On 7月6日, 上午7时38分, Immortal Nephi <Immortal_Ne....@satx.rr.com> wrote:
> I design a class for general purpose. I do not allow a client to read
> or modify interface and implemention. I allow them to write a new non-
> member function outside of class' interface and implmention. The
> problem is that non-member function cannot access private data member.
>
> The friend keyword is not the solution. I am aware that friend
> keyword allows the non-member function to access private data member.
>
> If you define more than two non-member functions, you will want one of
> some non-member function to be pointed to the member function pointer
> at run-time. The client can decide to remove first non-member
> function from the member function pointer and add another non-member
> function at run-time. The member function pointer array is not
> necessary according to my design.
>
> Does it mean that it is the only solution to access public data member
> only?
>
> Take a look at my example code below.
>
> #ifndef A_H
> #define A_H
>
> class A
> {
> public:
> typedef void (*Func_Ptr)(A &);
>
> A(void);
> ~A(void);
>
> void Run(void);
> void Set_Func_Ptr(Func_Ptr);
>
> private:
> friend void Non_Member_Func(A &);
>
> Func_Ptr Member_Func_Ptr;
>
> public:
> int W;
> int X;
>
> private:
> int Y;
> int Z;
>
> };
>
> #endif // A_H
>
> // A_CPP
> #include "A.h"
>
> A::A(void)
> {
> W = 1;
> X = 2;
> Y = 3;
> Z = 4;
>
> }
>
> A::~A(void)
> {
>
> }
>
> void A::Run(void)
> {
> Non_Member_Func( *this );
> Member_Func_Ptr( *this );
>
> }
>
> void A::Set_Func_Ptr(Func_Ptr Ptr)
> {
> Member_Func_Ptr = Ptr;
>
> }
>
> // MAIN_CPP
> #include <stdio.h>
> #include "A.h"
>
> void Non_Member_Func(A &a)
> {
> a.W *= 2; // Can access public
> a.X *= 2; // can access public
> a.Y *= 2; // can access private
> a.Z *= 2; // can access private
>
> printf("W: %d\nX: %d\nY: %d\nZ: %d\n", a.W, a.X, a.Y, a.Z);
>
> }
>
> void Member_Func(A &a)
> {
> a.W *= 2;
> a.X *= 2;
>
> // a.Y += 4; // Error can't access private
> // a.Z += 6; // Error can't access private
>
> printf("W: %d\nX: %d\n", a.W, a.X);
>
> }
>
> void Member_Func2(A &a)
> {
> a.W *= 4;
> a.X *= 4;
>
> // a.Y += 4; // Error can't access private
> // a.Z += 6; // Error can't access private
>
> printf("W: %d\nX: %d\n", a.W, a.X);
>
> }
>
> int main(void)
> {
> A a;
>
> Non_Member_Func( a );
>
> a.Set_Func_Ptr( Member_Func );
> a.Run();
>
> a.Set_Func_Ptr( Member_Func2) ;
> a.Run();
>
> return 0;
>
> }
I don't know if I understand your purpose to design such a class. It's
weird to allow client to write a non-member function to access private
member of a general purpose interface class. If you design a class for
general purpose, you may only make interface ( or public member )
exposed to client users rather than implementation ( private member).
If the user can write non-member to directly access private member,
why C++ need access control? except you use some trickery by access
its memory directly.
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