How to return an ArrayList after allocate in function?

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You’re almost there. When you call the defer it will unconditionally deinit the ArrayList when the scope ends; int his case when the function ends. So you are returning a uninitialized ArrayList. To return the initialized ArrayList while properly uninitializing it in case of error in the function, use errdefer:

fn isReturn(allocator: Allocator) !std.ArrayList(i32) {
    var list = std.ArrayList(i32).init(allocator);
    errdefer list.deinit();

    try list.append(1);
    try list.append(2);

    std.debug.print("List: {any}\n", .{list.items});

    return list;
}

The caller of the function is now responsable for calling deinit on the returned ArrayList.

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