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

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 
// #include 
{
    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 
{
    foo_vec::const_iterator it
        = std::find_if(
                    tokens.begin(),
                    tokens.end(),
                    boost::bind(
                                std::logical_or(),
                                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 
// #include 
// #include 
namespace phx = boost::phoenix;

struct match_beginning_impl
{
    template 
    struct result
    {
        typedef bool type;
    };

    template 
    bool operator()(Arg1 arg1, Arg2 arg2) const
    {
        // Do stuff
    }
};
phx::function 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.