Dereferencing an invalid pointer, then taking the address of the result

Consider:

int* ptr = (int*)0xDEADBEEF;
cout << (void*)&*ptr;

How illegal is the *, given that it's used in conjunction with an immediate & and given that there are no overloaded op&/op* in play?

(This has particular ramifications for addressing a past-the-end array element &myArray[n], an expression which is explicitly equivalent to &*(myArray+n). This Q&A addresses the wider case but I don't feel that it ever really satisfied the above question.)

Solution 1:

According to the specification, the effect of dereferencing an invalid pointer itself produces undefined behaviour. It doesn't matter what you do after dereferencing it.

Solution 2:

Assuming the variable `ptr' does not contain a pointer to a valid object, the undefined behavior occurs if the program necessitates the lvalue-to-rvalue conversion of the expression `*ptr', as specified in [conv.lval] (ISO/IEC 14882:2011, page 82, 4.1 [#1]).

During the evaluation of `&*ptr' the program does not necessitate the lvalue-to-rvalue conversion of the subexpression `*ptr', according to [expr.unary.op] (ISO/IEC 14882:2011, page 109, 5.3.1 [#3])

Hence, it is legal.

Solution 3:

It is legal. Why wouldn't it be? You're just setting a value to a pointer, and then accessing to it. However, assigning the value by hand must be obviously specified as undefined behavior, but that's the most a general specification can say. Then, you use it in some embedded software controller, and it will give you the correct memory-mapped value for some device...