访问班级中的私人成员

Question

I need to access an object from a DLL, do some manipulations to the object and feed the object to another function. The trouble is the fields I need to change are private.

I don't want to change the private modifier for the fields in the original class because the class was written a long time ago and is used in many places. However, the place where I am manipulating the class I need most of the fields without protection (it is a hack). What is the best way to do it?

Note: I am not allowed to change the original class

Answer 1

What is compiled in the DLL is not really important in this case, what matters is the header file you include. I suggest you change the header file of the class you're interested in so that the variables you need are public .

Member access is checked by compiler and not by the linker, so all that matters is how the class is declared. This does not require you to recompile the DLL or change the implementation of the class, but to just modify the header file (or a copy of the header file).

Answer 2

一种方法：编写Getter / Setter函数

Answer 3

If you can see the original class you can make a mock class with the same bit pattern and cast the original object to the mock class object

Example below

Original class

class ClassWithHiddenVariables
{
private:
    int a;
    double m;

}; 

Mock class

 class ExposeAnotherClass
    {
    public:
        int a_exposed;
        double m_exposed;
    };

When you want to see members of the ClassWithHiddenVariables object, use reinterpret_cast to cast to ExposeAnotherClass

 ClassWithHiddenVariables obj;
    obj.SetVariables(10, 20.02);    
    ExposeAnotherClass *ptrExposedClass;
    ptrExposedClass = reinterpret_cast<ExposeAnotherClass*>(&obj);  
    cout<<ptrExposedClass->a_exposed<<"\n"<<ptrExposedClass->m_exposed;

Answer 4

The "Forger" hack from Exceptional C++ Style, Item 15 will work. Although, it is illegal as it violates the One Definition Rule so I'd advise against doing this in production code.

From page 106

// Example 15-1: Lies and Forgery
//
class X {
    // instead of including x.h, manually (and illegally) duplicates X's
    // definition, and adds a line such as:
    friend ::Hijack( X& );
};

void Hijack( X& x ) {
    x.private_ = 2;
}

Note how class X's private member "private_" is accessible in Hijack.