问题
Apparently ->* doesn't work automagically if you overload ->, and has to be overloaded manually.
Why iterators for standard containers don't overload ->* in addition to ->, forcing usage of (*iter).*mem_ptr instead of iter->*mem_ptr?
#include <iostream>
#include <vector>
struct S
{
int x;
};
int main()
{
std::vector<S> vec = {{42}};
auto mem_ptr = &S::x;
std::cout << (*vec.begin()).*mem_ptr << '\n'; // This line compiles.
std::cout << vec.begin()->*mem_ptr << '\n'; // This line doesn't compile.
}
回答1:
With the caveat that these questions aren't typically answerable, here are a few reasons why operator->*() may not be overloaded. Although it's possible the real answer is that nobody thought of it. And if this, to you, is an important missing language feature, you could always submit a proposal.
For starters, ptr->*pmd just isn't a very commonly used expression in general. So the fact that you cannot write it->*pmd isn't something that most people miss, especially when (*it).*pmd accomplishes exactly the same goal at the cost of just 2 extra characters. The potential upside here seems fairly small. Still, iterators should be consistent with pointers, so it would make sense. But...
Pointers to members aren't just pointers to member data, we can also have pointers to member functions and can write (ptr->*pmf)() today, where ptr->*pmf by itself is ill-formed. You can't get those semantics at all with operator->* - to get the call operation to work, ptr->*pmf would have to basically return a lambda. So now, this actually becomes fairly complicated - unless you want to just support ptr->*pmd. With any approach, you're inconsistent with pointers.
For input iterators, you don't want to support operator->*() at all since it would yield an immediately dangling reference.
To me, personally, the cost (figuring out how to specify these operators, for which iterators, and what to do about pointers to member functions) doesn't really seem worth the benefit (saving 2 characters in an expression that's rarely written).
来源：https://stackoverflow.com/questions/48626039/why-standard-container-iterators-dont-overload