Shallow And Deep Copy In C++

Introduction

Understanding shallow and deep copy in C++ is essential for writing safe, efficient code, especially when dealing with classes that manage resources such as dynamic memory, file handles, or network sockets. When an object is copied, the compiler can generate a default copy constructor and copy‑assignment operator that perform a member‑wise copy. This default behavior works fine for simple data members, but it can lead to subtle bugs when the class owns pointers or other non‑trivial resources. In this article we explore the difference between shallow and deep copying, show when each approach is appropriate, and demonstrate how to implement a correct deep copy using the Rule of Three/Five.

What Is Copying in C++?

Copying an object means creating a new instance that has the same observable state as the original. The language provides two special member functions for this purpose:

• Copy constructor – MyClass(const MyClass& other);
• Copy‑assignment operator – MyClass& operator=(const MyClass& other);

If you do not declare these functions, the compiler implicitly defines them, performing a shallow copy of each non‑static data member. Whether that is sufficient depends on the semantics of the members.

Shallow Copy

A shallow copy duplicates the values of an object’s data members exactly as they appear. For primitive types (int, double, bool) this is fine. For pointer members, however, the pointer value itself is copied, not the memory it points to. As a result, both the original and the copy end up referring to the same resource.

When Shallow Copy Is Acceptable

• The class contains only value‑type members (no owning pointers).
• The class is designed to be immutable after construction, so sharing resources does not cause mutation conflicts.
• The class explicitly documents that copies share ownership (e.g., a reference‑counted smart pointer).

Example of a Shallow Copy

class Buffer {
public:
    Buffer(std::size_t size) : data_(new char[size]), size_(size) {}
    // Compiler‑generated copy constructor & assignment → shallow copy
private:
    char* data_;   // owning raw pointer
    std::size_t size_;
};

If we write:

Buffer a(10);
Buffer b = a;   // shallow copy: b.data_ points to the same memory as a.data_

Both a and b now own the same block. When either object’s destructor runs, it deletes the memory, leaving the other object with a dangling pointer—a classic double‑free or use‑after‑free bug That's the part that actually makes a difference..

Deep Copy

A deep copy creates a new, independent copy of any resources owned by the object. For pointer members, this means allocating new memory and copying the contents pointed to by the original pointer. After a deep copy, modifying the copy does not affect the original, and each object can safely release its own resources in its destructor.

When Deep Copy Is Required

• The class owns dynamically allocated memory or other exclusive resources.
• The class’s copy semantics should give each instance its own independent state.
• The class follows value semantics (like std::string or std::vector).

Implementing a Deep Copy

We must provide our own copy constructor and copy‑assignment operator (and, ideally, a destructor) to manage the resource correctly.

class Buffer {
public:
    // Constructor
    explicit Buffer(std::size_t size) : data_(new char[size]), size_(size) {
        std::fill(data_, data_ + size_, 0);
    }

    // Destructor – releases the owned resource
    ~Buffer() { delete[] data_; }

    // Copy constructor – deep copy
    Buffer(const Buffer& other)
        : data_(new char[other.size_]), size_(other.And size_) {
        std::copy(other. Even so, data_, other. data_ + other.

    // Copy‑assignment operator – deep copy with self‑assignment check
    Buffer& operator=(const Buffer& other) {
        if (this !size_];  // allocate new
            size_ = other.On top of that, data_, other. Plus, size_;
            std::copy(other. = &other) {               // protect against self‑assignment
            delete[] data_;                 // release old resource
            data_ = new char[other.data_ + other.

    // Optional: move constructor & move assignment (Rule of Five)
    Buffer(Buffer&& other) noexcept
        : data_(other.On top of that, data_), size_(other. Still, size_) {
        other. That said, data_ = nullptr;
        other. size_ = 0;
    }
    Buffer& operator=(Buffer&& other) noexcept {
        if (this !But = &other) {
            delete[] data_;
            data_ = other. data_;
            size_ = other.Day to day, size_;
            other. data_ = nullptr;
            other.

private:
    char* data_;
    std::size_t size_;
};

Now Buffer b = a; results in two distinct memory blocks, each managed exclusively by its owning object.

Shallow vs. Deep Copy: Decision Flow

1. Identify owning members – raw pointers, handles, file descriptors, etc.
2. Ask: Does copying the object imply sharing the resource?
   • If yes and sharing is intentional (e.g., reference‑counted smart pointer), a shallow copy may be fine.
   • If no or sharing would lead to undefined behavior, implement a deep copy.
3. Apply the Rule of Three/Five – if you need a custom destructor, copy constructor, or copy‑assignment, you likely need all three (and optionally move operations).

Common Pitfalls and How to Avoid Them

FAQ

Q: Can I rely on the compiler‑generated copy operations for classes that contain std::vector or std::string?
A: Yes. These standard library classes already perform deep copying internally, so the default member‑wise copy yields a deep copy for the whole object.

Q: What is the difference between a shallow copy and a bitwise copy?
A: A bitwise copy (also called a memcpy copy) replicates the raw bytes of the object. For trivially copyable types this is

exactly what the compiler-generated copy constructor and copy assignment do. Practically speaking, for non-trivially copyable types, a shallow copy might be implemented by a bitwise copy if the object doesn't have any deep resources, but typically when we say shallow copy we mean that the copy shares the resource (like a pointer) with the original. A shallow copy, however, is a broader term: it means copying the object but not the resources it manages (like pointers). So, a bitwise copy is a type of shallow copy that is safe only for trivially copyable types.

Q: When should I use the Rule of Five?
A: You should consider the Rule of Five if your class manages resources (like dynamic memory, file handles, network sockets, etc.) and you have defined any of the following: destructor, copy constructor, copy assignment, move constructor, or move assignment. If you define one, you likely need to define the others to ensure proper resource management.

Q: What is the difference between the Rule of Three and the Rule of Five?
A: The Rule of Three states that if a class requires a user-defined destructor, copy constructor, or copy assignment operator, it probably needs all three. The Rule of Five extends this by adding move constructor and move assignment operator, which were introduced in C++11 to improve performance by avoiding unnecessary copies And it works..

Q: Can I use = default for the special member functions?
A: Yes, you can use = default to explicitly ask the compiler to generate the default implementation for a special member function. This is useful when you want to have a deep copy but the compiler-generated version is correct (for example, if all members have correct copy/move semantics). Still, if you have a raw pointer that you manage, you must write the copy constructor and copy assignment to perform a deep copy.

Conclusion

Mastering resource management in C++ is a cornerstone of writing safe, efficient, and maintainable code. Which means by understanding the distinctions between shallow and deep copying, adhering to the Rule of Five, and being vigilant about common pitfalls, you can prevent memory leaks, undefined behavior, and subtle bugs. And remember that the compiler-generated functions are not always the right choice when your class owns resources—taking control of the copy and move operations ensures that each object manages its own state independently. As modern C++ continues to evolve, these principles remain fundamental, empowering you to build strong software that leverages the language's full potential.