Question

I’d like to do something like this:

boost::regex re("tryme");
ifstream iss;
iss.open("file.txt");
istream_iterator<string> eos;     
istream_iterator<string> iit (iss);

find(iit,eos,bind2nd(boost::regex_match),re));

The errors are following:

Could not find a match for ‘bind2nd<_Fn2,_Ty>(bool (*) (BidiIterator,BidiIterator,match_results &,const basic_regex &,unsigned long))’

Could not find a match for ‘find(istream_iterator,int>,istream_iterator,int>,undefined,regex)’

Could you please help me to do it correctly? Thanks.

Answer 1

The first problem is that std::find() tries to match values, i.e., you need to replace std::find() by std::find_if(). That’s simple to do.

The next problem is that boost::regex_match isn’t one function but a family of functions. std::bind2nd() has no idea which member of this family you want to match against. Also, the function overload you apparently want to use takes three, not two, arguments: the last argument of type boost::match_flag_type is defaulted. I got it to work with std::bind() using this:

std::bind(static_cast<bool (*)(std::string const&,
                               boost::regex const&,
                               boost::match_flag_type)>(&boost::regex_match),
         std::placeholders::_1, re);

If you really want to use std::bind2nd() it is probably easiest to create a simple forwarding function:

bool my_regex_match(std::string s, boost::regex const& r) {
    return boost::regex_match(s, r);
}

void f() {
    boost::regex re("tryme");
    std::ifstream iss(file.txt);
    std::find_if(std::istream_iterator<std::string>(iss),
             std::istream_iterator<std::string>(),
             std::bind2nd(std::ptr_fun(&my_regex_match), re));
}

The std::bind2nd() template cannot really work with raw function pointers. This is why std::ptr_fun() needs to be used. In general, when any of the standard functions called *_fun() need to be used, the fun actually stops: these functions cannot cope with functions taking arguments by reference. Thus, my_regex_match() takes its std::string argument by value: std::bind2nd() would try to create a function object taking a reference to reference as argument otherwise. This is another reason why you might want to use std::bind() or boost::bind().