我是说，const_cast接着修改一个ref-to-const绑定到一个临时是好吗？ [英] Am I right in saying that const_cast followed by modification on a ref-to-const bound to a temporary is okay?

问题描述

我想查看我对此事的理解和结论。

---

在IRC上， / p>

是否可以接受 const_cast a const 引用绑定到临时对象？

翻译 -const绑定到一个临时，他想放弃其 const - 修改它。

我的回答是，我问了一个类似的问题< a>以前，其中共识似乎是暂时本身不是固有的 const ，因此，你可以抛弃 const 一个引用你有他们，并通过结果修改它们。并且，只要原始引用 - const 仍然存在，这不会影响临时的生命周期。

这是：

  int main（）
 {
 const int& x = int（3）; 
 
 int& y = const_cast< int&>（x）; 
 y = 4; 
 
 cout<< X; 
} 
 //输出：4 
 // ^法律和安全
  

我可以吗？

---

^{（当然，事物完全！）}

解决方案

否。

首先，据我所知，它是否为字面值是
无关紧要。非类类型的R值总是具有非cv限定的
类型（§3.10/ 9），然而，在§8.5.3（引用的初始化）中，我们
有：

对类型cv1 T1的引用由类型cv2 T2的表达式初始化如下：

[...]

-

否则，临时类型为cv1 T1 是使用非引用副本初始化（8.5）的规则从初始化器表达式创建和初始化的。然后引用绑定到临时。如果T1与T2的引用相关，则cv1必须与cv2相同cv-qualification as，或更大的cvqualification
。

（上述所有点都涉及左值或类类型。）

在我们的例子中，我们有：

  int const& x = ...; 
  

所以 cv1 T1 是 int const ，我们创建的临时对象类型为
int const 。这是一个顶级const（在对象上），所以任何尝试
修改它是未定义的行为。

至少，这是我的解释。我希望标准更清楚这一点。

I would like to check my understanding and conclusions on this matter.

---

On IRC, it was asked:

Is it acceptable to const_cast a const reference that's bound to a temporary object?

Translating: he has a ref-to-const bound to a temporary, and he wants to cast away its const-ness to modify it.

My response was that I'd asked a similar question previously, where the consensus seemed to be that temporaries themselves are not inherently const, and thus that you can cast off the const-ness of a reference you have to them, and modify them through the result. And, as long as that original ref-to-const still exists, this won't affect the temporary's lifetime.

That is:

int main()
{
   const int& x = int(3);

   int& y = const_cast<int&>(x);
   y = 4;

   cout << x;
}
// Output: 4
// ^ Legal and safe

Am I right?

---

^{(Of course, whether or not such code is actually advisable is another matter entirely!)}

解决方案

No.

First, as far as I can tell, whether it is a literal or not is irrelevant. Rvalues of non-class types always have non-cv qualified types (§3.10/9), however, in §8.5.3 (initialization of a reference), we have:

A reference to type "cv1 T1" is initialized by an expression of type "cv2 T2" as follows:

[...]

--

Otherwise, a temporary of type "cv1 T1" is created and initialized from the initializer expression using the rules for a non-reference copy initialization (8.5). The reference is then bound to the temporary. If T1 is reference-related to T2, cv1 must be the same cv-qualification as, or greater cvqualification than, cv2; otherwise, the program is ill-formed.

(All of the preceding points concern either lvalues or class types.)

In our case, we have:

int const& x = ...;

So cv1 T1 is int const, and the temporary object we create has type int const. This is a top level const (on the object), so any attempt to modify it is undefined behavior.

At least, that's my interpretation. I wish the standard were a bit clearer about this.

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