形成对对象的引用是否构成访问? [英] Does forming a reference to an object constitute access?

问题描述

对对象的引用构成访问吗?

Does forming a reference to an object constitute access?

GCC和Clang当前正在做的事情:

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
}

我的问题是关于陈述的

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

• 根据抽象机器，这是否读取了 ptr 指向的对象的值?
• 如果 ptr == nullptr ，此语句是否孤立地成为未定义的行为?
  • 是的，对空指针的间接寻址是UB- https://stackoverflow.com/a/59205697/1614051
  • 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

    请注意，我专门询问有关形成引用的问题，而不是有关使用它来读取值的问题.

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

    编辑09/12/2019:接受以下答案:

    • int const volatile&是吗?ref = * ptr; 读取指向对象的值?
      • 不.
      • 是的，未定义空指针上的 * ptr .
      • 不，仅构成引用并不违反严格的别名.
      • 大概是 reinterpret_cast -引用正确的类型是允许的和有效的.
      • No, just forming the reference does not violate strict aliasing.
      • Presumably reinterpret_cast-ing the reference to the correct type is allowed and valid.

      推荐答案

      [basic.compound]/3 ...指针类型的每个值都是以下之一:

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

      (3.1)-指向对象或函数的指针(据说该指针指向该对象或函数)，或者

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

      (3.2)-超出对象末尾的指针(8.7)，或者

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

      (3.3)-该类型的空指针值(7.11)，或者

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

      (3.4)—无效的指针值.

      (3.4) — an invalid pointer value.

      [expr.unary.op]/1 一元 * 运算符执行 indirect :应用该表达式的表达式应为指向对象类型的指针，或者指向函数类型的指针，结果是一个左值，表示表达式所指向的对象或函数.

      [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.

      因此，表达式 * ptr 的含义仅针对指向对象或函数的指针 ptr 定义，即，其值小于 [basic.compound]/(3.1).在所有其他情况下，此表达式都表现出不确定的行为.

      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.

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