What do we need unary_function and binary_function for?

Question

I read the tutorials about the binary and unary functions. I understood the structure of them, but I couldn't imagine in which case I need these functions. Can you give an example for usage of them.

http://www.cplusplus.com/reference/std/functional/unary_function/

http://www.cplusplus.com/reference/std/functional/binary_function/

Answer 1

These aren't functions, these are classes (structs, actually, but doesn't matter). When you define your own binary functions to use with STL algorithms, you derive them from these classes in order to automatically get all the typedefs.

E.g.

struct SomeFancyUnaryFunction: public std::unary_function<Arg_t, Result_t>
{
   Result_t operator ()(Arg_t const &)
   {
      ...
   }
};

now you don't need to manually provide the typedefs for argument_type, result_type etc. These structs, just like the iterator struct are there just for our convenience, in order to reuse the typedefs needed for algorithms.

Update for C++11:

As of C++11, the new std::bind does not really need any typedefs, so there are, in a way, obsolete.

Answer 2

Basically, they provide all the typedefs necessary to allow composition of higher-order functions from unary and binary function objects using function adaptors. For example, this allows using a binary functor where a unary is needed, binding one of the arguments to a literal value:

std::find_if( begin, end, std::bind1st(greater<int>(),42) );

std::bind1st relies on the functor passed to it to provide those types.

AFAIK the new std::bind doesn't need them, so it seems in new code you can use std::bindand do away with them.