可以空引用吗?
这段代码是否有效(和定义的行为)?
Is this piece of code valid (and defined behavior)?
int &nullReference = *(int*)0;
g++ 和 clang++ 编译时没有任何警告，即使使用 -Wall、-Wextra、-std=c++98, -pedantic, -Weffc++...
Both g++ and clang++ compile it without any warning, even when using -Wall, -Wextra, -std=c++98, -pedantic, -Weffc++...
当然引用实际上不是空的，因为它不能被访问(这意味着取消引用一个空指针)，但我们可以通过检查它的地址来检查它是否为空:
Of course the reference is not actually null, since it cannot be accessed (it would mean dereferencing a null pointer), but we could check whether it's null or not by checking its address:
if( & nullReference == 0 ) // null reference
推荐答案
引用不是指针.
8.3.2/1:
一个引用应该被初始化为引用有效的对象或函数.[注意:特别是空引用不能存在于一个明确定义的程序，因为只有这样创建这样的引用将是将其绑定到通过获得的对象"取消引用一个空指针，它导致未定义的行为.作为在 9.6 中描述，引用不能直接绑定到位域.]
A reference shall be initialized to refer to a valid object or function. [Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the "object" obtained by dereferencing a null pointer, which causes undefined behavior. As described in 9.6, a reference cannot be bound directly to a bit-field. ]
1.9/4:
描述了某些其他操作在本国际标准中作为未定义(例如，效果取消引用空指针)
Certain other operations are described in this International Standard as undefined (for example, the effect of dereferencing the null pointer)
正如约翰内斯在一个已删除的答案中所说的那样，是否应该将取消引用空指针"明确地声明为未定义的行为存在一些疑问.但这并不是引起怀疑的情况之一，因为空指针肯定不会指向有效的对象或函数"，并且标准委员会内不希望引入空引用.
As Johannes says in a deleted answer, there's some doubt whether "dereferencing a null pointer" should be categorically stated to be undefined behavior. But this isn't one of the cases that raise doubts, since a null pointer certainly does not point to a "valid object or function", and there is no desire within the standards committee to introduce null references.
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