How to determine if anytype is std.ArrayList?

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1

I’m writing a msgpack serialization library and have something I’m struggling with. Because of the flexible type generation at comptime, I’m not sure how to detect if some field of a struct is, for example, ArrayList, so that I can serialize it as an array, not as a generic struct. Is something like that even possible?

2

A trick I’ve done in the past is to extract the T you pass to ArrayList and reinstantiate the type. Then compare if the type you get is the same as ArrayList

fn isArraylist(comptime T: type) bool {
    if (@typeInfo(T) != .@"struct" or !@hasDecl(T, "Slice"))
        return false;

    const Slice = T.Slice;
    const ptr_info = switch (@typeInfo(Slice)) {
        .pointer => |info| info,
        else => return false,
    };

    return T == std.ArrayListAlignedUnmanaged(ptr_info.child, null) or
        T == std.ArrayListAlignedUnmanaged(ptr_info.child, ptr_info.alignment) or
        T == std.ArrayListAligned(ptr_info.child, null) or
        T == std.ArrayListAligned(ptr_info.child, ptr_info.alignment);
}

test {
    try std.testing.expect(!isArraylist(u8));
    try std.testing.expect(!isArraylist([]const u8));
    try std.testing.expect(isArraylist(std.ArrayList(u8)));
}

const std = @import("std");
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