Arg1, typename Arg2>
    struct result
    {
        typedef bool type;
    };

    template <typename Arg1, typename Arg2>
    bool operator()(Arg1 arg1, Arg2 arg2) const
    {
        // Do stuff
    }
};
phx::function<match_beginning_impl> match_beginning;

using phx::arg_names::arg1;

foo_vec::const_iterator it
    = std::find_if(
                tokens.begin(),
                tokens.end(),
                match_beginning(arg1, "-b") || !match_beginning(arg1, "-")
                );

However to accomplish your task it probably makes more sense to employ different tools - for example: regular expressions (Boost Regex or Boost Xpressive). If you want to handle the command line options then use Boost Program Options.