Understanding the C++ Copy Constructor (C++23)

In C++23, the copy constructor remains a non-static member function of the form `T(const T&)` (or with cv-qualifiers and/or ref-qualifiers) that initializes a new object as a bit-for-bit or member-wise copy of an existing object of the same type. The language continues to implicitly declare a copy constructor when no user-defined copy constructor, move constructor, copy assignment, move assignment, or destructor exists, though such implicit generation is deprecated since C++11 if certain user-declared special members are present. Guaranteed copy elision rules introduced in C++17 and unchanged in C++23 eliminate many observable calls to copy constructors during return-by-value, initialization from temporaries, and throw expressions, allowing objects to be constructed directly in their final storage location. C++23's deducing this feature (explicit object parameters) enables more flexible and uniform member function definitions, though it is rarely applied directly to copy constructors themselves since they conventionally take a const lvalue reference parameter rather than using an explicit `this`. Overall, modern C++23 code benefits from stronger move semantics, mandatory elision, and concepts for better constraints, making copy constructors less frequently invoked in performance-critical paths while preserving their classical role when deep copying or explicit control is required.

What a copy constructor does

A copy constructor builds a new object by copying from an existing object of the same type. It defines what it means to “clone” an object. In its conventional form:

Type::Type(const Type& other);

If you do not define a copy constructor, the compiler may generate one that performs a memberwise copy: each member is copied in order. This is correct for many “value-like” classes, but it is often wrong when the class owns resources (like dynamically allocated memory via a raw pointer).

Why copy constructors matter: pass-by-value creates a copy

Suppose we want to count occurrences of a character in a stack without changing the original stack. One classic approach is to pass the stack by value. That creates a local copy inside the function, so we can pop from the copy freely.

int cnt_char(char c, ch_stack s) {
    int count = 0;
    while (!s.empty()) {
        count += (c == s.pop());  // pop from the local copy
    }
    return count;
}

What this line is doing

count += (c == s.pop());

1. s.pop() removes and returns the top element of the local copy of the stack.
2. The returned element is compared to c. The expression (c == s.pop()) is true or false.
3. In arithmetic contexts, true converts to 1 and false converts to 0, so count increments by 1 only when the characters match.

The key idea: because s is passed by value, the function must create a local copy of the argument. Creating that local copy requires a copy constructor (unless the type is trivially copyable or the copy is optimized away).

A clearer “expanded” form (correct types)

If ch_stack stores characters, then pop() should return a char, not a char*. The one-liner can be expanded like this:

char just_popped = s.pop();
if (c == just_popped) {
    ++count;
}

When copy constructors are used

Copy construction happens in these common situations:

1. Initialization from another object: Type b(a); or Type b = a;
2. Pass-by-value parameters: f(Type x) creates a copy of the argument (conceptually).
3. Returning by value: Type make() may copy or move a local object into the return value (often optimized).

Pass-by-reference: no copy is created

If you pass an object by reference, you operate on the original object—no copy constructor is invoked:

void print_line2(String& a)
{
    cout << a;
    a = "\n";
    cout << a;
}

String name("Fred");
print_line2(name);          // no copy; modifies name directly
cout << name;               // value is now changed

Modern C++23 guidance: Rule of Zero, Three, and Five

The most important modern rule is the Rule of Zero: if your class uses RAII members (like std::string, std::vector, std::unique_ptr), you usually do not need to write a destructor, copy constructor, or copy assignment operator at all. The compiler-generated versions will be correct.

If your class manually owns resources (for example, it stores a raw char* that it deletes), then you typically need the Rule of Three (copy constructor, copy assignment, destructor), and in modern C++ the Rule of Five adds move constructor and move assignment.

Example: legacy String-style deep copy (educational)

The following pattern illustrates why a custom copy constructor is needed when a class owns a buffer. In modern C++ you should use std::string instead, but this is a good learning example:

Copy Constructor
ClassName::ClassName(const ClassName& parameter) {
    statements
}

String::String(const String& right)
{
    len = right.length();
    buffer = new char[len + 1];

    for (int i = 0; i < len; ++i)
        buffer[i] = right[i];

    buffer[len] = '\0';
}

When the compiler-generated copy constructor is enough

Practical C++23 takeaways

• Pass-by-value creates a copy (conceptually). It’s useful when you want to work on a local clone.
• Prefer const& when you want to avoid copying large objects.
• Prefer RAII members (std::string, std::vector) so copy behavior is correct automatically.
• If you own resources manually, commit to the Rule of Three/Five and define copy/move semantics deliberately.