在父級中使用受保護數據，傳入子級

Question

如何在傳入派生類時訪問父級類中的數據，該類是受保護的。

class parent 
{ 
    protected: 
     int a; 
}; 

class child : public parent 
{ 
    void addOne(parent * &); 
}; 

void child::addOne(parent * & parentClass) 
{ 
    parentClass->a += 1; 
} 

int main() 
{ 
    parent a; 
    child b; 

    parent* ap = &a; 

    b.addOne(ap); 
} 

Answer 1

您無法通過指針/對基類的引用訪問受保護的數據。這是爲了防止您打破其他派生類對該數據可能具有的不變量。

class parent 
{ 
    void f(); 
    // let's pretend parent has these invariants: 
    // after f(), a shall be 0 
    // a shall never be < 0. 

    protected: 
     int a; 
}; 

class child : public parent 
{ 
public: 
    void addOne(parent * &); 
}; 


class stronger_child : public parent 
{ 
public: 
    stronger_child(int new_a) { 
     if(new_a > 2) a = 0; 
     else a = new_a; 
    } 
    // this class holds a stronger invariant on a: it's not greater than 2! 
    // possible functions that depend on this invariant not depicted :) 
}; 

void child::addOne(parent * & parentClass) 
{ 
    // parentClass could be another sibling! 
    parentClass->a += 1; 
} 

int main() 
{ 
    stronger_child a(2); 
    child b; 

    parent* ap = &a; 

    b.addOne(ap); // oops! breaks stronger_child's invariants! 
}