Accessing a Private variable - c++

Question

I'm trying to get my head around encapsulation in C++. I have this program working when everything is in public, but when I move the vector to private, I'm having trouble accessing the vector with my public functions. I've tried using a friend and I can't get it to work. How can I get my addElement function to see the vector?
#include <iostream>
#include <vector>
using namespace std;
class MySet{
private:
vector<int> elements;
public:
MySet();
friend void addElement(int value);
};
MySet::MySet (){
vector <int> elements;
}
void MySet::addElement(int value){
elements.push_back(value);
}
void Print(vector<int>& v) {
vector<int>::iterator it;
for(it = v.begin(); it != v.end(); ++it) {
cout << (*it) << '\n';
}
}
int main(int argc, char *argv[]){
int value;
MySet set;
cout << "Enter your numbers,(enter -1 to end)" << endl;
while(cin){
cin>> value;
if(value==-1)
break;
set.addElement(value);
}
Print(set.elements);
system("PAUSE");
}

Answer 1

Basically you want to make a Print a method instead of trying to access the private member variable. In general trying to access the private variable directly is usually a good sign that it shouldn't be private in the first place or that you need to change your design. In this case changing your design would most likely lead to the best outcome:
using namespace std;
class MySet{
private:
vector<int> elements;
public:
MySet();
friend void addElement(int value);
void print() {
vector<int>::iterator it;
for(it = this->elements.begin(); it != this_.elements.end(); ++it){
cout << (*it) << '\n';
}
}
};
Now you can print it like so:
set.print();
And this avoids the issues with using the private member.
Note that we no longer need to pass in the particular instance of MySet to the print function, this is because the class already has that information it needs via the this pointer.
The general idea is that you store the state as private then give a bunch of public functions for the users of the class the manipulate that state. The users shouldn't manipulate the state directly though, so if you find you are doing this step back and reconsider your design.