CODEWITHSUNDEEP
c++nullpointer-arithmetic
Jarek C
Jarek C

Reputation: 1261

Is it allowed to add a zero to a null pointer?

I know that pointer arithmetic is disallowed for null pointers. But imagine I have something like this:

class MyArray {
  int *arrayBegin;  // pointer to the first array item, NULL for an empty array
  unsigned arraySize;   // size of the array, zero for an empty array
public:
  int *begin() const { return arrayBegin; }
  int *end() const { return arrayBegin + arraySize; }  // possible? (arrayBegin may be null)

Is it possible (allowed) to have the above end() implementation? Or is it necessary to have:

  int *end() const { return (arraySize == 0) ? nullptr : (arrayBegin + arraySize); }

to avoid pointer arithmetic with nullptr because arrayBegin is null for an empty array (despite arraySize also being zero in this case)?

I know it's possible to store int *end; instead of unsigned size; and let size be computed as end-begin - but then comes the same issue: Is it allowed to compute nullptr - nullptr?

I would especially appreciate standard references.

Upvotes: 16

Views: 1054

Answers (1)

Sergey Strukov
Sergey Strukov

Reputation: 401

Yes, you can add zero to the null pointer and subtract one null pointer from another. Quoting Additive operators [expr.add] section of the C++ standard:

When an expression J that has integral type is added to or subtracted from an expression P of pointer type, the result has the type of P.

  • If P evaluates to a null pointer value and J evaluates to 0, the result is a null pointer value.

Upvotes: 17

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