Virtual functions are among the foundational concepts of object-oriented programming in C++. They provide developers with robust tools to ensure polymorphism, allowing classes to specify behavior that derived classes may override with their own implementations. However, alongside virtual functions, the modern C++ standard (since C++11) introduced “deleted virtual functions,” an intriguing yet somewhat perplexing language feature. It’s no surprise that the concept of deleted virtual functions often confuses even seasoned developers. In today’s post, we’ll demystify the concept, explore why you might use deleted virtual functions, and layout practical scenarios where they enhance your object-oriented designs.

Recap – Virtual Functions in a Nutshell

Before jumping deep into deleted virtual functions, let’s quickly refresh our understanding of what virtual functions exactly are.

Definition and Purpose of Virtual Functions:

In C++, virtual functions offer developers the power of runtime polymorphism. A base class defines a virtual function, and any derived class may override it to implement its own functionality.

Consider a simple example:

#include <iostream>

class Animal {
public:
    virtual void speak() {
        std::cout << "Animal sound!" << std::endl;
    }
};

class Dog : public Animal {
public:
    void speak() override {
        std::cout << "Woof!" << std::endl;
    }
};

int main() {
    Animal* animal = new Dog();
    animal->speak(); // Outputs: Woof!
    delete animal;
}

In this example, speak() is a virtual function in the base class Animal, and we override its behavior in the derived class Dog. This structure is fundamental to leverage runtime polymorphism effectively.

Virtual functions are typically used when:

• You have a hierarchy of related objects.
• Derived classes are expected to specify their own implementations.
• You need polymorphic behavior in your software.

Deleted Functions – A Quick Overview

Before understanding “deleted virtual functions,” let’s grasp the simpler concept of “deleted functions,” introduced with C++11.

In modern C++, the delete keyword can explicitly disable certain functions, making any attempt to call these deleted functions lead to compile-time errors. Doing this makes your APIs safer and intentions clearer.

Here’s an example:

class Example {
public:
    Example(int x) {} // Allowed constructor

    Example(double) = delete; // Deleted constructor
};

int main() {
    Example ex(5);      // OK
    Example ey(5.5);    // Compile-time error!
}

The deleted function tells programmers explicitly, “You can’t use this function,” and the compiler enforces this intent strictly.

What Are Deleted Virtual Functions Exactly?

Deleted virtual functions, quite simply put, merge two distinct C++ language features: virtual functions and deleted functions. This is where you specify a virtual function in your base class, then explicitly delete it using the = delete syntax.

Here’s the syntax:

class Base {
public:
    virtual void doSomething() = delete;
};

When developers first encounter this construct, confusion frequently arises—if a function is virtual meant for overriding, what’s the purpose of deleting it?

The key to understanding deleted virtual functions lies in the intent behind them—not usage itself, but the explicit prevention of incorrect usage.

Why Would You Delete a Virtual Function?

You may initially struggle to find scenarios justifying deleted virtual functions. But the language standardization committee and advanced C++ practitioners had specific use cases for this feature:

1. Preventing Derived Classes from Implementing Certain Methods

You deliberately signal derived classes and engineers working on derived classes that certain behaviors are not permitted within your class hierarchy clearly and explicitly.

2. Explicitly Declaring Interface Constraints

Deleted virtual functions assist in clearly communicating the intent within your interfaces. Certain methods might not make sense in submitted derived implementations, and deleting those virtual functions emphasizes that reality.

3. Clarifying and Enforcing Your Design Explicitly

Deleted virtual functions eliminate ambiguity around whether future derived classes can provide implementations or not, thus clarifying API boundaries.

