how to use binder and bind2nd functors?

Question

How to use binder2nd, bind2nd, and bind1st? More specifically when to use them and are they necessary? Also, I'm looking for some examples.

Answer 1

They're never, strictly speaking, necessary, as you could always define your own custom functor object; but they're very convenient exactly in order to avoid having to define custom functors in simple cases. For example, say you want to count the items in a std::vector<int> that are > 10. You COULD of course code...:

std::count_if(v.begin(), v.end(), gt10()) 

after defining:

class gt10: std::unary_function<int, bool>
{
public:
    result_type operator()(argument_type i)
    {
        return (result_type)(i > 10);
    }
};

but consider how much more convenient it is to code, instead:

std::count_if(v.begin(), v.end(), std::bind1st(std::less<int>(), 10)) 

without any auxiliary functor class needing to be defined!-)

Answer 2

Binders are the C++ way of doing currying. BTW, check out Boost Bind library