C++ private Keyword

The private keyword in C++ is used to define the access level of class members. Members declared as private are accessible only within the class in which they are declared. They cannot be accessed directly from outside the class, including by objects of the class or derived classes.

By default, all members of a class are private unless explicitly specified as public or protected.

Syntax

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class ClassName {
private:
    // Private members
};

class: The keyword used to define a class.
private: The access specifier indicating that the members below it are private.
members: The data members or member functions that are accessible only within the class.

Examples

Example 1: Accessing Private Members through Public Functions

In this example, you will learn how private members can be accessed indirectly using public member functions.

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#include <iostream>
using namespace std;

class Person {
private:
    string name; // Private member

public:
    // Setter function to modify private member
    void setName(const string& newName) {
        name = newName;
    }

    // Getter function to access private member
    string getName() const {
        return name;
    }
};

int main() {
    Person person;
    person.setName("Alice"); // Modify private member through public function
    cout << "Name: " << person.getName() << endl; // Access private member through public function
    return 0;
}

Output:

Name: Alice

Explanation:

The name member is declared as private, so it cannot be accessed directly outside the class.
The public functions setName and getName provide controlled access to the private member.
The main function uses these public methods to modify and access the private member.

Example 2: Attempting to Access Private Members Directly

In this example, you will learn how the error is generated when trying to access private members directly.

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#include <iostream>
using namespace std;

class Rectangle {
private:
    int length, width; // Private members

public:
    Rectangle(int l, int w) : length(l), width(w) {}

    int area() const {
        return length * width; // Access private members within the class
    }
};

int main() {
    Rectangle rect(5, 3);
    // Uncommenting the line below will cause a compilation error
    // cout << "Length: " << rect.length << endl; // Direct access to private member
    cout << "Area: " << rect.area() << endl; // Use public method to access private members
    return 0;
}

Output:

Area: 15

Explanation:

The length and width members are private, so direct access outside the class is not allowed.
An attempt to access these private members directly in the main function results in a compilation error.
The area public method is used to calculate and return the area of the rectangle, allowing indirect access to the private members.

Key Points about `private` Keyword

The private keyword restricts access to members within the class in which they are declared.
Private members cannot be accessed directly from outside the class, ensuring encapsulation.
Public methods (getters and setters) are commonly used to provide controlled access to private members.
By default, all members of a class are private unless explicitly specified as public or protected.
Encapsulation provided by private helps in hiding implementation details and ensuring data integrity.

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C++ private Keyword