If Only the Virtual Destructor is Declared as Default, Is the Copy Constructor Implicitly Generated?

Introduction

In the world of C++ programming, understanding the intricacies of constructors and destructors is crucial for creating efficient and effective code. One question that often puzzles developers is whether declaring a virtual destructor as default implies the implicit generation of a copy constructor. In this article, we’ll delve into the depths of C++ syntax and semantics to provide a clear answer to this question.

The Role of Virtual Destructors

Before we dive into the main topic, let’s quickly recap the purpose of virtual destructors. A virtual destructor is a special type of destructor that ensures proper cleanup of objects in an inheritance hierarchy. When a base class has a virtual destructor, it allows derived classes to inherit its behavior and correctly release resources when an object is destroyed.

class Base {
public:
    virtual ~Base() { /* ... */ }
};

class Derived : public Base {
public:
    ~Derived() { /* ... */ }
};

Default Virtual Destructors

A default virtual destructor is a virtual destructor that is declared with the `= default` syntax. This syntax tells the compiler to generate a default implementation for the destructor.

class Base {
public:
    virtual ~Base() = default;
};

class Derived : public Base {
public:
    ~Derived() { /* ... */ }
};

The Copy Constructor Conundrum

Now, let’s get back to our main question: If only the virtual destructor is declared as default, is the copy constructor implicitly generated? The short answer is: no, the copy constructor is not implicitly generated.

To understand why, let’s examine the rules of implicit constructor generation in C++. The compiler will implicitly generate a default constructor, copy constructor, move constructor, and copy assignment operator, as well as a move assignment operator, if and only if:

• The class does not have a user-declared constructor.
• The class does not have a user-declared copy or move constructor.
• The class does not have a user-declared copy or move assignment operator.

In our example, since we’ve declared a virtual destructor as default, the compiler will not implicitly generate a copy constructor.

class Base {
public:
    virtual ~Base() = default;
};

class Derived : public Base {
public:
    ~Derived() { /* ... */ }
};

// Trying to use the copy constructor will result in an error
Derived d1;
Derived d2 = d1; // Error: no copy constructor available

Why the Compiler Doesn’t Generate a Copy Constructor

The reason the compiler doesn’t generate a copy constructor in this case is that the presence of a user-declared virtual destructor implies that the class is intended to be a base class. As a base class, it’s likely that the class will have additional resources that need to be managed, and the compiler can’t assume how to correctly copy those resources.

Moreover, the `= default` syntax only tells the compiler to generate a default implementation for the virtual destructor, not to imply any additional constructors or assignment operators.

What to Do Instead

If you need to create a copy of an object, you’ll need to explicitly declare and implement a copy constructor or use the `= default` syntax to request the compiler to generate one. Here’s an example:

class Base {
public:
    Base(const Base& other) = default;
    virtual ~Base() = default;
};

class Derived : public Base {
public:
    Derived(const Derived& other) = default;
    ~Derived() { /* ... */ }
};

// Now the copy constructor is available
Derived d1;
Derived d2 = d1; // Okay, copy constructor is generated

Best Practices

To avoid potential issues with constructor generation, follow these best practices:

1. Always declare a virtual destructor in your base classes.
2. If you need a copy constructor, explicitly declare and implement it.
3. Use the `= default` syntax only when you’re sure the compiler can generate a correct implementation.
4. Be mindful of the rules of implicit constructor generation and don’t rely on the compiler to generate constructors without understanding the implications.

Scenario	Implicit Constructor Generation
No user-declared constructors	Default constructor, copy constructor, move constructor, and assignment operators are generated
User-declared virtual destructor (no `= default`)	No constructors or assignment operators are generated
User-declared virtual destructor with `= default`	Only the virtual destructor is generated, no other constructors or assignment operators

Conclusion

In conclusion, declaring a virtual destructor as default does not imply the implicit generation of a copy constructor. To ensure proper copying of objects, you need to explicitly declare and implement a copy constructor or use the `= default` syntax to request the compiler to generate one. By following best practices and understanding the rules of implicit constructor generation, you can create robust and efficient C++ code.

Remember, a well-crafted destructor is essential for managing resources, but it’s not a substitute for a properly implemented copy constructor.

/* ... */

// Happy coding!

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