C++ Academy · Lesson

const Member Functions

Mark read-only methods.

const Member Functions is a free C++ Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C++ Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

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.

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.

Frequently asked questions

Is the “const Member Functions” lesson free?

Yes — the full text of “const Member Functions” is free to read here on the web, and the C++ Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C++ Academy course, upgrade to CoddyKit PRO.

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; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

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.

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