From cppreference, I found that:
Class template std::function is a general-purpose polymorphic function wrapper. Instances of std::function can store, copy, and invoke any Callable target -- functions, lambda expressions, bind expressions, or other function objects, as well as pointers to member functions and pointers to data members.
I cannot see why a std::function should be able to store such a pointer and I've never heard before about that feature.
Is it really possible, I missed something or that's an error in the documentation?
How should the operator() behave in such a case?
As from the documentation:
Invokes the stored callable function target with the parameters args.
Anyway, there is no stored callable function target to invoke here. Am I wrong?
To be honest, I cannot even figure out what's the right syntax for such a function, otherwise I'd have written an example to test it.
How could the following template be used to define a pointer to data member?
template< class R, class... Args >
class function<R(Args...)>
The effect of a call to the function call operator of std::function<R(ArgTypes...)>:
R operator()(ArgTypes... args) const
is equivalent to (§ 20.9.11.2.4 [func.wrap.func.inv]/p1):
INVOKE<R>(f, std::forward<ArgTypes>(args)...)
whose definition includes the following bullet (§ 20.9.2 [func.require]/p1):
Define INVOKE(f, t1, t2, ..., tN) as follows:
[...]
1.3 —
t1.*fwhenN == 1andfis a pointer to member data of a classTandt1is an object of typeTor a reference to an object of typeTor a reference to an object of a type derived fromT;
then, when f is a pointer to a data member stored in an internal invoker of a std::function, then the std::function itself should define a single argument, e.g.:
std::function<int(std::pair<int,int>)> f = &std::pair<int,int>::first;
f(std::make_pair(1, 2));