What is an undefined reference/unresolved external symbol error and how do I fix it?

An “undefined reference” error (also referred to as an “unresolved external symbol” error) in C++ occurs when the compiler can’t find the implementation or definition of a function or variable that has been declared. This typically happens during the linking phase when your code is being converted into an executable.
To fix this error:

1. Check Function Definitions: Ensure that the function or variable you’re using has a proper definition or implementation. If you’ve declared a function in a header file, make sure there’s a corresponding implementation in a source file.
2. Include Source Files: Make sure that all relevant source files are included in your project or build process. For example, if you have a separate .cpp file with the function definition, it needs to be compiled and linked with the rest of your code.
3. Check Function Signatures: Verify that the function or variable’s declaration (signature) matches its definition. Any discrepancies in the number or types of parameters can lead to undefined reference errors.
4. Library Linking: If you’re using external libraries, ensure that you’ve properly linked them during the compilation and linking process. Specify the necessary library flags or options to the compiler.
5. Namespace Issues: Check for namespace issues, especially if you’re using namespaces. The function or variable might be in a different namespace.
6. Typos and Case Sensitivity: Carefully inspect your code for typos, as C++ is case-sensitive. A simple typo in the function or variable name can cause this error.
7. Build Cleanly: Sometimes, these errors can persist if there are remnants of old object files or previous builds. Try cleaning your project and rebuilding it from scratch.

By addressing these common issues, you should be able to fix the “undefined reference” or “unresolved external symbol” error in C++.