How to get a user's input into a class's member variable and dereference it

Question

class cookie{
public:
    cookie() = default;
    int*p_member{};
    int case{};
private:
};

#include <iostream>
using namespace std;
int main(){
    cookie cold;
    cout << "Type what you want into the cookie p_member variable " << endl;
    std::cin >> cold.*p_member; // this doesn't work 
}

I wanna know how to get access to the classes pointer variable put data inside it and then derefrence it.

why is the member a pointer? You need an `int` to store an integer. A pointer is not sufficient, your pointer points nowhere — 463035818_is_not_an_ai, Sep 08 '22 at 09:20
@463035818_is_not_a_number how come, it doesn't point anywhere ? — Ekene Nwokedi, Sep 08 '22 at 09:23
you do not make it point anywhere. The pointer is not initialized to point to a `double` — 463035818_is_not_an_ai, Sep 08 '22 at 09:23
Is this about a member that is a pointer or a pointer-to-member? — Bartek Banachewicz, Sep 08 '22 at 09:24
@463035818_is_not_a_number The pointer is a nullptr because i want to put something inside it , int the main function. — Ekene Nwokedi, Sep 08 '22 at 09:24
a pointer to a `int` stores an adress of an `int`, it does not store an `int`. There is no `int` in your code to which you could assign — 463035818_is_not_an_ai, Sep 08 '22 at 09:26
@BartekBanachewicz it's about a member that is a pointer.I would like to assign a value to the member, and then dereference the member so i could print it out. — Ekene Nwokedi, Sep 08 '22 at 09:26
"I would like to assign a value to the member" you can assign to it the address of an `int`, but you need an `int` first — 463035818_is_not_an_ai, Sep 08 '22 at 09:28
@463035818_is_not_a_number so i can't assign it directly like a normal pointer. — Ekene Nwokedi, Sep 08 '22 at 09:29
You need to first assing the address to the pointer, and that address has to be of a memory location that's accessible to you. In this case the easiest way would be to create an `int` variable and get its address via the address-of operator. After you have obtained the address, you can initialize the data in that memory region and subsequently read from it via your dereferenced pointer. — Bartek Banachewicz, Sep 08 '22 at 09:29

score 0 · Accepted Answer · answered Sep 08 '22 at 09:49

First things first, make sure that you're not dereferencing a null or uninitialized pointer. Otherwise you'll have undefined behavior.

it's about a member that is a pointer.I would like to assign a value to the member, and then dereference the member so i could print it out.

You can use a pointer to member syntax for this task as shown below:

class cookie{
public:
    
    int var;
    int cookie::*p_member=&cookie::var; //p_member can point to an int data member of an cookie object 
    
};

int main(){
    cookie cold{};
    
    cout << "Type what you want into the cookie p_member variable " << endl;
    std::cin >> cold.*cold.p_member;  //take input from user directly into var using member pointer syntax
    
    std::cout << "var: " << cold.var << std::endl; //print var to confirm that it is correctly set
}

Working demo

How to get a user's input into a class's member variable and dereference it

1 Answers1