What is std::ref useful in this function?

std::reference_wrapper::operator()In some cases, the function performed is a bit more "magic" than the function of a direct function call. Its effect is specified as (quoting N4296 [refwrap.invoke]):

template <class... ArgTypes>
result_of_t<T&(ArgTypes&&... )>
operator()(ArgTypes&&... args) const;

Returns: INVOKE(get(), std::forward<ArgTypes>(args)...). (20.9.2)

where get()returns a reference to the contents of the reference_wrapperwrapper . INVOKEDescribed in 20.9.2 [func.require]:

Defined INVOKE(f, t1, t2, ..., tN)as follows:

(1.1) — (t1.*f)(t2, ..., tN)when fis a pointer to a member function of a class Tand t1is Tan object of type Tor a reference to an object of type or a reference to an object of type T;

(1.2) — ((*t1).*f)(t2, ..., tN)when fis a pointer to a member function of a class Tand t1is not one of the types described in the previous item;

(1.3) t1.*f—when N == 1and fis a pointer to member data of a class, Tand t1is an object of type or a reference to Tan object of that type Tor a reference to an object of type T;

(1.4) (*t1).*f—when N == 1sum fis a pointer to member data of a class Tand t1is not one of the types described in the previous item;

(1.5) - f(t1, t2, ..., tN)in all other cases.

A consequence of calling ref(f)rather than simply fbeing that pointers to member functions and pointers to member data can be "called" using the appropriate object pointer/reference as an argument. E.g,

struct A { void foo(); };
struct B : A {};
struct C : B {};
for_each_arg(&A::foo, A{}, B{}, C{}, std::make_unique<A>());

will be called foo, and the Atemporary object, Band Cthe held object unique_ptr( DEMO ). The reasons why you would rather use ref(f)than use obviously depend on the context in which it fis used for_each_arg.