protected: C++ access control works on per-class basis, not on per-object basis
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簡単な質問です
同級生は筆記試験に参加して、問題を投げて、女の子は私と討論して、女の子はstackoverflowの上で答えを見つけました......
以下に引用する.http://stackoverflow.com/questions/6986798/subtle-c-inheritance-error-with-protected-fields ==========================================================================
Below is a subtle example of accessing an instance's protected field x. B is a subclass of A so any variable of type B is also of type A. Why can B::foo() access b's x field, but not a's x field?
Why can the B::foo() access the members of the contained class B pointer b?The rule is:In C++ access control works on per-class basis, not on per-object basis.So an instance of class B will always have access to all the members of another instance of class B.
Since child is inheriting parent, child gets x. Hence you can access x directly in foo() method of child. This is the concept of protected variables. You can access protected variables of parent in child directly. Note : Here I am saying you can access x directly but not through A's object! Whats the difference ? Since, x is protected, you cannot access A's protected objects outside A. Doesnt matter where it is - If its main or Child . That's why you are not able to access in the following way.
Here comes an interesting concept. You can access a private variable of a class using its object with in the class!
// BASE：：foo() p val? ， ，
class BASE
{
private:
int val;
public:void foo(BASE *p)
{
int w = p->val;
}
};同級生は筆記試験に参加して、問題を投げて、女の子は私と討論して、女の子はstackoverflowの上で答えを見つけました......
以下に引用する.http://stackoverflow.com/questions/6986798/subtle-c-inheritance-error-with-protected-fields ==========================================================================
Below is a subtle example of accessing an instance's protected field x. B is a subclass of A so any variable of type B is also of type A. Why can B::foo() access b's x field, but not a's x field?
class A
{
protected:
int x;
};
class B : public A
{
protected:
A *a;
B *b;
public:
void foo()
{
int u = x; // OK : accessing inherited protected field x
int v = b->x; // OK : accessing b's protected field x
int w = a->x; // ERROR : accessing a's protected field x
}
};Why can the B::foo() access the members of the contained class B pointer b?The rule is:In C++ access control works on per-class basis, not on per-object basis.So an instance of class B will always have access to all the members of another instance of class B.
class A
{
protected:
int x;
};
class B : public A
{
public:
void foo()
{
int u = x; // OK : accessing inherited protected field
}
};Since child is inheriting parent, child gets x. Hence you can access x directly in foo() method of child. This is the concept of protected variables. You can access protected variables of parent in child directly. Note : Here I am saying you can access x directly but not through A's object! Whats the difference ? Since, x is protected, you cannot access A's protected objects outside A. Doesnt matter where it is - If its main or Child . That's why you are not able to access in the following way.
Here comes an interesting concept. You can access a private variable of a class using its object with in the class!
class dummy
{
private :
int x;
public:
void foo()
{
dummy *d;
int y = d->x; // Even though x is private, still you can access x from object of d - But only with in this class. You cannot do the same outside the class.
}
};