使用 reinterpret_cast 访问私人数据

Question

这似乎成功编译并访问了私有数据。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，但我认为它没有解决我的问题。

Answer 1

不，行为未定义。为了使 reintepret_cast 有意义，这两个对象必须可以相互转换

[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 ]

可能适用的唯一要点是关于标准布局的要点类。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,

[...]

有一个紧迫的问题。对象的任何非静态数据成员本身必须是标准布局。不能保证 std::string 是标准布局类型。所以行为是未定义的。

Answer 2

是的，这很好，条件是 std::string（因此 class foo）是标准布局（它 is in libstdc++, libc++ and MSVC STL). Per class.mem/26 :

If a standard-layout class object has any non-static data members, its address is the same as the address of its first non-static data member [...] [Note: The object and its first subobject are pointer-interconvertible ([basic.compound], [expr.static.cast]). — end note]

和basic.compund/4：

Two objects a and b are pointer-interconvertible if: [...]

one is a standard-layout class object and the other is the first non-static data member of that object [...]

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.

显然，这只适用于第一个非静态数据成员。

使用 reinterpret_cast 访问私人数据

Accessing private data with reinterpret_cast

c++

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