问题描述
我想检查一下我对此事的理解和结论.
I would like to check my understanding and conclusions on this matter.
在 IRC 上，有人问:
On IRC, it was asked:
const_cast 是否可以接受绑定到临时对象的 const 引用?
Is it acceptable to
const_castaconstreference that's bound to a temporary object?
翻译:他有一个 ref-to-const 绑定到一个临时的，他想抛弃它的 const-ness 来修改它.
Translating: he has a ref-to-const bound to a temporary, and he wants to cast away its const-ness to modify it.
我的回答是我问过 a之前类似的问题，其中的共识似乎是临时人员本身并不是天生的const，因此您可以摆脱引用的const-ness您必须对它们进行修改，并通过结果对其进行修改.而且，只要原来的 ref-to-const 仍然存在，就不会影响临时对象的生命周期.
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.
即:
int main()
{
const int& x = int(3);
int& y = const_cast<int&>(x);
y = 4;
cout << x;
}
// Output: 4
// ^ Legal and safe
我说的对吗?
(当然，这样的代码是否真正可取完全是另一回事！)
推荐答案
没有.
首先，据我所知，它是否是文字是无关的.非类类型的右值始终具有非 cv 限定types(第 3.10/9 节)，然而，在第 8.5.3 节(引用的初始化)中，我们有:
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:
对cv1 T1"类型的引用由cv2 T2"类型的表达式初始化，如下所示:
A reference to type "cv1 T1" is initialized by an expression of type "cv2 T2" as follows:
[...]
--
否则，将使用非引用复制初始化 (8.5) 的规则从初始化表达式创建和初始化cv1 T1"类型的临时变量.然后将引用绑定到临时文件.如果 T1 与 T2 引用相关，则 cv1 必须与 cv-qualification 相同或更高比，cv2；否则，程序是非良构的.
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.
(前面的所有观点都涉及左值或类类型.)
(All of the preceding points concern either lvalues or class types.)
在我们的例子中，我们有:
In our case, we have:
int const& x = ...;
所以cv1 T1是int const，而我们创建的临时对象有类型int 常量.这是一个顶级常量(在对象上)，所以任何尝试修改它是未定义的行为.
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.
至少，这是我的解释.我希望标准对此更加清晰.
At least, that's my interpretation. I wish the standard were a bit clearer about this.
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