C++ 遍历的幂等性：指针？参考 ？指针

Question

我的理解

• 在 C++ 中，普通值可以使用 转换为指向其自身的指针&value，或通过隐式转换转换为对其自身的引用。

• 与 Rust 不同，引用并不是实际的一等类型，而只是某些类型声明站点接受的二进制限定符。引用可以透明地使用，就好像它们是它们的裁判一样，隐藏内部指针¹。因此，我们可以&refToValue立即获取指向裁判的指针。

问题

如果我将指针转换为引用，再转换回指针，我能保证恢复到原始指针吗？Rust 中就是这种情况，但 Rust 再次将一级引用定义为通过指针抽象的“普通”类型。

// Sample code by @chi
T x;
T* ptr1 = &x;
T& ref = *ptr1;
T* ptr2 = &ref;
assert(ptr1 == ptr2); // Is that always true according to the spec?

_{1：我知道 C++ 标准不要求将引用实现为指针，但我看不出如何以任何其他方式实现它们（不考虑上下文特定的优化），特别是如果我的最后一个问题的答案是“是的”。}

Answer 1

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If I convert a pointer, to a reference, back to a pointer, am I guaranteed to get the original pointer back?

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T x;\nT* ptr1 = &x;\nT& ref = *ptr1;\nT* ptr2 = &ref;\n

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Yes.

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Full explanation on the initialisations

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• Per [expr.unary.op]/3.2, T* ptr1 = &x initializes ptr1 as a pointer to T pointing to the designated object (x).
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The operand of the unary & operator shall be an lvalue of some type T. The result is a prvalue.
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\n(3.2)\nOtherwise, the result has type \xe2\x80\x9cpointer to T\xe2\x80\x9d and points to the designated object ([intro.memory]) or function ([basic.compound]). If the operand names an explicit object member function ([dcl.fct]), the operand shall be a qualified-id.

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• Per [expr.unary.op]/1 and [dcl.ref]/5, T& ref = *ptr1 initializes ref to refer to a valid object, namely x as *ptr1 performs indirection on ptr1 and yields an lvalue denoting the object to which ptr1 points, namely x.
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[expr.unary.op]/1 The unary * operator performs indirection. Its operand shall be a prvalue of type \xe2\x80\x9cpointer to T\xe2\x80\x9d, where T is an object or function type. The operator yields an lvalue of type T denoting the object or function to which the operand points.

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[dcl.ref]/5 A reference shall be initialized to refer to a valid object or function.

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• Finally, per [expr.unary.op]/3.2 again, T* ptr2 = &ref initializes ptr2 as a pointer to T pointing to the designated object (x).
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What about the pointer comparison?

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• As per the equality of the two pointers, I raise [basic.compound]/3.4:
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A value of a pointer type that is a pointer to or past the end of an object represents the address of the first byte in memory ([intro.memory]) occupied by the object35 or the first byte in memory after the end of the storage occupied by the object, respectively.

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和[expr.eq]/3.2：

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[参考 == 的结果，按照 /1 中的定义]
\nif the pointers are both null, both point to the same function, or both represent the same address, they compare equal.

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ptr1意思是，无论和的实际值如何ptr2:

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ptr1 == ptr2\n

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产量true.

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