I used to think that adding an integral type to a pointer (provided that the the pointer points to an array of a certain size etc. etc.) is always well defined, regardless of the integral type. The C++11 standard says ([expr.add]):
When an expression that has integral type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the pointer operand points to an element of an array object, and the array is large enough, the result points to an element offset from the original element such that the difference of the subscripts of the resulting and original array elements equals the integral expression. In other words, if the expression P points to the i -th element of an array object, the expressions (P)+N (equivalently, N+(P)) and (P)-N (where N has the value n ) point to, respectively, the i + n -th and i − n -th elements of the array object, provided they exist. Moreover, if the expression P points to the last element of an array object, the expression (P)+1 points one past the last element of the array object, and if the expression Q points one past the last element of an array object, the expression (Q)-1 points to the last element of the array object. If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined.
On the other hand, it was brought to my attention recently that the built-in add operators for pointers are defined in terms of
ptrdiff_t, which is a signed type (see 13.6/13). This seems to hint that if one does a
malloc() with a very large (unsigned) size and then tries to reach the end of the allocated space via a pointer addition with a
std::size_t value, this might result in undefined behaviour because the unsigned
std::size_t will be converted to
ptrdiff_t which is potentially UB.
I imagine similar issues would arise, e.g., in the
std::vector, which is implemented in terms of an unsigned
size_type. In general, it seems to me like this would make practically impossible to fully use the memory storage available on a platform.
It's worth noting that nor GCC nor Clang complain about signed-unsigned integral conversions with all the relevant diagnostic turned on when adding unsigned values to pointers.
Am I missing something?
EDIT: I'd like to clarify that I am talking about additions involving a pointer and an integral type (not two pointers).
EDIT2: an equivalent way of formulating the question might be this. Does this code result in UB in the second line, if
ptrdiff_t has a smaller positive range than
char *ptr = static_cast<char * >(std::malloc(std::numeric_limits<std::size_t>::max())); auto end = ptr + std::numeric_limits<std::size_t>::max();