What is undefined reference to vtable for?

Undefined reference to vtable error occurs when the virtual table (vtable) is either missing or not defined. Every time a class is defined with a virtual function, the compiler generates a vtable for that class. The vtable is a lookup table that points to the virtual functions of the class. When the program is compiled, the compiler generates a reference to the vtable for each class that contains virtual functions.

When the linker fails to find the definition of the vtable, it throws an undefined reference to vtable error. This error typically occurs when the virtual function is declared in a base class, but its definition is missing in the derived class.

Some common causes of the undefined reference to vtable error are:

1. Class definition is incomplete
2. Function definition is missing or not implemented
3. Multiple inheritance ambiguity
4. Inconsistent compiler usage

To fix this error, make sure that the class definition contains all the necessary virtual functions and their definitions. Also, make sure that the derived class overrides all virtual functions from the base class. Check for any inconsistencies in the code or compiler usage that may be causing the error.