C++ Academy · Lesson

const Member Functions

Mark read-only methods.

Lesson 2 of 413 steps

const Member Functions is a free C++ Academy lesson on CoddyKit. This is lesson 2 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the C++ Academy learning path, and your progress syncs across the web and the CoddyKit app. The C++ Academy course includes 4 lessons in total.

What a const Member Function Is

A member function marked const promises not to modify the object's observable state. The keyword goes after the parameter list.

#include <iostream>

class Counter {
    int n = 0;
public:
    int value() const { return n; }   // read-only
    void inc() { ++n; }
};

int main() {
    Counter c;
    c.inc();
    std::cout << c.value() << "\n";
}

const Objects Call const Methods

On a const object you may only call const member functions. Non-const methods are forbidden.

#include <iostream>

class Counter {
    int n = 0;
public:
    int value() const { return n; }
    void inc() { ++n; }
};

int main() {
    const Counter c;
    std::cout << c.value() << "\n";   // OK
    // c.inc();                        // ERROR
}

this Becomes const

Inside a const member function, this is a pointer to const, so the compiler rejects any attempt to modify members.

Mark Every Read-Only Method const

Getters, queries, and printers should all be const. This lets them work on const objects and const references.

const Overloading

You can provide both a const and a non-const version of a method; the compiler picks based on whether the object is const.

#include <iostream>
#include <vector>

class Box {
    std::vector<int> v{1, 2, 3};
public:
    int& at(int i) { return v[i]; }              // mutable access
    const int& at(int i) const { return v[i]; }  // read-only access
};

int main() {
    Box b;
    b.at(0) = 99;
    std::cout << b.at(0) << "\n";
}

The mutable Keyword

mutable members may change even inside a const method — useful for caches or counters that are not part of logical state.

#include <iostream>

class Cache {
    mutable int hits = 0;
    int data = 42;
public:
    int get() const { ++hits; return data; }   // hits is mutable
    int hitCount() const { return hits; }
};

int main() {
    Cache c;
    c.get(); c.get();
    std::cout << c.hitCount() << "\n";
}

Logical vs Physical const

const protects the object's logical state; mutable lets implementation details like caches change without breaking that promise.

const Methods and const References

A function receiving a const& can only call const methods on it — another reason to mark queries const.

#include <iostream>

class Counter {
    int n = 5;
public:
    int value() const { return n; }
};

void report(const Counter& c) {
    std::cout << c.value() << "\n";   // needs value() to be const
}

int main() { Counter c; report(c); }

Returning const References

Const accessors often return const& to expose members without allowing modification or copying.

Do Not Cast Away const

Using const_cast to modify a truly const object is undefined behavior. Use mutable for legitimate exceptions instead.

const-Correct Classes

A well-designed class marks every non-mutating method const, so it works seamlessly in const contexts throughout a program.

Quick Check

Test your understanding of const member functions.

Answer it in the interactive lesson

Recap

You learned to mark read-only methods const, that const objects call only const methods, how to overload on const, and how mutable allows implementation-detail changes. Avoid const_cast; design classes const-correctly from the start.

Free to start

Learn C++ with an AI tutor — free

Write and run real code in your browser, get instant help from a 24/7 AI tutor, and pick up where you left off on the web or in the app.

Courses: 51
Lessons: 203

Start learning free View the full course

Frequently Asked Questions

Is the “const Member Functions” lesson free?

Yes — any 3 lessons of the C++ Academy learning path, including “const Member Functions”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The C++ Academy course includes 4 lessons in total.

What will I learn in “const Member Functions”?

Mark read-only methods. You practise C++ Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C++ Academy?

No prior experience is required. C++ Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 4.

How long does the “const Member Functions” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C++ Academy lesson?

Yes. Every C++ Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

← Back to C++ Academy