Resolving the unresolved external symbol Error in C++ with Static Methods

Learn how to fix the `unresolved external symbol` error in C++ when working with static methods and mutex locks in this comprehensive guide.
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This video is based on the question https://stackoverflow.com/q/76221598/ asked by the user 'Justin Mathew' ( https://stackoverflow.com/u/1132408/ ) and on the answer https://stackoverflow.com/a/76221651/ provided by the user 'Karen Baghdasaryan' ( https://stackoverflow.com/u/15262489/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the unresolved external symbol Error in C++

If you're working with C++ and encounter the error message:

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

You're not alone. This error can be confusing, especially if you're new to C++. It usually points to an issue related to static members in your class. In this post, we will explore why this error occurs and how to resolve it effectively.

The Problem

In the code snippet shared by a user on a programming forum, the pricercache class declares a static member variable entry_mutex, which is utilized for thread synchronization. However, the error indicates that while entry_mutex is declared, it has not been defined anywhere in the code, leading to the linker failing to resolve the symbol.

Sample Code

Here’s the relevant section of the user's code:

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

This line declares entry_mutex, but without a definition, the linker raises an error.

The Solution

To fix the unresolved external symbol error, you need to provide a definition for the static member. Below are the steps to resolve this issue:

Step 1: Define the Static Member

Right after your class definition, you should define the static variable like so:

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

This line goes outside your class declaration (preferably at the bottom of your implementation file) and lets the compiler know where to allocate memory for this static member.

Step 2: Placement of the Definition

Make sure you place this definition in the same file where your class is defined, or if you're managing a separate implementation file, it should be included there.

Complete Example

Here’s how your modified code would look with the definition included:

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

Conclusion

By defining the static member entry_mutex, we ensure that the compiler can link to this variable correctly. Remember, whenever you declare a static member in a class, you must also define it outside the class to avoid linking errors like unresolved external symbol.

If you're implementing a container class, consider keeping your code organized and make sure all static members are properly defined. Happy coding!

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