Why Am I Getting 'Undefined Reference to Virtual Tables and Constructors' Linker Errors in GCC C ?

GCC C Linker Errors: Undefined Reference to Virtual Tables and Constructors

In C , virtual functions allow derived classes to override methods defined in their base classes. When linking a program that uses virtual functions, the linker requires the definitions of these methods to be present in the object files. Failure to provide these definitions results in undefined reference errors, such as:

• Undefined reference to 'vtable for XXX'
• Undefined reference to 'ClassName::ClassName()'

Possible Causes

These errors can occur for various reasons, including:

1. Overridden Virtual Functions without Definitions: As mentioned earlier, every virtual function must have a definition in the final class that uses it. If you declare a virtual function in a child class but do not define it, the linker will report an undefined reference error.
2. Missing Header Files: The header file(s) containing the class definitions may not be properly included in the source files that use them.
3. Incorrect Library Linkage: Ensure that you are linking to the correct library containing the implementations of the classes you are using.

Confirming Static Libraries and Class Availability

1. Library Bitness: To check if static libraries are 64-bit, use the file command:

   file -L <library_path>

   It should show 64-bit x86-64 shared object if the library is 64-bit.

2. Class Availability: Use the objdump -t command to check if a library contains a specific class:

   objdump -t <library_path> | grep <class_name>

   If the class is present, it will be listed in the output.

Example Issue

As shown in the provided error log, the undefined reference to SomeClass::close() indicates that the class has been used but not defined. Similarly, the undefined references to SomeClass::SomeClass() and vtable for SomeOtherClass suggest that the constructor and virtual table for the SomeOtherClass class are not defined in the object files.

Solution

To resolve this issue, ensure that all virtual functions are defined in the appropriate child classes, the required header files are included, and the correct libraries are linked to your program.

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