指针不起作用

Question

My code:

#include<iostream>

using namespace std;

struct element
{
    int liczba;
    element *nastepny;
    element();
};

element::element()
{
    nastepny=0;
}

int main()
{
    element pierwszy;
    pierwszy.liczba=1;
    element drugi;
    (*nastepny).pierwszy=2;
    drugi.liczba=2;
    return 0;
}

It says that *nastepny was not declared, but I do not know how is it possible. I created structure with *nastepny. If I write element *nastepny, it says that element has no member named pierwszy. SOmething is going wrong because pierwszy is declared at the beginning of main function.

Answer 1

(*nastepny).pierwszy=2;

That makes no sense as nastepny is not declared statically or within its current or any parent scope. It is a member of your structure and only exists as a part of and instance of one.

Also, you cannot simply assign two to the address of some unallocated pointer. You have a pointer, but it does not yet point anywhere valid. So, either:

drugi.nastepny = malloc(sizeof *drugi.nastepny);
*drugi.nastepny = whatever;

To allocate space dynamically, or...

drugi.nastepny = &some_variable;

But watch for lifetime issues on that last one.

Answer 2

The error happens in

(*nastepny).pierwszy=2;
  ^^^^^^^^

nastepny is not declared in this scope. The member of the struct doesn't make it accessible outside the struct (instances).

I guess you are meaning something like this:

drugi.nastepny = &pierwszy;
drugi.nastepny->liczba = 2;

Answer 3

我建议这样做：

pierwszy.nastepny = &drugi;

Answer 4

I'm not sure how you expect to get an object when you deference your pointer when you never assigned it a memory address?

If you want to get a pointer to the object itself then you can use the this pointer. However in this scenario such an application of it would make no sense.

The only assignment you provided to the element pointer was 0 (NULL), so it's not pointing to any element object.