How do I get my "level" class to obatain private information from another class?

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I have a class named "data" in which i perform some work with files where i have 4 private variables.I want to use this data in another class named "level".How can i use those private variables in another class?

What I have tried:

I've tried getting acces with an object by returning the value via a function but it says something about the "<< operator",i assume that it has something to do with overloading the "<<" but i don't know how to do that and don't know if it works between classes

Posted 9-Mar-17 7:33am

Member 13049206

Updated 9-Mar-17 10:01am

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[no name] 9-Mar-17 13:56pm

You use public properties.

Member 13049206 9-Mar-17 14:04pm

What do you mean

[no name] 9-Mar-17 14:27pm

I mean exactly what I said.

Richard MacCutchan 9-Mar-17 14:33pm

"it says something about"
So you want us to guess what "it" is, and what "it" said, and when or where? Why not show the actual code, and the exact error message(s), and people may have some clue as to what you are talking about?

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Maciej Los · Accepted Answer · 2017-03-09T08:42:00

Solution 1

Start with basics:
Variable Scope in C++[^]
C++ Classes and Objects[^]

Posted 9-Mar-17 8:42am

Maciej Los

Comments

CPallini 9-Mar-17 16:09pm

A savy suggestion.
5.

Maciej Los 9-Mar-17 16:14pm

Thank you, Carlo.

CPallini · Accepted Answer · 2017-03-09T10:00:00

An obvious solution, already suggested, is implementing getter and setter (if you new both read and write access to the variables) methods in class data and then use it in class level.

C++

#include <iostream>
using namespace std;

class data
{
  int d; // the private variable d
public:
  data(int d):d(d){}

  int get_d() const {return d;} // grant read access to d
  void set_d(int d){this->d = d;} // grant write access to d
};


class level
{
public:
  // read access to d
  void show_d(const data & dt)
  {
    cout << dt.get_d() << endl;
  }
  // write access to d
  void inc_d(data & dt)
  {
    dt.set_d( dt.get_d() + 1);
  }
};

int main()
{
  data dt(5);

  level lv;
  lv.show_d( dt);
  lv.inc_d(dt);
  lv.show_d(dt);
}

This way, however, the private variable is exposed (via getter and setter) to all the data class consumers.

Another approach uses the friend[^] specifier to grant to level class full access to all data class members.

C++

#include <iostream>
using namespace std;

class level; // forward declaration of level class
class data
{
  int d; // the private variable d
public:
  data(int d):d(d){}
  friend class level; // grant to level class full access to private members
};


class level
{
public:
  // read access to d
  void show_d(const data & dt)
  {
    cout << dt.d << endl;
  }
  // write access to d
  void inc_d(data & dt)
  {
    ++dt.d;
  }
};

int main()
{
  data dt(5);

  level lv;
  lv.show_d( dt);
  lv.inc_d(dt);
  lv.show_d(dt);
}