How to Access Private Members of a Class in C++

1. Use private Access Specifier to Encapsulate Class Members in C++
2. Use public Functions to Retrieve Private Members of a Class in C++

This article will explain several methods of how to access private members of a class in C++.

Use private Access Specifier to Encapsulate Class Members in C++

Access specifiers are employed to implement a core feature of object-oriented programming - called encapsulation. As a result, we restrict the direct access to specific data members of the class and essentially construct an interface to operate on class data. The C++ provides several access specifier keywords like: public, private and protected, which usually precede the members of the class that need to be qualified with corresponding accessibility.

Members defined after the private specifier are accessible to member functions only and can’t be referred directly from the code that uses the class. A class is usually constructed with two sides in mind - the designer of the class and the user of the class. The latter is usually the one who is affected by the encapsulation. If the programmer defines members before the first access specifier, their accessibility is by default set to private when the class keyword is used, and public on the struct keyword.

In the following example, we implement a class called BaseClass with two string data members declared as private, so to access the values of these members, the designer of the class should include public functions that retrieve them. Notice that we can remove the public keyword from the code below and still have getUsername and getName functions as public members.

Another perk of the encapsulation is the flexibility to modify the internal class structure without worrying about the user side’s compatibility issues. As long as the interface, thus, public functions don’t change, the user of the class does not need to modify her code.

#include <iostream>
#include <string>
#include <utility>
#include <vector>

using std::cout;
using std::endl;
using std::string;
using std::vector;

class BaseClass {
 public:
  BaseClass() = default;
  explicit BaseClass(string user, string nm)
      : username(std::move(user)), name(std::move(nm)) {}

  ~BaseClass() = default;

  string& getUsername() { return username; };
  string& getName() { return name; };

 private:
  string username;
  string name;
};

int main() {
  BaseClass base("buddy", "Buddy Bean");

  cout << "base -> name: " << base.getName() << endl;
  cout << "base -> username: " << base.getUsername() << endl;

  exit(EXIT_SUCCESS);
}

Output:

base -> name: Buddy Bean
base -> username: buddy

Use public Functions to Retrieve Private Members of a Class in C++

private members can be modified using the class interface functions, e.g changeUsername function takes string argument from the user of the class and it stores its value to the private member - username. Note that there is also a friend keyword that distinguishes other classes and functions allowed to access the private members of the class. These functions can be external, not part of the class mentioned above. Mind though, the incorrect usage of access specifiers most likely results in compiler errors.

#include <iostream>
#include <string>
#include <utility>
#include <vector>

using std::cout;
using std::endl;
using std::string;
using std::vector;

class BaseClass {
 public:
  BaseClass() = default;
  explicit BaseClass(string user, string nm)
      : username(std::move(user)), name(std::move(nm)) {}

  ~BaseClass() = default;

  void changeUsername(const string& s) { username.assign(s); };

  string& getUsername() { return username; };
  string& getName() { return name; };

 private:
  string username;
  string name;
};

int main() {
  BaseClass base("buddy", "Buddy Bean");

  cout << "base -> name: " << base.getName() << endl;
  cout << "base -> username: " << base.getUsername() << endl;
  base.changeUsername("jolly");
  cout << "base -> username: " << base.getUsername() << endl;

  exit(EXIT_SUCCESS);
}

Output:

base -> name: Buddy Bean
base -> username: buddy
base -> username: jolly