Скачать или смотреть How to Pass a Local Instance to a Global Callback Function in C++

Learn how to effectively give a global callback function in C++ access to a local instance, utilizing GLFW's window user pointer.
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This video is based on the question https://stackoverflow.com/q/74089773/ asked by the user 'Roman Lebedev' ( https://stackoverflow.com/u/19952410/ ) and on the answer https://stackoverflow.com/a/74089822/ provided by the user '273K' ( https://stackoverflow.com/u/6752050/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Pass a Local Instance to a Global Callback Function in C++

When developing applications in C++, especially those utilizing GLFW for handling windows and input events, we often encounter scenarios where we need to manage interactions between global callback functions and local variables. A specific question arises: How can I give a global callback function a local instance? This blog will provide a clear solution to that problem, equipping you with the knowledge to effectively manage object instances in callback functions.

Understanding the Problem

In your C++ program using GLFW, you might have a global callback function defined for handling keyboard input. Here's a basic example:

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This callback function is in the global namespace, which means it cannot directly access local instances created in another function, like main(). For instance, in your main() function, you might have declared an object of the Sphere class as follows:

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The challenge is to communicate information about lightSphere to the global key_callback. You want to avoid declaring the sphere globally simply to make it accessible.

The Solution: Using GLFW's User Pointer

Fortunately, GLFW provides a neat way to overcome this limitation through the use of window user pointers. Let's break down the solution step-by-step:

Step 1: Set the User Pointer

Before setting the key callback, we set the user pointer for the GLFW window. This allows us to attach any data (in our case, an instance of Sphere) to the window.

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Here, we pass the address of lightSphere to glfwSetWindowUserPointer. This means we are storing a reference to our local instance in the GLFW window, which can later be retrieved within the callback.

Step 2: Retrieve the User Pointer in the Callback

Once the user pointer is set, we can modify our key_callback function to retrieve the Sphere object using the window user pointer:

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Step 3: Complete Example

Here's how the complete solution looks in the context of your program:

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Conclusion

By utilizing GLFW's window user pointer, you efficiently pass local instances to global callback functions without cluttering your global namespace. This is not only a clean solution but also adheres to good programming practices by keeping your code modular and organized. Next time you face the challenge of accessing local instances within global callback functions, remember this approach! Happy coding!