unary_function and binary_function are deprecated in C++ 11, and removed in C++ 17. What should we use instead?

Question

I have been reading Effective STL by Meyers.

I came across some sections which mention function adapter objects, such as not1, bind1st, bind2nd. There are apparently a range of such function adapter objects, however I have never encountered these before. Another example is mem_fun and mem_fun_ref.

Many of these inherit from unary_function and binary_function. Some links are provided below.

https://en.cppreference.com/w/cpp/utility/functional/unary_function

https://en.cppreference.com/w/cpp/utility/functional/binary_function

https://en.cppreference.com/w/cpp/utility/functional/mem_fun

These objects are depreciated in C++ 11, which probably explains why I had not encountered them before.

Why are they deprecated, and what replaces them? My instinct tells me that a lambda can replace such function objects, and while reading the book it did occur to me that some of the sorting operations on STL containers can be described using a lambda instead of a function object.

• Does a lambda completely replace these "adapter functions".
• If so, are function objects still useful post C++ 11? If they have been made obsolete in the STL, does this mean function objects are obsolete everywhere in C++ 11 or do they still have uses?

As a final comment, are there any modern books on STL which serve as a good replacement for Meyers Effective STL? I have learned a lot from reading his book but there are probably more sections than I currently realize which contain obsolete information.

Answer 1

You can use one of these:

• Lambda expression
• std::bind
• std::mem_fn

Does a lambda completely replace these "adapter functions".

Since C++14, lambdas can do everything that the deprecated and later removed functor templates can do.

If so, are function objects still useful post C++ 11?

Lambdas are function objects.

If you mean to ask whether it's still useful to write named function class templates: Yes, it's still occasionally useful.

---

Lambdas can be just as reusable by writing a named function that returns the lambda.

Can you add ... an example?

Here's an example:

constexpr auto
reusable_lambda() noexcept
{
    return []{
        return "Just an example";
    };
}