C++ Academy · Lesson

Rule of Three/Five

Manage copy and move correctly.

Lesson 3 of 413 steps

Rule of Three/Five is a free C++ Academy lesson on CoddyKit. This is lesson 3 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.

The Rule of Three

If a class needs a custom destructor, it almost always also needs a custom copy constructor and copy assignment. These three travel together.

Why Three?

A class managing a raw resource must define how it is destroyed, copied, and assigned — otherwise the compiler-generated shallow copies cause double frees.

#include <iostream>

class Bad {
    int* p;
public:
    Bad(int v) : p(new int(v)) {}
    ~Bad() { delete p; }
    // default copy => two objects delete the same p!
};

Custom Copy Constructor

A correct copy constructor performs a deep copy, allocating its own resource rather than sharing.

#include <iostream>

class Box {
    int* p;
public:
    Box(int v) : p(new int(v)) {}
    Box(const Box& o) : p(new int(*o.p)) {}   // deep copy
    ~Box() { delete p; }
    int get() const { return *p; }
};

int main() {
    Box a(5);
    Box b = a;   // independent copy
    std::cout << b.get() << "\n";
}

Custom Copy Assignment

Copy assignment must free the old resource, guard against self-assignment, and deep-copy the new value.

Box& operator=(const Box& o) {
    if (this != &o) {              // self-assignment guard
        delete p;
        p = new int(*o.p);
    }
    return *this;
}

The Rule of Five

C++11 adds move operations. If you define any of the five (destructor, copy ctor, copy assign, move ctor, move assign), consider defining all five.

Move Constructor

A move constructor steals the resource from a temporary and leaves the source empty — far cheaper than copying.

Box(Box&& o) noexcept : p(o.p) {
    o.p = nullptr;   // source left empty
}

Move Assignment

Move assignment releases the current resource then takes ownership of the source's.

Box& operator=(Box&& o) noexcept {
    if (this != &o) {
        delete p;
        p = o.p;
        o.p = nullptr;
    }
    return *this;
}

Mark Moves noexcept

Move operations should be noexcept so containers like std::vector can use them safely when reallocating.

The Rule of Zero

Best of all: own nothing raw. Use std::unique_ptr, std::vector, or std::string and you can define none of the five — the compiler defaults are correct.

#include <memory>

class Good {
    std::unique_ptr<int> p;   // owns the resource for you
public:
    Good(int v) : p(std::make_unique<int>(v)) {}
    // no destructor, no copy/move needed
};

Deleting Operations

If copying makes no sense, delete it: Box(const Box&) = delete;. This makes the type move-only or non-copyable on purpose.

Choosing the Right Rule

Aim for the Rule of Zero. Fall back to the Rule of Five only when you genuinely wrap a raw resource the standard library does not cover.

Quick Check

Test your understanding of the Rule of Three/Five.

Answer it in the interactive lesson

Recap

You learned the Rule of Three/Five: managing a raw resource means defining destructor, copy, and move operations consistently. Mark moves noexcept. Best of all, follow the Rule of Zero by using standard RAII types and defining none.

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Frequently Asked Questions

Is the “Rule of Three/Five” lesson free?

Yes — any 3 lessons of the C++ Academy learning path, including “Rule of Three/Five”, 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 “Rule of Three/Five”?

Manage copy and move correctly. 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 3 of 4.

How long does the “Rule of Three/Five” lesson take?

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