特定类型的指针可以分配给指向包含与其成员之一相同类型的 union 体的指针吗？

Question

假设我有以下代码，定义了一个结构和一个 union :

struct Foo
{
    int i;
    float f;
};


union Bar
{
    struct Foo foo;
    char buf[10];
};

将指针分配给struct Foo类型的对象来代替指向union Bar类型的指针并随后这样使用它是否安全:

struct Foo fooVar;

union Bar* barPtrVar = (union Bar*) &fooVar;

bar->foo.i = 1000;
bar->foo.f = 0.5f;

只要我们确定只使用union Bar的foo成员？

我记得在 C 中，指向结构的指针与指向它的第一个成员的指针相同，因此它们有时可以互换，因此尽管类似的逻辑可能适用于此处。

Answer 1

即使您从 buf 成员读取(在某种程度上)，这种用法也是安全的。

C standard 第 6.5p7 节给出以下别名规则:

An object shall have its stored value accessed only by an lvalue expression that has one of the following types:

• a type compatible with the effective type of the object,
• a qualified version of a type compatible with the effective type of the object,
• a type that is the signed or unsigned type corresponding to the effective type of the object,
• a type that is the signed or unsigned type corresponding to a qualified version of the effective type of the object,
• an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), or
• a character type.

您的用例属于倒数第二个要点，并从 buf 成员读取到 buf[sizeof(struct Foo)-1] 根据最后一个要点是允许的。