How to Call Task Class Methods from an extern 'C' Function in WASM

Learn how to resolve name mangling issues when calling class instance methods from functions marked with `extern "C"` in WebAssembly.
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Calling Mangled Class Instance Methods from a Function Marked extern "C"

When working with C+ + and compiling to WebAssembly (WASM), programmers might encounter challenges related to name mangling. This issue is especially pertinent when trying to call class instance methods from functions designated with extern "C". In this guide, we will dive into the problem of calling a method from a class instance within an extern "C" function and explore an effective solution to bypass this complication.

The Problem Explained

What is Name Mangling?

Name mangling is a technique employed by C+ + compilers to encode additional information into function names. This allows the linker to distinguish between overloaded functions or functions belonging to a particular class. For instance, given the following class:

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

The test method of the Task class gets a mangled name during compilation. As such, if you try to call this method from a function marked with extern "C"—which is intended to avoid name mangling—you may receive an error like this:

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

This error occurs because the C-compatible function lacks the mangled name required to locate the test method.

The Solution

Understanding the extern "C" Directive

The extern "C" directive tells the compiler not to mangle names. This makes the declared functions callable from other languages like JavaScript, which is essential when compiling for WebAssembly. However, this directive should only be used for plain functions, not for classes or member functions.

Implementing a Relay Function

To successfully call a non-static member function like Task::test from an extern "C" function, you can use a plain C-style function as a relay. This approach allows you to bypass the name mangling issue while still making use of the class methods.

Here's an example of how to implement this solution:

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

Steps to Fix the Error

Create a Relay Function: Declare a relay function in your extern "C" block that takes a pointer to the Task object and a pointer to an integer for the c parameter.

Call the Member Function: Inside this relay function, call the test method of the Task class using the object pointer and dereference the integer pointer to obtain the original value.

Usage in WASM: When you compile your code to WebAssembly, ensure you call the relay function task_test from your JavaScript code instead.

Example Implementation

Consider the following complete implementation combining the class, method, and relay function:

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

Conclusion

Navigating name mangling when calling class methods from extern "C" functions is a common challenge in C+ + programming, especially when targeting WASM. By implementing a relay function that properly handles class instances, you can effectively avoid these errors and enable smooth inter-language communication. Remember, the key takeaway is to utilize pointers in C-style functions when dealing with classes to ensure compatibility and successful linking.

If you have further questions or need assistance with your C+ + projects, feel free to reach out!