c++ - 使用 reinterpret_cast 访问私有(private)数据

Question

这似乎可以成功编译和访问私有(private)数据。这是定义明确的行为吗？

#include <iostream>
#include <string>

using std::string;

class foo {
    string private_data = "Hello World";
};

int main()
{
    foo f;
    auto* pprivate_data = reinterpret_cast<string*>(&f);
    std::cout << *pprivate_data << '\n';
}

This问题有点相似，但我相信它不能解决我的问题。

Answer 1

不，行为未定义。对于这样的reintepret_cast为了有意义，这两个对象必须是可相互转换的

[basic.compound]

4 Two objects a and b are pointer-interconvertible if:

• they are the same object, or
• one is a union object and the other is a non-static data member of that object ([class.union]), or
• one is a standard-layout class object and the other is the first non-static data member of that object, or, if the object has no non-static data members, any base class subobject of that object ([class.mem]), or
• there exists an object c such that a and c are pointer-interconvertible, and c and b are pointer-interconvertible.

If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_­cast. [ Note: An array object and its first element are not pointer-interconvertible, even though they have the same address. — end note ]

唯一可能适用的项目是关于标准布局类的项目。如果我们查阅该定义，我们会看到

[class.prop]

3 A class S is a standard-layout class if it:

• has no non-static data members of type non-standard-layout class (or array of such types) or reference,
• [...]

有一个直接的问题。对象的任何非静态数据成员本身必须是标准布局。没有保证std::string是标准布局类型。所以行为是不确定的。