C++ vtable Error — Complete Guide

C++ vtable Error — Complete Guide

· 10분 읽기 · 수정 Intermediate Problem-Solving
C++ vtable Error — Complete Guide

이 글의 핵심

C++ vtable error C++, vtable, "undefined, Introduction: "undefined reference to vtable for MyClass" explained in detail with practical examples.

Introduction: “undefined reference to vtable for MyClass"

"Declared virtual function but got linker error”

In C++, declaring virtual function without defining it causes vtable linker error.

// ❌ vtable error
class Base {
public:
    virtual void foo();  // Declaration only
    virtual ~Base();     // Declaration only
};

// Link error:
// undefined reference to `vtable for Base'

What This Guide Covers:

  • What is vtable?
  • vtable error causes
  • Solutions
  • Pure virtual functions

1. What is vtable?

vtable (Virtual Table)

vtable is virtual function table that determines which function to call at runtime.

class Base {
public:
    virtual void foo() {
        std::cout << "Base::foo\n";
    }
    
    virtual void bar() {
        std::cout << "Base::bar\n";
    }
};

class Derived : public Base {
public:
    void foo() override {
        std::cout << "Derived::foo\n";
    }
};

// vtable structure (conceptual)
// Base vtable:
//   [0] -> Base::foo
//   [1] -> Base::bar
//
// Derived vtable:
//   [0] -> Derived::foo
//   [1] -> Base::bar

2. vtable Error Causes

Cause 1: Missing Virtual Function Definition

// ❌ No definition
// base.h
class Base {
public:
    virtual void foo();  // Declaration only
    virtual ~Base();     // Declaration only
};

// Link error: undefined reference to `vtable for Base'

Cause 2: Missing Destructor Definition

// ❌ No destructor definition
class Base {
public:
    virtual ~Base();  // Declaration only
    
    virtual void foo() {
        std::cout << "foo\n";
    }
};

// Link error: undefined reference to `vtable for Base'

3. Solutions

Solution 1: Add Definition in .cpp

// ✅ base.h
class Base {
public:
    virtual void foo();
    virtual ~Base();
};

// ✅ base.cpp
void Base::foo() {
    std::cout << "Base::foo\n";
}

Base::~Base() {
    std::cout << "~Base\n";
}

Solution 2: Inline Definition in Header

// ✅ base.h
class Base {
public:
    virtual void foo() {
        std::cout << "Base::foo\n";
    }
    
    virtual ~Base() {
        std::cout << "~Base\n";
    }
};

Solution 3: Use = default

// ✅ base.h
class Base {
public:
    virtual void foo() {
        std::cout << "Base::foo\n";
    }
    
    virtual ~Base() = default;  // Default implementation
};

4. Pure Virtual Functions

Pure Virtual Function with = 0

// ✅ Pure virtual function
class Base {
public:
    virtual void foo() = 0;  // Pure virtual
    virtual ~Base() = default;
};

class Derived : public Base {
public:
    void foo() override {
        std::cout << "Derived::foo\n";
    }
};

// Base b;  // Compile error: abstract class
Derived d;  // OK

Summary

Key Points

  1. vtable: Virtual function table for runtime dispatch
  2. Error cause: Virtual function declared but not defined
  3. Solution: Provide definition in .cpp or inline
  4. Pure virtual: Use = 0 for abstract interface
  5. Destructor: Always define virtual destructor

When to Use

✅ Provide definition when:

  • Declaring virtual functions
  • Using virtual destructor
  • Implementing polymorphic base class

❌ Don’t:

  • Declare virtual function without definition
  • Forget destructor definition
  • Mix up pure virtual (= 0) with regular virtual

Best Practices

  • ✅ Define all declared virtual functions
  • ✅ Use = default for simple destructors
  • ✅ Use = 0 for pure virtual functions
  • ❌ Don’t declare virtual without definition
  • ❌ Don’t forget virtual destructor definition

Master vtable for error-free C++ polymorphism! 🚀

Frequently Asked Questions (FAQ)

Q. When would I use this in practice?

A. Everything about C++ vtable Error : from basic concepts to practical applications.

Q. What should I read before this?

A. Follow the previous article or related articles links at the bottom of each post to learn in sequence. See the C++ series index for the full picture.

Q. Where can I study this more deeply?

A. Check cppreference and the relevant library’s official documentation. The reference links at the end of the article are also worth using.

Other articles related to this topic.

Keywords Covered in This Article (Related Search Terms)

This article covers C++, vtable, linker-error, virtual, virtual-function, undefined-reference.