C++
Summary - Encapsulation
Combine private, getter/setter and const to write an Account class.
Summary - Encapsulation
In this section we deepened encapsulation. private members hide data; getters and setters provide controlled access and validation. const methods promise immutability; constant objects can only call these.
mutable members can be modified inside const methods; this is useful for caching. friend functions can access private members from outside; useful for operator overloading. Now we will bring these together in a task.
Exercise Define the Sensor class with encapsulation
Define a Sensor class. Under private: add int reading; and mutable int reads;. Under public: add void setReading(int r) { if (r >= 0) reading = r; }, int getReading() const { reads++; return reading; } and int getReads() const { return reads; }. In main create an instance, call setReading(30), call getReading() twice and print getReads() with std::cout.
Show example solution
#include <iostream>
class Sensor {
int reading;
mutable int reads;
public:
void setReading(int r) {
if (r >= 0) reading = r;
}
int getReading() const {
reads++;
return reading;
}
int getReads() const {
return reads;
}
};
int main() {
Sensor s;
s.setReading(30);
s.getReading();
s.getReading();
std::cout << s.getReads();
return 0;
}
Exercise Add a friend function to Sensor
Take the Sensor class above. Under public: add the friend declaration friend void report(const Sensor&);. Outside the class write void report(const Sensor& s) { std::cout << "Reading: " << s.reading; }. In main create Sensor s;, call setReading(25) and call report(s).
Show example solution
#include <iostream>
class Sensor {
int reading;
mutable int reads;
public:
void setReading(int r) {
if (r >= 0) reading = r;
}
int getReading() const {
reads++;
return reading;
}
int getReads() const {
return reads;
}
friend void report(const Sensor&);
};
void report(const Sensor& s) {
std::cout << "Reading: " << s.reading;
}
int main() {
Sensor s;
s.setReading(25);
report(s);
return 0;
}
End of section
You completed the encapsulation section. You can now hide data with private, provide controlled access with getters and setters, promise immutability with const methods, and manage side state like caches with mutable.
With friend you granted access to external functions when needed. These five OOP topics — classes, constructors, inheritance, library design and encapsulation — form the foundation of C++'s object-oriented side. The next sections cover advanced topics like templates, smart pointers and the STL.
Test yourself
-
Why is a getter method made const?
int getX() const { return x; }
- To make it faster
- To make it private
- To state it will not modify the object
- To allow calling from outside
Show answer
To state it will not modify the object — const states that the method will not modify the object.
-
Write the mutable keyword
private: ____ int calls;
Show answer
private: mutable int calls;
Next lesson: Static member variables
