c – 临时对象最初是const吗？

这个代码是UB吗？

struct A
{
 void nonconst() {}
};

const A& a = A{};
const_cast<A&>(a).nonconst();

换句话说,(临时)对象最初是const吗？我已经查看了标准,但找不到答案,所以会对相关部分的引用表示赞赏.

编辑：对于那些说A {}不是const的人,你可以做A {}.nonconst()吗？

解决方法:

参考a的初始化由[dcl.init.ref]/5(粗矿)给出：

Otherwise, if the initializer expression

• is an rvalue (but not a bit-field)[…]

then the value of the initializer expression in the first case and the result of the conversion in the second case is called the converted initializer.
If the converted initializer is a prvalue, its type T4 is adjusted to type “cv1 T4” ([conv.qual]) and the temporary materialization conversion ([conv.rval]) is applied.

所以这意味着初始化引用的类型prvalue表达式A {}被调整为const A.

然后[conv.rval]说：

A prvalue of type T can be converted to an xvalue of type T.
This conversion initializes a temporary object ([class.temporary]) of type T.

因此,绑定到引用的临时对象的类型与调整后的prvalue类型相同：const A.

所以代码const_cast< A&>(a).nonconst();是未定义的行为.

标签：temporary-objects,c,language-lawyer,undefined-behavior,const
来源： https://codeday.me/bug/20191004/1853671.html