Question

I have the following code:

#include <iostream>
using namespace std;

struct S {
    S(int i): I(i) { }
    int & v () { return I; }
    private :
    int I;
};

S s1 (1);
int main() {

    cout << s1.v() << "\n";

    return 0;
}

And I get the output:

1

I am confused as to how it is working. For example with s1 it is calling the constructor S(int i) but how is it sub-classing I and calling its constructor when there is no I class and how is the private variable I getting assigned a number when there has been nothing assigned to it? Also, if v() returns int& (a reference therefore, I would think it would print out a memory location but it gives 1 consistently).

Sorry if this sounds stupid can’t figure it out.

Answer 1

It seems you have multiple questions:

1. The struct is not subclassing I, it is simply using Initialization Lists to construct the I variable. The same could have been done with the constructor S(int i) { I = i;}.

2. It is getting assigned in the constructor, see #1.

3. You are confusing references and pointers. References pretend to act like regular value passed numbers. Pointers, returning int * with a function such as int * v () { return &I; } would print out the address of the variable unless you dereference them with the * symbol. References automatically dereference themselves.