ProgramGuru

Read an Object Without Mutation

Read an Object Without Mutation

This identifies the current object inside an ordinary member function.

A const member function promises not to modify ordinary instance state through that receiver, allowing read-only reporting.

Before this lesson, review Preserve a Mark Rule when needed.

What you’ll learn

  • Use this and const methods to read an Object Without Mutation.
  • Explain how the complete example produces 72.
  • Make the practice change and explain why the result changes or stays the same.

Follow a complete C++ example

A const member function promises not to modify ordinary instance state through that receiver, allowing read-only reporting.

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 StudentRecord  {
  int marks_;
  public: explicit StudentRecord(int marks) : marks_(marks)  {
  }
  int mark() const  {
    return marks_;
  }
}
;
int main()  {
  const StudentRecord student(72);
  std::cout << student.mark() << '\n';
  return 0;
}

Expected output

72
Run in C++ compiler →

Connect the code to its result

C++17 · source used in this explanation

#include <iostream>

class StudentRecord  {
  int marks_;
  public: explicit StudentRecord(int marks) : marks_(marks)  {
  }
  int mark() const  {
    return marks_;
  }
}
;
int main()  {
  const StudentRecord student(72);
  std::cout << student.mark() << '\n';
  return 0;
}

Expected output

72

This identifies the current object inside an ordinary member function. A const member function promises not to modify ordinary instance state through that receiver, allowing read-only reporting.

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. In this example, distinguish what the program calculates or changes from what it actually displays.

Important mistakes and boundaries

Const does not prevent every external side effect or changes to mutable members. A non-const member cannot normally be called on a const object.

Change the school example

C++17 · edit this complete example

#include <iostream>

class StudentRecord  {
  int marks_;
  public: explicit StudentRecord(int marks) : marks_(marks)  {
  }
  int mark() const  {
    return marks_;
  }
}
;
int main()  {
  const StudentRecord student(72);
  std::cout << student.mark() << '\n';
  return 0;
}

Your task: Construct a const student with 90. 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: Construct a const student with 90. The read-only report returns 90.

This identifies the current object inside an ordinary member function. A const member function promises not to modify ordinary instance state through that receiver, allowing read-only reporting.

Const does not prevent every external side effect or changes to mutable members. A non-const member cannot normally be called on a const object.

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 StudentRecord  {
  int marks_;
  public: explicit StudentRecord(int marks) : marks_(marks)  {
  }
  int mark() const  {
    return marks_;
  }
}
;
int main()  {
  const StudentRecord student(90);
  std::cout << student.mark() << '\n';
  return 0;
}

Expected output

90
Run in C++ compiler →

Check your understanding

C++17 · original program for this question

#include <iostream>

class StudentRecord  {
  int marks_;
  public: explicit StudentRecord(int marks) : marks_(marks)  {
  }
  int mark() const  {
    return marks_;
  }
}
;
int main()  {
  const StudentRecord student(72);
  std::cout << student.mark() << '\n';
  return 0;
}
Before making the practice edit, what does the original C++17 program print?
Choose one answer. You can try again.

Answer and explanation

72

This identifies the current object inside an ordinary member function. A const member function promises not to modify ordinary instance state through that receiver, allowing read-only reporting. Const does not prevent every external side effect or changes to mutable members. A non-const member cannot normally be called on a const object.

What to remember

This identifies the current object inside an ordinary member function. Const does not prevent every external side effect or changes to mutable members. A non-const member cannot normally be called on a const object.

Useful connections: Preserve a Mark Rule, 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.

    Next lesson: Store Shared Class Data →