Private copy constructor with unamed argument?

Understanding Private Copy Constructors with Unnamed Arguments in C++

Explore the nuances of private copy constructors in C++, focusing on their role in preventing object copying and the implications of using unnamed arguments.

In C++, the copy constructor is a special constructor that creates a new object as a copy of an existing object. By default, if you don't define one, the compiler provides a public default copy constructor. However, there are scenarios where you might want to prevent objects of a certain class from being copied. This is where a private copy constructor comes into play. This article delves into the purpose and implementation of private copy constructors, particularly when declared with an unnamed argument, and how they effectively disable object copying.

The Role of a Private Copy Constructor

Making a copy constructor private is a common technique to prevent objects of a class from being copied. If a class has a private copy constructor, any attempt to copy an object of that class (e.g., by passing it by value to a function, returning it by value, or using the copy initialization syntax) will result in a compile-time error. This is a powerful mechanism for enforcing design constraints, especially for classes that manage unique resources or have complex state that should not be duplicated.

class NonCopyable {
private:
    NonCopyable(const NonCopyable&); // Private copy constructor
    // NonCopyable& operator=(const NonCopyable&); // Private copy assignment operator

public:
    NonCopyable() { /* ... */ }
    // Other member functions
};

void func(NonCopyable obj) { /* ... */ } // Compile-time error: cannot access private copy constructor

int main() {
    NonCopyable a;
    // NonCopyable b = a; // Compile-time error
    // func(a);         // Compile-time error
    return 0;
}

Example of a class with a private copy constructor to prevent copying.

Unnamed Arguments: A Historical Perspective

The question often arises about declaring a private copy constructor with an unnamed argument, like NonCopyable(const NonCopyable&);. In C++, it is perfectly valid to declare function parameters without names if those parameters are not used within the function's definition. For a private copy constructor that is intentionally left undefined (or defined to throw an exception), the parameter name is indeed unnecessary because the constructor's body will never be executed by legitimate code. This practice is more about signaling intent and reducing verbosity, rather than having a functional difference.

classDiagram
    class MyClass {
        -MyClass(const MyClass&)
        +MyClass()
        +doSomething()
    }

    MyClass --> MyClass : prevents copy
    MyClass : +MyClass()
    MyClass : +doSomething()

Class diagram illustrating a private copy constructor preventing object copying.

Modern C++: = delete for Non-Copyable Classes

With C++11, the = delete specifier provides a cleaner and more explicit way to declare functions as deleted, effectively preventing their use. This is the recommended approach for making classes non-copyable, as it clearly communicates intent and provides better compiler diagnostics than a private, undeclared copy constructor. The compiler will generate an error if you try to use a deleted function, just as it would for a private one, but the error message is often more informative.

class ModernNonCopyable {
public:
    ModernNonCopyable() { /* ... */ }
    ModernNonCopyable(const ModernNonCopyable&) = delete; // Explicitly deleted copy constructor
    ModernNonCopyable& operator=(const ModernNonCopyable&) = delete; // Explicitly deleted copy assignment operator
    // Other member functions
};

void func(ModernNonCopyable obj) { /* ... */ } // Compile-time error

int main() {
    ModernNonCopyable a;
    // ModernNonCopyable b = a; // Compile-time error
    // func(a);                 // Compile-time error
    return 0;
}

Using = delete to explicitly prevent copying in modern C++.

In summary, a private copy constructor with an unnamed argument serves the purpose of preventing object copying by making the constructor inaccessible. While historically a valid and common technique, modern C++ provides the = delete specifier as a more explicit and idiomatic way to achieve the same goal, offering improved readability and compiler feedback. Both methods effectively enforce the non-copyable nature of a class, but = delete is generally preferred in contemporary C++ development.