c++ - 这种类型的双关语定义明确吗？

Question

阅读 this answer 中的引述关于严格的别名规则，我在 C++11 中看到以下内容:

If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:

• ...

• an aggregate or union type that includes one of the aforementioned types among its elements or non-static data members (including, recursively, an element or non-static data member of a subaggregate or contained union),

• ...

所以我认为下面的代码没有违反严格的别名规则:

#include <iostream>
#include <cstdint>
#include <climits>
#include <limits>

struct PunnerToUInt32
{
    std::uint32_t ui32;
    float fl;
};

int main()
{
    static_assert(std::numeric_limits<float>::is_iec559 &&
                  sizeof(float)==4 && CHAR_BIT==8,"Oops");
    float x;
    std::uint32_t* p_x_as_uint32=&reinterpret_cast<PunnerToUInt32*>(&x)->ui32;
    *p_x_as_uint32=5;
    std::cout << x << "\n";
}

好的，满足严格的别名规则。由于任何其他原因，这是否仍然表现出未定义的行为？

Answer 1

你不能这样做:&reinterpret_cast<PunnerToUInt32*>(&x)

关于 reinterpret_cast 的规则状态:

When a pointer or reference to object whose dynamic type is DynamicType is reinterpret_cast (or C-style cast) to a pointer or reference to object of a different type AliasedType, the cast always succeeds, but the resulting pointer or reference may only be used to access the object if one of the following is true:

• AliasedType is (possibly cv-qualified) DynamicType
• AliasedType and DynamicType are both (possibly multi-level, possibly cv-qualified at each level) pointers to the same type T
• AliasedType is the (possibly cv-qualified) signed or unsigned variant of DynamicType
• AliasedType is an aggregate type or a union type which holds one of the aforementioned types as an element or non-static member (including, recursively, elements of subaggregates and non-static data members of the contained unions): this makes it safe to obtain a usable pointer to a struct or union given a pointer to its non-static member or element.
• AliasedType is a (possibly cv-qualified) base class of DynamicType
• AliasedType is char or unsigned char: this permits examination of the object representation of any object as an array of unsigned char

因为这些都不适用于 DynamicType 的组合正在float和 AliasedType正在PunnerToUInt32该指针不能用于访问您正在执行的对象。使行为未定义。

有关详细信息，请参阅:Why Doesn't reinterpret_cast Force copy_n for Casts between Same-Sized Types?

编辑:

分解第 4^th 个子弹 int 大小块产量:

1. "AliasedType "
   此处取为PunnerToUInt32

2. “是聚合类型还是 union 类型”
   PunnerToUInt32合格，因为它符合 aggregate type 的资格:

   • array type
   • class type (typically, struct or union), that has
     • no private or protected non-static data members
     • no user-provided constructors, including those inherited from public bases (explicitly defaulted or deleted constructors are allowed)
     • no virtual, private, or protected base classes
     • no virtual member functions

3. “将上述类型之一作为元素或非静态成员(递归地包括子聚合的元素和包含的 union 的非静态数据成员)”
   再次 PunnerToUInt32符合条件，因为它是 float fl成员

4. “这使得获取指向结构或 union 的可用指针变得安全”
   这是最后的正确部分 AliassedType是 PunnerToUInt32
5. “给定一个指向其非静态成员或元素的指针”
   这是一个违规行为，因为 DynamicType这是 x不是 PunnerToUInt32 的成员

因为违反第​​ 5 部分操作这个指针是未定义的行为。

如果您喜欢一些推荐读物，可以查看 Empty Base Optimization如果不是，我会在此处为您提供主要相关性:

Empty base optimization is required for StandardLayoutTypes in order to maintain the requirement that the pointer to a standard-layout object, converted using reinterpret_cast, points to its initial member

因此你可以利用reinterpret_cast这样做的第 4 个 项目符号:

PunnerToUInt32 x = {13, 42.0F};
auto y = reinterpret_cast<PunnerToUInt32*>(&x.ui32);

Live Example