TITLE: non-derivable classes

(Source: comp.lang.c++.moderated, 15 May 2001)


ALEXANDRESCU: Andrei Alexandrescu

In CUJ on May 2001, Shanker Chandrabose gives a great tip on making classes
non-derivable.

It's very strange that until now everybody used to go for the friend-based
solution:

class NonDerivableHelper
{
    NonDerivableHelper() {}
    friend class NonDerivable;
};

class NonDerivable : private virtual NonDerivableHelper
{
    ... your code here ...
};

This idiom was discussed a number of times around here. People agree it's a
pity you can't make NonDerivableHelper a library class because friends who
are template arguments are not allowed.

Anyway, Chandrabose comes with an idiom that doesn't require friend:

class NonDerivableHelper
{
protected:
    NonDerivableHelper() {}
};

class NonDerivable : private virtual NonDerivableHelper
{
    ... your code here ...
};

Cool! I wonder if there's any flaw in his idea.

Now I think the next step is to put this in a library and to optimize away
the virtual inheritance overhead:

// library header

namespace Private
{
    class NonDerivableHelper
    {
    protected:
        NonDerivableHelper() {}
    };
}

#ifdef NDEBUG
#define FINAL_CLASS
#else
#define FINAL_CLASS : private virtual Private::NonDerivableHelper
#endif

// application code

class NonDerivable FINAL_CLASS
{
    ... your code here ...
};



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Several people wrote to say that the code recently presented did not
prevent deriving from a class. This is true. To fix the code so that
it works as expected make two changes as marked below. Now class D1
fails to compile as desired.


#define FINAL_CLASS : private virtual Private::NonDerivableHelper

namespace Private
{
    class NonDerivableHelper
    {
    protected:
        NonDerivableHelper(int) {} <<<<<<<<< ADDED CTOR ARGUMENT
    };
}

class NonDerivable FINAL_CLASS
{
   int x;

public:
	NonDerivable() : NonDerivableHelper(0) {} <<<<<<<<< PASS A DUMMY VALUE
  virtual void foo() { printf("In NonDerivable\n"); }
};


class D1: public NonDerivable
{
public:
	D1() : NonDerivable() {};
	virtual void foo() { printf("In D1\n"); NonDerivable::foo(); }
};


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-----Original Message-----
From: cpptips-admin@cpptips.hyperformix.com
[mailto:cpptips-admin@cpptips.hyperformix.com] On Behalf Of Allan Clarke
Sent: 2001/05/25 Friday 7:43 AM
To: cpptips
Subject: [cpptips] final class again


Several people wrote to say that the code recently presented did not
prevent deriving from a class. This is true. To fix the code so that
it works as expected make two changes as marked below. Now class D1
fails to compile as desired.


#define FINAL_CLASS : private virtual Private::NonDerivableHelper

namespace Private
{
    class NonDerivableHelper
    {
    protected:
        NonDerivableHelper(int) {} <<<<<<<<< ADDED CTOR ARGUMENT
    };
}

class NonDerivable FINAL_CLASS
{
   int x;

public:
	NonDerivable() : NonDerivableHelper(0) {} <<<<<<<<< PASS A DUMMY VALUE
  virtual void foo() { printf("In NonDerivable\n"); }
};


class D1: public NonDerivable
{
public:
	D1() : NonDerivable() {};
	virtual void foo() { printf("In D1\n"); NonDerivable::foo(); }
};


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