How to Remove undefined symbol Error in Objective-C

Описание: Learn how to effectively resolve the `undefined symbol` error in Objective-C by following our simple step-by-step guide.
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This video is based on the question https://stackoverflow.com/q/77484919/ asked by the user 'tombuarts' ( https://stackoverflow.com/u/733711/ ) and on the answer https://stackoverflow.com/a/77487543/ provided by the user 'Mykyta iOS' ( https://stackoverflow.com/u/22459244/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Remove undefined symbol Error in Objective-C

If you've ever encountered the frustrating undefined symbol error while working with Objective-C, you are not alone. This error typically shows up during the linking phase of building your project, causing the build to fail and resulting in the following error message:

[[See Video to Reveal this Text or Code Snippet]]

Understanding the Problem

In this case, the issue arises when you declare a global variable, ue, in your global.h file as extern int ue;. You might also include this header file across various view controllers in your project. However, the moment you try to assign a value to this variable, the build system throws the undefined symbol error.

What Does extern Mean?

The extern modifier is crucial to understanding this problem. It indicates that the variable ue is declared in a different file, meaning you should not instantiate it (assign it a value) in the same location where you declare it. Hence, when you try to give it a value in the same scope, you're causing a conflict that leads to the error.

Solutions to Resolve the Error

Fortunately, there are a couple of straightforward solutions to get rid of this pesky error. Let’s go through each option step-by-step.

Option 1: Use static Instead of extern

One easy way to resolve this is to declare your variable as static in your global.h file. This confines the variable's scope to the file it is declared in, allowing you to assign a value without causing linker issues.

Here’s how you can do it:

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Option 2: Create a Dedicated Implementation File

If you want to keep the extern modifier for your variable, you will need to set up a dedicated Objective-C class containing both global.h and global.m files. Here's how:

Create Two Files:

global.h for declarations.

global.m for definitions.

Modify global.h:
In this file, you should only declare the variable like so:

[[See Video to Reveal this Text or Code Snippet]]

Modify global.m:
In the implementation file, actually assign a value to the variable:

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Conclusion

By following either of these methods, you should be able to resolve the undefined symbol error in your Objective-C project effectively. Whether you choose to switch to static for simplicity or create dedicated implementation files for better organization, understand that managing variable scope is key to successful coding in Objective-C.

Happy coding, and may you run into fewer undefined symbol errors in your future projects!