Does forming a reference to an object constitute access?

问题

Does forming a reference to an object constitute access?

Here's what GCC and Clang currently do:

void test(int const volatile* ptr) noexcept {
  *ptr;  // movl (%rdi), eax  // Reads *ptr
  [[maybe_unused]] int const volatile& ref = *ptr;  // Does not read *ptr
}

My question is specifically about the statement

  [[maybe_unused]] int const volatile& ref = *ptr;

• According to the abstract machine, does this read the value of the object pointed to by ptr?
• Would this statement, in isolation, be undefined behavior if ptr == nullptr?
  • Yes, indirection on null pointer is UB - https://stackoverflow.com/a/59205697/1614051
• Would it be an aliasing violation if ptr pointed to something other than an int?

Note that I ask specifically about forming the reference, and not about using it to read the value.

Edit 09/12/2019: Accepting the following answers:

• Does int const volatile& ref = *ptr; read the value of the pointed-to object?
  • No.
• Is this undefined when ptr == nullptr?
  • Yes, *ptr on a null pointer is undefined.
• Is forming the reference an aliasing violation if ptr points to an object of different type?
  • No, just forming the reference does not violate strict aliasing.
  • Presumably reinterpret_cast-ing the reference to the correct type is allowed and valid.

回答1:

[basic.compound]/3 ... Every value of pointer type is one of the following:

(3.1) — a pointer to an object or function (the pointer is said to point to the object or function), or

(3.2) — a pointer past the end of an object (8.7), or

(3.3) — the null pointer value (7.11) for that type, or

(3.4) — an invalid pointer value.

---

[expr.unary.op]/1 The unary * operator performs indirection: the expression to which it is applied shall be a pointer to an object type, or a pointer to a function type and the result is an lvalue referring to the object or function to which the expression points.

Thus, the meaning of the expression *ptr is only defined for pointer ptr that points to an object or function - that is, a pointer whose value falls under [basic.compound]/(3.1). In all other cases, this expression exhibits undefined behavior.

来源：https://stackoverflow.com/questions/59205336/does-forming-a-reference-to-an-object-constitute-access