How to implement Callbacks in Fusion language?

[ghost]

I have read the reference but I can't find anything about it. Please help. Thank you.

[pfusik]

public abstract class MyClass
{
    protected abstract void MyCallback();
}

or:

public class MyClass
{
    protected virtual void MyCallback()
    {
    }
}

and subclass in your target language.

[BenBE]

C/C++ function pointers are less powerful, because they contain no object reference.

I need to object:

• For C the common idiom is providing a void* ptr for user data, which is in turn used to reference some "object" …

• For C++: Use std::function and use lambda functions with context binding:

std::function<void()> cb_function = [this]() {
    this->matter();
};

Or use the variant from C. Might be surprising, but that also works on C++ …

The Java-style callbacks are just plain BS, because they force arguments when none would be needed otherwise.

[pfusik]

I was referring to function pointers themselves. What you describe is a pair of a function pointer and a data pointer. That is what initial versions of Fusion used.

they force arguments when none would be needed otherwise

I don't understand what are you referring to here?

[BenBE]

they force arguments when none would be needed otherwise

I don't understand what are you referring to here?

The Java-style callbacks always carry an implicit reference to the object they are created on. Thus you always have to provide for this extra, implicit argument in the underlying target language even in cases where you would not normally need it (e.g. I just need some function called once an event takes place) or where explicit placement needs to be ensured (e.g. most Enum* APIs on Windows).

And while you can't pass std::function to those APIs directly either, there's normally a few lines of code you can auto-generate to create a matching wrapper (which wouldn't even hurt in the case of fut as you auto-generate code anyway). That aside, IIRC there's even a macro/API available on Windows, which creates this wrapper at runtime …

[ghost]

I don't know that much in the detail. But I have the same opinion. Java-style callbacks are BS. Poor Java doesn't support function pointers so it's impossible to pass a method as parameter to another method. Java developers, which so keen on design patterns, created this own design pattern to overcome Java's drawback. It's an incomplete patchwork. I don't know why you praised it.

C# nowadays is a bloated unfathomable mess. But please recall that it used to be a more reasonable version of Java. It supports delegates from the beginning. You decided to follow C# syntax, but adopted Java's no unsigned types and Java's poor workaround for callbacks. I have no idea.

[pfusik]

@BenBE Calling a function with an object reference vs no object reference is a difference of one CPU instruction. Unrelevant for performance. Easily optimizable with LTO. What's more, my experience is that most callbacks do need their context data.

@iahung2 I'm not going to join this Java-vs-C# flamewar, especially that you admitted to not know Java.

Both: Could you please show your code snippet so we can discuss if any Fusion syntax improvements would make a considerable difference?

[pfusik]

Another example:

public abstract class ThreeIntsFull
{
    protected abstract void OnFull!();
}

public class ThreeIntsContainer
{
    int[3] Content;
    int Count = 0;

    public void Append(int value, ThreeIntsFull! fullCallback)
    {
        if (Count >= 3)
            fullCallback.OnFull();
        else
            Content[Count++] = value;
    }
}

or:

public abstract class ThreeIntsContainer
{
    int[3] Content;
    int Count = 0;
    protected abstract void OnFull!();

    public void Append(int value)
    {
        if (Count >= 3)
            OnFull();
        else
            Content[Count++] = value;
    }
}