How to use and access static pointer-to-member value s_ptm inside class?

Question

We can declare and use instance-level pointer-to-member value/func inside class. And using obj.*(obj.ptm) or ptr->*(ptr->ptm) pattern to access.

But if declare them as static type-level s_ptm value/func, how to initialize and access/use them?

class C
{
  public:
     static int s_i;
     static int C::* s_ptm; //declare static ptm value OK here
};

Q: How to initialize and access s_ptm?

coliru.stacked-crooked.com/a/44fa362afb8462ef

Does this answer your question? https://stackoverflow.com/questions/185844/how-to-initialize-private-static-members-in-c — 463035818_is_not_an_ai, Jun 20 '20 at 16:21
But we can declare s_ptm inside class C and `int C::* i =nullptr` to initialize it outside of class. Why `no way to use` it? — Larry, Jun 20 '20 at 16:29
it points to a member of `C` of type `int`, but `C` has no member of type `int` — 463035818_is_not_an_ai, Jun 20 '20 at 16:36
why non-static `int C::* i` is ok? if C has no member of type int — Larry, Jun 20 '20 at 16:46
it doesnt matter if it is static or not. If `C` has no `int` member then a pointer to a `int` member of `C` is of little use. What do you acutally want to achieve? What did you try and what error do you get? — 463035818_is_not_an_ai, Jun 20 '20 at 16:47
Ok, I add `static int s_i` to class C. And use `int C::* i=& C::s_i;` to get `error: cannot initialize a variable of type 'int C::*' with an rvalue of type 'int *'` — Larry, Jun 20 '20 at 16:54
please include a [mcve] and the error message in the question. — 463035818_is_not_an_ai, Jun 20 '20 at 16:56
what do you actually want to achieve? Do you want to get a pointer to `C::s_i` ? Or do you want to initialize `s_ptm` ? Why do you need a pointer to member? — 463035818_is_not_an_ai, Jun 20 '20 at 17:00
https://coliru.stacked-crooked.com/a/44fa362afb8462ef check out here — Larry, Jun 20 '20 at 17:04
why cant you include that together with the error message in the question? — 463035818_is_not_an_ai, Jun 20 '20 at 17:07

463035818_is_not_an_ai · Accepted Answer · 2020-06-20T17:58:38.913

0

Pointers to static members are just plain pointers. You cannot assign a pointer to a static member to a pointer to member. The good news is: You don't need it. Pointers to members enable you to point to a member and then given an instance you can access its member via that pointer. For a static member this isnt needed, because all instances share the same static member.

To initialize s_ptm you need an int member first:

struct C {
    static int s_i;
    static int C::* s_ptm; //declare static ptm value OK here
    int c;
};

int C::* C::s_ptm = &C::c;     // OK
//int C::* C::s_ptm = &C::s_i; // NOPE !
int* p = &C::s_i;              // OK

However, with only a single int member a pointer to member of type int is no that useful. Instead of accessing C::c via the pointer you can access it by its name. Pointers to members are useful when there is more than one member of the same type or in a generic context.

edited Jun 20 '20 at 17:58

answered Jun 20 '20 at 17:06

463035818_is_not_an_ai

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I know a static ptm point to a instance member is not that useful. But out of curiosity, what the result of de-ref of `C::s_ptm`? how to de-ref it? `*(C::s_ptm)` not work – Larry Jun 20 '20 at 18:02
@Larry what is the meaning of "not work" ? Works for me here: https://godbolt.org/z/8btrCF – 463035818_is_not_an_ai Jun 20 '20 at 18:32
Oops, I forgot `obj.*(C::s_ptm)` != `*(C::s_ptm)` – Larry Jun 20 '20 at 19:14

How to use and access static pointer-to-member value s_ptm inside class?

1 Answers1