如何使用活页夹和bind2nd函子？

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