Question

I have made a small example in order to understand how boost::bind () works with collections. I have a class called Data:

class Data
{
public:
    void print ();
    const std::string& get () const;
    std::string& get () ;
};

I have created a std::vector of Data objects called samples and I am able to use bind in the same way as std::mem_fun_ref works.

std::for_each (samples.begin (),samples.end (),std::mem_fun_ref (&Data::print));
std::for_each (samples.begin (),samples.end (),boost::bind (&Data::print,_1));

The basic idea is that the bind returns a function object of type bind_t<RetType=void, ObjType=Data, ArgType=void>. The member function as the first parameter allows the compiler to deduce RetType, ObjType and ArgType. The placeholder _1 corresponds to the data object which must be provided by the algorithm.

Then std::for_each calls the function object “for each” element in the following way:

for ( ; first!=last; ++first ) f(*first);

bind_t::operator(ObjType& obj) is invoked and its definition should be something like this:

return (obj.*_method ());

I have crated a class called Filter that performs some processing over a data element.

class Filter
{
    void filter (Data& data);
    ...
};

If I want to apply the filter over the data elements in the vector I call bind in the following way:

std::for_each (samples.begin (),samples.end (),boost::bind (&Filter::filter,filter,_1));

for_each passes a Data object to bind_t::operator(). In this case the function object already has the object and just need the parameter so in this case placeholder _1 refers to the argument.

Here comes my question:

How can use bind if I have to iterate over a std::map rather than a vector?

(Sorry for all the explanation, I just want to make sure that I understand the way in which bind works)

Answer 1

#include <boost/bind.hpp>
#include <algorithm>
int main()
{
  struct Sample
  {
    int i_;
    double d_;
  };
  typedef std::map<int, Sample> Samples;
  struct Filter
  {
    void filter(const Sample &s)
    {
    }
  };
  Filter filter;
  Samples samples;
  std::for_each(samples.begin(), samples.end(), boost::bind(&Filter::filter, filter, boost::bind(&Samples::value_type::second, _1))); 
}