Deep Dive: Practical Examples and Scenarios

Let’s look at real-world examples highlighting the practical benefit of deleted virtual functions in C++:

Example Scenario 1: Preventing Accidental Override of Unsuitable Methods

Conceptually, your interface might mistakenly offer a functionality unsuitable for certain derived types. Here’s how deleting the virtual function saves you from mistakes:

class NetworkInterface {
public:
    virtual void connectViaBluetooth() = delete; // Prevent Bluetooth for specific connections.
};

class EthernetAdapter : public NetworkInterface {
    // Compiler forbids overriding 'connectViaBluetooth()'
};

By explicitly deleting connectViaBluetooth(), you’re clarifying intent: Ethernet-based adapters can’t use Bluetooth connections. The compiler thus clearly documents and enforces this constraint.

Example Scenario 2: Explicitly Communicating Design Constraints

Let’s frame another practical scenario highlighting your API design explicitly:

class Shape {
public:
    virtual void resize(double factor) = delete; // Explicitly indicates not all shapes are scalable
};

class FixedDimensionShape : public Shape {
    // resizing impossible, the compiler enforces it
};

Here, you explicitly prevent resizing certain shapes. Any class that derives from Shape will clearly understand that attempting to add resize functionality is disallowed.

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Common Pitfalls and Best Practices

While deleted virtual functions have their positives, developers should beware of their pitfalls. Here are common caveats:

Pitfalls:

• Misusing deleted virtual functions can confuse newcomers to your codebase.
• Abusing deleted virtual functions excessively makes it tougher to change hierarchy later.

Best Practices:

• Use deleted virtual functions sparingly, to clearly communicate certain constraints.
• Document clearly why you’re deleting certain methods.
• Ensure alternative well-defined behavior exists when removing fundamental functionality.

Alternatives to Deleted Virtual Functions (If Any)

In certain cases, explicit:

• Private or protected virtual methods.
• Throwing exceptions or asserting from default implementations.

could also convey intent without explicitly “deleting” functions. Consider carefully which approach suits your use case.

• Private/Protected Virtual Methods: Good alternative if you still might internally handle them, restricting external API heavily.
• Throwing exceptions: Useful if you detect invalid usage at runtime safely, rather than compile-time prevention.

Pros & Cons Summary

Pros:

• Clearly demonstrates programmer intent regarding class design.
• Ensures less probable bugs from incorrect implementations inherited accidentally.
• Makes codebase self-documenting more explicitly.

Cons:

• May confuse beginners unfamiliar with deleted virtual functions.
• Too rigid approach, potentially complicating future redesigning efforts.
• Requires well-thought-out reasons for acceptance.

FAQs Section

What exactly is a deleted virtual function?

A deleted virtual function is a virtual member explicitly marked as deleted (= delete). It prevents derived classes from accidentally providing implementations or using that function.

Can I override a deleted virtual function in derived classes?

No. Once a virtual function is explicitly deleted, derived classes can’t override or provide their own implementations. Attempting to do so triggers compile-time errors.

When should I consider using deleted virtual functions in my projects?

Use them whenever it makes sense to explicitly forbid certain functionalities from becoming part of derived classes, clearly expressing limitations of your hierarchy or interface.

Do deleted virtual functions affect polymorphism and inheritance?

Yes. They restrict inherited functionality explicitly. They help shape clearer functional boundaries of class hierarchies—but don’t inherently break polymorphism if correctly managed.

Are deleted virtual functions related to pure virtual functions?

Both pure virtual and deleted virtual functions explicitly indicate constraints—but pure virtual functions mandate overrides, whereas deleted virtual functions forbid them.

Can deleted virtual functions cause runtime errors?

No. Deleted functions typically cause compile-time errors when invoked or if anyone attempts overriding them. They do not produce runtime errors directly.

Conclusion

Deleted virtual functions represent a powerful, subtle facility in modern C++ that enables developers to create clearer, safer, and more maintainable programming APIs. Though initially confusing, once mastered, they represent exceptional clarity in communication and design enforcement. Always practice discretion and clear documentation, helping fellow team members navigate around and benefit from your clear and purposeful class designs.

Call to Action

Have you ever struggled to understand or effectively use deleted virtual functions? Share your own challenges, practical use cases, or experiences with the community by leaving a comment below! For further deep-dives into modern C++, consider exploring official cppreference.com documentation.