7 ms·

If null references are a mistake, isn't initializing an array index variable to -1 a mistake too?

• itronitron 4y ago
  Yes, I suppose null references are only a problem if you are unable to write a conditional statement.
  • josephg 4y ago
    The problem with null references is you have to write those conditional statements everywhere, otherwise your program might crash. You generally know as the programmer which pointers you expect to be nullable and which should always be an object. But the compiler has no idea, so it can’t help make sure you have null checks in all the places you need them.
    • GoblinSlayer 4y ago
      And what to do if null check fails? Call abort()?
  • tadfisher 4y ago
    null references are a problem when: - null is a valid member of every type - there's no way to constrain a reference to only non-null members of the referenced type - the compiler allows dereferencing values of a nullable type Weirdly, all three are true of C, C++ and Java.
    • GoblinSlayer 4y ago
      Null is a valid member of every type only in Java. C and C++ have value types.
• lmm 4y ago
  Yes. Make illegal states unrepresentable.
  • AnimalMuppet 4y ago
    And how exactly do you propose to do that? Here's a variable length array. Valid indexes are either [0...length-1] or [1...length], depending on which indexing arrangement you prefer. You can make -1 impossible by using an unsigned type. But for the index of a variable length array, how do you make length+1 unrepresentable?
    • lmm 4y ago
      Use some kind of range type, or, better, a type that represents "index into this array" (which needs some form of dependent typing, but is very much doable).
      • AnimalMuppet 4y ago
        A range type has the problem that it's not necessarily valid indexes for this array - it could be for some other array. "Index into this array" is, as you say, better (presumably it keeps a pointer or a reference or something to the array), but even that isn't bulletproof. Something like realloc could shrink the array but keep the same pointer, or the array could be deallocated and a new one allocated that happened to get the same address. What you really need is that the array keeps track of its own limits - something like std::vector or Java's ArrayList. But then you're back to the overhead of checking each access. So... I'm still not seeing how you're making illegal states unrepresentable.
        • lmm 4y ago
          > "Index into this array" is, as you say, better (presumably it keeps a pointer or a reference or something to the array), but even that isn't bulletproof. Something like realloc could shrink the array but keep the same pointer, or the array could be deallocated and a new one allocated that happened to get the same address. You can address that with some concept of lifetimes or ownership. > What you really need is that the array keeps track of its own limits - something like std::vector or Java's ArrayList. But then you're back to the overhead of checking each access. IME that's less good than having a dedicated type for "index into this array", even if the latter isn't perfect. A bounds-checked access using an offset from a different array might not be reading from uninitialized memory, but it's still overwhelmingly likely to be a programming bug.