Possible to get the address of tagged union prior to initialization? | Zig Programming Language | Page 1

trail portal Oct 20, 2023, 9:39 AM

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I am heap allocating a tagged union, but one of the fields of this union requires an address to "self", so to speak.

const myUnion = try gpa.create(MyUnion);
myUnion.* = .{
    .known_tag = .{
        .some_field = .{ .ptr = &myUnion.known_tag, .vtable = ... },
    }
};

This currently panics with "cannot access union field 'known_tag' when '...' is active"

I get what Zig is trying to do here, but also, it would be annoying to assign some_field = undefined and then update it on the next line.

cold umbra Oct 20, 2023, 9:50 AM

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try @unionInit

trail portal Oct 20, 2023, 9:56 AM

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Why would that help in this case?

cold umbra Oct 20, 2023, 10:10 AM

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ah

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thats strange

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are you sure you assigned everything?

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inside known_tag?

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if you dont it wont switch

uneven siren Oct 20, 2023, 10:55 AM

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const myUnion = try gpa.create(MyUnion);
myUnion.* = .{.known_tag = undefined};
myUnion.* = .{
    .known_tag = .{
        .some_field = .{ .ptr = &myUnion.known_tag, .vtable = ... },
    }
};

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@trail portal

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that way you set it to that tag before getting the ptr

trail portal Oct 20, 2023, 10:55 AM

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Yup, I realise this works. It's just a bit annoying 🤷‍♂️

uneven siren Oct 20, 2023, 10:56 AM

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yes

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it sucks a little

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why do you need the known_tag ptr in some_field to begin with

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are you sure you can't use @fieldParentPtr()

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that would be simpler', not have this issue and make your struct smaller

robust rapids Oct 20, 2023, 11:09 AM

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Wait, wouldnt:

const myUnion = try gpa.create(MyUnion);
myUnion.* = .{
    .known_tag = .{
        .some_field = .{ .ptr = myUnion, .vtable = ... },
    }
};

work fine?

#Possible to get the address of tagged union prior to initialization?