How to Access Structure Members as Pointers

Question

I am trying to cin a value to the birthMonth variable in my structure. My understanding is that structure members are stored sequentially. If that is the case, I cannot find out why using the line

cin >> *((short*)((people) + sizeof(Person::name)));

does not store the input value into the structure element. Am I wrong about the members of a structure being stored sequentially?

main.cpp

#include <iostream>
using namespace std;

struct Person {
    char name[15];
    short birthMonth;
};
int main() {
    Person people[2];
    cout << "Birth Month: ";
    cin >> *((short*)(people + sizeof(Person::name)));
    cout << people[0].birthMonth;
}

Sample Run:

Birth Month: 7
0
Program ended with exit code: 0

Answer 1

Because of memory alignment and padding, C++ actually doesn't give any guarantee that a struct's member is located according to the cumulative size of previous members. If you're trying to write to the first Person 's birthMonth , it's as simple as:

cin >> people[0].birthMonth;

EDIT: to actually address the question's title, if you want an int* pointer to a specific person's birthMonth , it's done using the address-of operator:

int* birthMonth_ptr = &people[0].birthMonth;

But if you want a pointer to any Person 's birthMonth member, you can use a pointer-to-member :

// Note that no Person object is used when defining
int Person::*member_ptr = &Person::birthMonth;

// To use the pointer though, you need an object
int a_persons_birthdate = people[0].*member_ptr;

Also note that a pointer-to-member is typed differently from a regular pointer, and can't be cast to one.

Answer 2

To understand why your code does not work, add a few lines to print couple of addresses.

short* ptr1 = (short*)(people + sizeof(Person::name));
short* ptr2 = &(people[0].birthMonth);
std::cout << "ptr1: " << ptr1 << ", ptr2: " << ptr2 << std::endl;

You will notice that due to alignment requirements, ptr1 and ptr2 are different.

Further reading: https://en.wikipedia.org/wiki/Data_structure_alignment .