Why Do I Get \'Undefined Symbols\' and \'Typeinfo\' Errors in C and How Can I Fix Them?

Resolving "Undefined Symbols" and "Typeinfo" Errors in C

Encountering errors such as "Undefined symbols" and "Typeinfo" during compilation can be puzzling. This article aims to shed light on these errors and provide a solution.

Understanding Vtables and Typeinfo

• Vtable (virtual method table): A data structure containing pointers to the virtual methods of a class. It allows polymorphic calls to virtual methods, where a derived class can override methods from its base class.
• Typeinfo: Information about the class type, such as its name and methods. This information is used for dynamic type identification and runtime type information access.

Error Analysis

The given error message indicates that the symbols for the Obstacle class's vtable and typeinfo are missing. This can be caused by several reasons:

1. Missing Pure Virtual Methods: An abstract base class requires all its virtual methods to be declared as "pure virtual," i.e., using = 0 at the end of their declaration. If a pure virtual method is not overridden by a derived class, it indicates a missing implementation.
2. Undefined Non-virtual Methods: If an abstract base class contains any non-pure virtual functions, the compiler assumes they are implemented somewhere. If they are not implemented, the necessary internal structures (vtable and typeinfo) will be missing.

Solution

To resolve the error, you should:

1. Declare Pure Virtual Methods: Ensure that the Obstacle class's virtual methods are declared as pure virtual.
2. Implement Non-virtual Methods: If the Obstacle class contains any non-pure virtual functions, make sure they are implemented in a derived class.

Once these conditions are met, the compiler will be able to generate the necessary vtable and typeinfo structures, and the compilation error should be resolved.

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