关于shared_ptr和指向成员运算符` - > *`和`std :: bind`的指针

Question

Recently I discovered that shared_ptr does not have pointer to member operator ->* . I created simple example:

template <typename Pointer, typename Function, typename... Args>
auto invoke1(Pointer p, Function f, Args... args) -> decltype((p->*f)(args...))
{
  return (p->*f)(args...);
}
struct A { 
    void g() { std::cout << "A::g()\n"; } 
};
int main() {
  A a;
  invoke1(&a, &A::g); // works!!
  std::shared_ptr<A> sa = std::make_shared<A>();
  invoke1(sa, &A::g); // compile error!!
}

Q1: Why is so? Why shared_ptr does not have this operator?

I added such operator for shared_ptr and the example started to work:

template <typename T, typename Result>
auto operator ->* (std::shared_ptr<T> pointer, Result (T::*function)()) ->decltype(std::bind(function, pointer))
{
    return std::bind(function, pointer);
}
template <typename T, typename Result, typename Arg1>
auto operator ->* (std::shared_ptr<T> pointer, Result (T::*function)(Arg1 arg1)) ->decltype(std::bind(function, pointer, std::placeholders::_1))
{
    return std::bind(function, pointer, std::placeholders::_1);
}

Q2: Is this right implementation for this operator? Are there somewhere any "gold" rules how to implement such operator, probably either I reinvented the wheel or go in completely wrong direction, what do you think? Is there a way to have a single function implementing this operator instead of as many function as there are placeholders in std...

After that I came to conclusion that std::bind can be used in my invoke method.

template <typename Pointer, typename Function, typename... Args>
auto invoke2(Pointer p, Function f, Args... args) 
                     -> decltype(std::bind(f, p, args...)())
{
   return std::bind(f, p, args...)();
}

In this way my example also works without need to add operator ->* to shared_ptr .

Q3: So, is std::bind now considered as a replacement for operator->* ?

Answer 1

In a nut shell: yes std::bind is a replacement for member function pointers.

Why? because member function pointers are awful, and their only purposes is to implement delegates, which is why std::bind and std::function do

For reference on how member function pointers are implemented, see my previous answer here . In simplest terms, member function pointers are crippled by the standard because they do not allow for calls after casts; this makes them quite pointless for the sort of behavior 90% of people want from member function pointers: delegates.

For this reason, std::function is used to represent an abstract "callable" type, with std::bind being used to bind a this to the member function pointer. You should absolutely not mess with member function pointers, and instead use std::bind and std::function.

Answer 2

I believe the simplest soultion would be to replace 'structure dereference' ( -> ) operator with a pair of derefence( * ) and structure reference( . ) operators:

template <typename Pointer, typename Function, typename... Args>
auto invoke1(Pointer p, Function f, Args... args) -> decltype(((*p).*f)(args...))
{
  return ((*p).*f)(args...);
}

Answer 3

I believe shared_ptr does not have operator ->* because it's impossible to implement it for arbitrary number of arguments (which C++11 allows to do for other use cases). Also, you can easily add an overload of invoke function for smart pointers that calls get() , so complicating the interface is not desirable.