undefined reference to

"function name"

This error message usually occurs during the linking phase of compiling a program. It indicates that the compiler was unable to find a definition for a function that was referenced in the code.

Possible causes for this error include:

- The function was not declared or defined properly.
- The function was not included in the list of source files to be compiled.
- The library that contains the function was not linked properly.
- The function is located in a different namespace or class than where it was called.

To fix this error, you should verify that the function has been properly declared or defined, ensure that the necessary source files and libraries are included, and check for any namespace or class issues.