Preserve a Mark Rule
Preserve a Mark Rule
Encapsulation controls changes to state through meaningful operations.
A private mark and a checked setter let the school reject invalid updates without exposing direct field assignment.
Before this lesson, review Initialize Valid Objects when needed.
What you’ll learn
- Use encapsulation to preserve a Mark Rule.
- Explain how the complete example produces 72 → false.
- Make the practice change and explain why the result changes or stays the same.
Follow a complete C++ example
A private mark and a checked setter let the school reject invalid updates without exposing direct field assignment.
Read the declarations above main before tracing its statements. Each declared name supplies the value, type or operation used later. The std:: prefix selects a name from the standard library, and cout << sends a value to the output stream.
C++17 · main.cpp · complete program
#include <iostream>
class CheckedStudent {
int marks_ {
}
;
public: bool setMark(int marks) {
if (marks < 0 || marks > 100) return false;
marks_ = marks;
return true;
}
int mark() const {
return marks_;
}
}
;
int main() {
CheckedStudent student;
student.setMark(72);
bool accepted = student.setMark(101);
std::cout << student.mark() << '\n' << std::boolalpha << accepted << '\n';
return 0;
}
Expected output
72
falseConnect the code to its result
C++17 · source used in this explanation
#include <iostream>
class CheckedStudent {
int marks_ {
}
;
public: bool setMark(int marks) {
if (marks < 0 || marks > 100) return false;
marks_ = marks;
return true;
}
int mark() const {
return marks_;
}
}
;
int main() {
CheckedStudent student;
student.setMark(72);
bool accepted = student.setMark(101);
std::cout << student.mark() << '\n' << std::boolalpha << accepted << '\n';
return 0;
}
Expected output
72
falseEncapsulation controls changes to state through meaningful operations. A private mark and a checked setter let the school reject invalid updates without exposing direct field assignment.
Trace the result: Read the complete program below from its starting values to the output operations. Each std::cout statement displays the value supplied at that point; \n begins the next line. The resulting lines, in order, are 72 → false. In this example, distinguish what the program calculates or changes from what it actually displays.
Important mistakes and boundaries
Private access is a language boundary, not encryption. Getters and setters only help when they preserve real rules.
Change the school example
C++17 · edit this complete example
#include <iostream>
class CheckedStudent {
int marks_ {
}
;
public: bool setMark(int marks) {
if (marks < 0 || marks > 100) return false;
marks_ = marks;
return true;
}
int mark() const {
return marks_;
}
}
;
int main() {
CheckedStudent student;
student.setMark(72);
bool accepted = student.setMark(101);
std::cout << student.mark() << '\n' << std::boolalpha << accepted << '\n';
return 0;
}
Your task: Try setting 80 instead of 101. Before running your edited program, predict the new output and explain which stored value, condition or object changes. Keep the rest of the example intact.
Use the complete source repeated here; this task does not depend on remembering code from another section.
Worked answer and reasoning
Worked variation: Try setting 80 instead of 101. The stored mark becomes 80 and accepted is true.
Encapsulation controls changes to state through meaningful operations. A private mark and a checked setter let the school reject invalid updates without exposing direct field assignment.
Private access is a language boundary, not encryption. Getters and setters only help when they preserve real rules.
For a baseline run, save this source as main.cpp and use clang++ -std=c++17 -Wall -Wextra -pedantic main.cpp -o school, or equivalent g++ flags. Recompile after editing so that you run the updated executable.
The source and output below belong to the edited program. Compare them with the original example to see the consequence of this single change.
C++17 · worked variation · complete program
#include <iostream>
class CheckedStudent {
int marks_ {
}
;
public: bool setMark(int marks) {
if (marks < 0 || marks > 100) return false;
marks_ = marks;
return true;
}
int mark() const {
return marks_;
}
}
;
int main() {
CheckedStudent student;
student.setMark(72);
bool accepted = student.setMark(80);
std::cout << student.mark() << '\n' << std::boolalpha << accepted << '\n';
return 0;
}
Expected output
80
trueCheck your understanding
C++17 · original program for this question
#include <iostream>
class CheckedStudent {
int marks_ {
}
;
public: bool setMark(int marks) {
if (marks < 0 || marks > 100) return false;
marks_ = marks;
return true;
}
int mark() const {
return marks_;
}
}
;
int main() {
CheckedStudent student;
student.setMark(72);
bool accepted = student.setMark(101);
std::cout << student.mark() << '\n' << std::boolalpha << accepted << '\n';
return 0;
}
What to remember
Encapsulation controls changes to state through meaningful operations. Private access is a language boundary, not encryption. Getters and setters only help when they preserve real rules.
Useful connections: Initialize Valid Objects, Give a Task a Name.
Language and design guidance: C++ Core Guidelines and C++ working draft. The draft describes an evolving language; these examples deliberately target C++17.
Reaching the recap and completing the quizzes are separate milestones.