将数组作为结构访问与未定义行为

Question

假设我们有一个包含 4 个 float 值的结构和一个包含 4 个元素的 float 数组。

然后将数组作为 Foo 实例访问并通过该实例更改数组元素是否是未定义的行为？

struct Foo
{
    float a;
    float b;
    float c;
    float d;
};

float values[4] = { 1.0f, 1.0f, 1.0f, 1.0f };

int main()
{
    auto& floats = *reinterpret_cast<Foo*>(values);
    floats.a = 0.0f;
    floats.b = 0.0f;
    floats.c = 0.0f;
    floats.d = 0.0f;
}

在线编译和运行：http://cpp.sh/6y7m

Answer 1

是的，这确实是一个未定义的行为。它违反了所谓的严格别名规则——禁止通过 unrelated 指针访问对象（我不会详细说明什么是 unrelated在这里，除非特别要求）。

但是，浮点数组与结构无关。

这是标准 (3.10 / 10) 的摘录：

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:

— the dynamic type of the object,

— a cv-qualified version of the dynamic type of the object,

— a type similar (as defined in 4.4) to the dynamic type of the object,

— a type that is the signed or unsigned type corresponding to the dynamic type of the object,

— a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object,

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

— a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,

— a char or unsigned char type.

将数组作为结构访问与未定义行为

Accessing an array as a struct vs undefined behavior

c++

pointers

casting

undefined-behavior