C++:reinterpret_cast 的奇怪行为

Question

我有以下片段

#include <iostream>

using namespace std;

class foobar
{
    public:
    unsigned int a = 97;
    unsigned int b = 98;
    unsigned int c = 99;
};


class barfoo
{
    public:
    char a:32,b:32,c;

};


int main(){
    foobar * f = new foobar();
    barfoo * b = (reinterpret_cast<barfoo *>(f));
    cout << b->a << "-" << b->b << "-" << b->c;

}

输出:

a-b-c

但是，如果我在 barfoo 中指定了 c 的宽度，它就不起作用(所有值都是 0)。观察

#include <iostream>

using namespace std;

class foobar
{
    public:
    unsigned int a = 97;
    unsigned int b = 98;
    unsigned int c = 99;
};


class barfoo
{
    public:
    char a:32,b:32,c:32;

};


int main(){
    foobar * f = new foobar();
    barfoo * b = (reinterpret_cast<barfoo *>(f));
    cout << b->a << "-" << b->b << "-" << b->c;

}

输出:

--

字符全为0

知道为什么会这样吗？

ideone 链接 first snippet -- 工作

链接到ideone second snippet -- 不工作

编译器是gcc-5.1谢谢!!

Answer 1

此 reinterpret_cast 不满足已定义行为的要求，因此虽然对其结果的赋值是可接受的，但 b 的任何后续访问都是未定义的行为:

barfoo * b = (reinterpret_cast<barfoo *>(f));

A reinterpret_cast必须满足以下条件之一，其中 foobar 是 DynamicType，barfoo 是 AliasedType:

• 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.

由于 AliasedType 不属于任何这些条件，因此访问 reinterpret_cast 的结果是未定义的行为。