final class in c++

Question

class Temp
{
private:
    ~Temp() {}
    friend class Final;
};

class Final : virtual public Temp
{
public:
     void fun()
     {
         cout<<"In base";
     }
};

class Derived : public Final
{
};

void main()
{
    Derived obj;
    obj.fun();
}

The above code tries to achieve non-inheritable class (final). But using above code the object of derived can still be created, why?

The desired functionality is achieved only if ctor made private, my question is why it is not achievable in case of dtor private?

Answer 1

The C++ FAQ describes different ways to achieve this – but from your question I guess you’ve already read them. ;-)

(Also, main must always return int, never void.)

Answer 2

non-inheritable, final, classes do not exist in C++.

Answer 3

The derived class does not call the private destructor of the base class, hence it does not need visibility.

Make your constructors private and only provide a static generator function.

Answer 4

Well, for this program (pleasse provide correct, compilable examples)

#include <iostream>

class Temp
{
private:
    ~Temp() {}
    friend class Final;
};

class Final : virtual public Temp
{
public:
    void fun() { std::cout<<"In base"; }
};

class Derived : public Final {};

int main() {
    Derived obj;
    obj.fun();
}

Comeau Online says

Comeau C/C++ 4.3.10.1 (Oct  6 2008 11:28:09) for ONLINE_EVALUATION_BETA2
Copyright 1988-2008 Comeau Computing.  All rights reserved.
MODE:strict errors C++ C++0x_extensions

"ComeauTest.c", line 16: error: "Temp::~Temp()" (declared at line 6) is inaccessible
  class Derived : public Final {
                         ^
          detected during implicit generation of "Derived::Derived()" at line
                    21

"ComeauTest.c", line 16: error: "Temp::~Temp()" (declared at line 6) is inaccessible
  class Derived : public Final {
        ^
          detected during implicit generation of "Derived::~Derived()" at line
                    21

2 errors detected in the compilation of "ComeauTest.c".

Since, when in doubt, I always trust como (I have only ever found one error in it, but many in other compilers), I suppose VC9 (which accepts the code) is in error. (From that void main() I suppose you use VC, too.)

Answer 5

Ughm, am I missing something:

$ g++ -o inherit inherit.cpp 
inherit.cpp: In constructor ‘Derived::Derived()’:
inherit.cpp:9: error: ‘Temp::~Temp()’ is private
inherit.cpp:21: error: within this context
inherit.cpp: In function ‘int main()’:
inherit.cpp:25: note: synthesized method ‘Derived::Derived()’ first required here 
inherit.cpp: In destructor ‘Derived::~Derived()’:
inherit.cpp:9: error: ‘Temp::~Temp()’ is private
inherit.cpp:21: error: within this context
inherit.cpp: In function ‘int main()’:
inherit.cpp:25: note: synthesized method ‘Derived::~Derived()’ first required here