The "undefined reference to vtable" error is a common issue encountered by C++ programmers, especially when dealing with polymorphism and inheritance. In this tutorial, we will delve into the concept of vtables, explore the causes of this error, and provide step-by-step guidance on how to fix it.
What is a VTable?
A vtable, short for "virtual function table," is a mechanism used by C++ compilers to implement polymorphism. It’s essentially a lookup table that stores pointers to virtual functions in a class. When a virtual function is called, the program uses the vtable to determine which implementation to invoke.
Where and When is a VTable Generated?
A vtable is automatically generated by the compiler when it encounters a polymorphic class definition. The selection process for generating a vtable typically involves finding a single translation unit (i.e., a source file) that defines a non-inline, non-pure virtual function. If this process fails to pick any translation units, or if the selected object file is not linked, the vtable becomes an undefined reference.
Causes of Undefined Reference to VTable Errors
There are three primary causes of "undefined reference to vtable" errors:
- Missing definition for a virtual function: When a class declares a virtual function but fails to provide its definition.
- Object file not linked: When the object file containing the definition of a virtual function is not included in the link command.
- All virtual functions have inline definitions: In some cases, if all virtual functions are defined inside the class definition (i.e., as inline functions), the compiler might not generate a vtable.
How to Fix Undefined Reference to VTable Errors
To fix these errors, follow these steps:
- Identify the first non-inline virtual function: Look for the first virtual function in your class definition that is not defined inside the class (i.e., not an inline function) and is not pure virtual.
- Provide a definition for the function: If the definition is missing, add it to your code. Make sure to use a qualified name (e.g.,
ClassName::functionName) when defining the function outside the class definition. - Check the link command: Verify that the object file containing the function’s definition is included in the link command.
Example
Suppose you have a class with a virtual destructor declared as follows:
class A {
public:
virtual ~A() = default;
};
To fix an undefined reference to vtable error, you can modify the destructor declaration and provide its definition outside the class definition:
class A {
public:
virtual ~A();
};
A::~A() = default;
By following these steps and understanding how vtables are generated and used in C++, you should be able to resolve "undefined reference to vtable" errors in your code.
Best Practices
To avoid these errors, keep the following best practices in mind:
- Always provide definitions for virtual functions.
- Ensure that object files containing virtual function definitions are linked correctly.
- Avoid using inline definitions for all virtual functions in a class.
By adhering to these guidelines and understanding the concept of vtables, you can write more robust and maintainable C++ code.