Why have unary_function, binary_function been removed from C++11?

With variadic templates, a lot of general function composing can be expressed much more simply and consistently, so all of the old cruft is no longer necessary:

Do use:

• std::function
• std::bind
• std::mem_fn
• std::result_of
• lambdas

Don't use:

• std::unary_function, std::binary_function
• std::mem_fun
• std::bind1st, std::bind2nd

It isn't removed, it's just deprecated in C++11. It's still part of the C++11 standard. You can still use it in your own code. It was removed in C++17 though.

It isn't used in the standard any more because requiring implementations to derive from binary_function is over-specification.

Users should not care whether less derives from binary_function, they only need to care that it defines first_argument_type, second_argument_type and result_type. It should be up to the implementation how it provides those typedefs.

Forcing the implementation to derive from a specific type means that users might start relying on that derivation, which makes no sense and is not useful.

Edit

How can we improve this in c++11 without unary_function?

You don't need it.

template<typename adaptableFunction>
class unary_negate
{
   private:
       adaptableFunction fun_;
   public:
       unary_negate(const adaptableFunction& f):fun_(f){}

       template<typename T>
           auto operator()(const T& x)  -> decltype(!fun_(x))
           {
               return !fun_(x);
           }
}

In fact you can do even better, see not_fn: a generalized negator