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Swift: Why does UInt.max == -1

As the title states, lldb reports the value of UInt.max to be a UInt of -1, which seems highly illogical. Considering that let uint: UInt = -1 doesn't even compile, how is this even possible? I don't see any way to have a negative value of UInt at runtime because the initializer will crash if given a negative value. I want to know the actual maximum value of UInt.

Solution

  • The Int value of -1 and the UInt value UInt.max have the same bit representation in memory.

    You can see that if you do:

    let i = Int(bitPattern: UInt.max)  // i == -1

    and in the opposite direction:

    if UInt(bitPattern: Int(-1)) == UInt.max {
    print("same")
}

    Output:

    same

    The debugger is incorrectly displaying UInt.max as a signed Int. They have the same bit representation in memory (0xffffffffffffffff on a 64-bit system such as iPhone 6 and 0xffffffff on a 32-bit system such as iPhone 5), and the debugger apparently chooses to show that value as an Int.

    You can see the same issue if you do:

    print(String(format: "%d", UInt.max))  // prints "-1"

    It doesn't mean UInt.max is -1, just that both have the same representation in memory.

    To see the maximum value of UInt, do the following in an app or on a Swift Playground:

    print(UInt.max)

    This will print 18446744073709551615 on a 64-bit system (such as a Macintosh or iPhone 6) and 4294967295 on a 32-bit system (such as an iPhone 5).

    In lldb:

    (lldb) p String(UInt.max)
(String) $R0 = "18446744073709551615"
(lldb)