C++ 如何在不结交“朋友”的情况下访问类外的私有数据成员？
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How to access private data members outside the class without making "friend"s?
提问by Abhineet
I have a class Aas mentioned below:-
我有一个class A如下所述：-
class A{
int iData;
};
I neither want to create member function nor inherit the above class Anor change the specifier of iData.
我既不想创建成员函数，也不想继承上述内容，class A也不想更改iData.
My doubts:-
我的疑问：-
- How to access
iDataof an object sayobj1which is an instance ofclass A? - How to change or manipulate the
iDataof an objectobj1?
- 如何访问
iData一个对象说obj1哪个是 的实例class A？ - 如何更改或操作
iData对象的obj1？
Note: Don't use friend.
注意：不要使用friend.
采纳答案by Mat
You can't. That member is private, it's not visible outside the class. That's the whole point of the public/protected/private modifiers.
你不能。该成员是私有的，在课堂外是不可见的。这就是 public/protected/private 修饰符的重点。
(You could probably use dirty pointer tricks though, but my guess is that you'd enter undefined behavior territory pretty fast.)
（虽然你可能会使用脏指针技巧，但我的猜测是你会很快进入未定义的行为领域。）
回答by Sleiman Jneidi
Here's a way, not recommended though
这是一种方法，虽然不推荐
class Weak {
private:
string name;
public:
void setName(const string& name) {
this->name = name;
}
string getName()const {
return this->name;
}
};
struct Hacker {
string name;
};
int main(int argc, char** argv) {
Weak w;
w.setName("Jon");
cout << w.getName() << endl;
Hacker *hackit = reinterpret_cast<Hacker *>(&w);
hackit->name = "Hyman";
cout << w.getName() << endl;
}
回答by Alok Save
EDIT:
Just saw you edited the question to say that you don't want to use friend.
Then the answer is:
编辑：
刚刚看到您编辑了问题，说您不想使用朋友。
那么答案是：
NO you can't, atleast not in a portable way approved by the C++ standard.
不，你不能，至少不能以 C++ 标准批准的可移植方式。
The later part of the Answer, was previous to the Q edit & I leave it here for benefit of >those who would want to understand a few concepts & not just looking an Answer to the >Question.
答案的后面部分在 Q 编辑之前，我将它留在这里是为了让那些想要了解一些概念的人受益，而不仅仅是寻找问题的答案。
If you have members under a Private access specifier then those members are only accessible from within the class. No outside Access is allowed.
如果您在 Private 访问说明符下有成员，那么这些成员只能从类内部访问。不允许外部访问。
An Source Code Example:
源代码示例：
class MyClass
{
private:
int c;
public:
void doSomething()
{
c = 10; //Allowed
}
};
int main()
{
MyClass obj;
obj.c = 30; //Not Allowed, gives compiler error
obj.doSomething(); //Allowed
}
A Workaround: friendto rescue
To access the private member, you can declare a function/class as friend of that particular class, and then the member will be accessible inside that function or class object without access specifier check.
解决方法：friend挽救
要访问私有成员，您可以将函数/类声明为该特定类的朋友，然后该成员将可以在该函数或类对象内部访问，而无需检查访问说明符。
Modified Code Sample:
修改后的代码示例：
class MyClass
{
private:
int c;
public:
void doSomething()
{
c = 10; //Allowed
}
friend void MytrustedFriend();
};
void MytrustedFriend()
{
MyClass obj;
obj.c = 10; //Allowed
}
int main()
{
MyClass obj;
obj.c = 30; //Not Allowed, gives compiler error
obj.doSomething(); //Allowed
//Call the friend function
MytrustedFriend();
return 0;
}
回答by Ajay
Bad idea, don't do it ever - but here it is how it can be done:
坏主意，永远不要这样做 - 但这是如何做到的：
int main()
{
A aObj;
int* ptr;
ptr = (int*)&aObj;
// MODIFY!
*ptr = 100;
}
回答by Timmah
http://bloglitb.blogspot.com/2010/07/access-to-private-members-thats-easy.html
http://bloglitb.blogspot.com/2010/07/access-to-private-members-thats-easy.html
this guy's blog shows you how to do it using templates. With some modifications, you can adapt this method to access a private data member, although I found it tricky despite having 10+ years experience.
这个人的博客向您展示了如何使用模板来做到这一点。通过一些修改，您可以调整此方法来访问私有数据成员，尽管我发现尽管有 10 多年的经验但它很棘手。
I wanted to point out like everyone else, that there is an extremely few number of cases where doing this is legitimate. However, I want to point out one: I was writing unit tests for a software suite. A federal regulatory agency requires every single line of codeto be exercised and tested, withoutmodifying the original code. Due to (IMHO) poor design, a static constant was in the 'private' section, but I needed to use it in the unit test. So the method seemed to me like the best way to do it.
我想像其他人一样指出，在极少数情况下这样做是合法的。但是，我想指出一点：我正在为软件套件编写单元测试。联邦监管机构要求对每一行代码进行练习和测试，而无需修改原始代码。由于（恕我直言）糟糕的设计，静态常量位于“私有”部分，但我需要在单元测试中使用它。所以这个方法在我看来是最好的方法。
I'm sure the way could be simplified, and I'm sure there are other ways. I'm not posting this for the OP, since it's been 5 months, but hopefully this will be useful to some future googler.
我相信方法可以简化，我相信还有其他方法。我不是为 OP 发布这个，因为它已经 5 个月了，但希望这对未来的一些谷歌人有用。
回答by Alexander Drichel
In C++, almost everything is possible! If you have no way to get private data, then you have to hack. Do it only for testing!
在 C++ 中，几乎一切皆有可能！如果您无法获得私人数据，那么您必须破解。只做测试！
class A {
int iData;
};
int main ()
{
A a;
struct ATwin { int pubData; }; // define a twin class with public members
reinterpret_cast<ATwin*>( &a )->pubData = 42; // set or get value
return 0;
}
回答by RocketR
There's no legitimate way you can do it.
没有合法的方法可以做到这一点。
回答by n. 'pronouns' m.
iDatais a privatemember of the class. Now, the word privatehave a very definite meaning, in C++ as well as in real life. It means you can't touch it. It's not a recommendation, it's the law. If you don't change the class declaration, you are not allowed to manipulate that member in any way, shape or form.
iData是private班级的成员。现在，这个词private在 C++ 和现实生活中都有非常明确的含义。这意味着你不能碰它。这不是建议，这是法律。如果不更改类声明，则不允许以任何方式、形状或形式操纵该成员。
回答by iammilind
Start making friends of class A. e.g.
开始做friend第class A。例如
void foo ();
class A{
int iData;
friend void foo ();
};
Edit:
编辑：
If you can't change class Abody then A::iDatais not accessible with the given conditions in your question.
如果您无法更改class A正文，则A::iData无法使用问题中的给定条件访问。
