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Sealing Classes in C++

, 2 Sep 2009 MIT
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A method to create sealed classes in C++

Introduction

Some recent languages like C# and Java allow you to seal your classes easily using a keyword like sealed or final respectively. C++ doesn't have any such keyword for this purpose. However, it's possible to still do it using a trick. When using virtual inheritance, the initialization list of the most-derived-class's constructor directly invokes the virtual base class's constructor. This means that if we can hide access to the virtual base class's constructor, then we can prevent any class from deriving from it. This mimics the effect of being sealed.

Solution Attempt #1

To provide an easy way to seal classes, we can write a header file Sealed.h like this:

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

#define Sealed private virtual SealedBase 

Now to seal a class, say Penguin, we just need to derive it from Sealed, like this:

#include "Sealed.h"

class Penguin : Sealed
{
}; 

That's it. Penguin is now a sealed class. Let's try deriving another class, BigZ (Surf's Up (2007), anyone?) from Penguin

class BigZ : Penguin
{
};

BigZ bigZ; // error C2248 

Instantiating an object of BigZ should yield a compiler error. The MSVC++ 2005 compiler gives me the following error message:

error C2248: 'SealedBase::SealedBase' : cannot access inaccessible member 
declared in class 'SealedBase'

A Serious Flaw

All seems to be working well. However, one of my fellow programmers, Angelo Rohit, pointed out to me that this method has a serious flaw in it. Angelo says that if BigZ derives from Penguin and Sealed, then it will be possible to create objects of BigZ:

class BigZ : Penguin, Sealed
{
};

BigZ bigZ; // OK; no compiler error 

Why does this happen? BigZ derives from Sealed just like Penguin does, which means that it now has access to Sealed's constructor. And since Sealed is inherited virtually by both Penguin and BigZ, there is only one copy of it - which is now also accessible to BigZ. Bummer. We need to have a mechanism by which BigZ is forced to call the constructor of a class which it doesn't have access to.

Solution Attempt #2

After pondering over this for a while, I realized that if we can somehow generate different base classes every time Sealed is derived from, then it would work.

Let's rewrite the Sealed.h header to look like this:

template <int T>
class SealedBase
{
protected:
    SealedBase()
    {
    }
};

#define Sealed private virtual SealedBase<__COUNTER__>

What does this do? SealedBase is now a templated class which takes an integer as an argument. __COUNTER__ is a predefined macro which expands to an integer starting with 0 and incrementing by 1 every time it is used in a compiland. So every time Sealed is derived from, it generates a new SealedBase class using the incremental number which __COUNTER__ expands to.

Now let's go back to our BigZ class which derives from both Penguin and Sealed:

class BigZ : Penguin, Sealed
{
};

BigZ bigZ; // error C2248

This time around though, BigZ can't escape from the compiler. Penguin derives from SealedBase<number1> and BigZ derives from SealedBase<number2>, where number1 and number2 are two non-identical integers. So now BigZ has to invoke the constructor of SealedBase<number1>, which it doesn't have access to.

The MSVC++ 2005 compiler gives me the following error message:

error C2248: 'SealedBase<T>::SealedBase' : cannot access inaccessible member 
declared in class 'SealedBase<T>'
1> with
1> [
1> T=0
1> ]

Portability Issues

However, you might be thinking that since we're using a special predefined macro __COUNTER__ in our implementation, this code is not portable. Well, it's supported by MSVC++ (which I used to test the above code) and also by GCC (http://www.gnu.org/software/gcc/gcc-4.3/changes.html).

But what about compilers which don't?

A Portable Solution 

After a little thought, I came up with the following way:

In Sealed.h:

template <class T>
class SealedBase
{
protected:
    SealedBase()
    {
    }
};

#define Sealed(_CLASS_NAME_) private virtual SealedBase<_CLASS_NAME_>

And to seal a class:

#include "Sealed.h"

class Penguin : Sealed(Penguin)
{
};

When sealing a class, we need to mention that class's name to the Sealed macro. This enables the Sealed macro to generate a new version of SealedBase. This is less elegant than simply having to derive from Sealed, but is more portable, making it a good alternative for compilers which don't support the __COUNTER__ predefined macro.

Final Words

People who use MSVC++ or GCC can simply use Solution Attempt #2, as it is cleaner. People on other compilers, can use the Portable Solution. If you have any questions, suggestions, improvements, or simply want to say hi, please email me.

Thanks for reading!
Francis Xavier

References

  1. C++ Q&A: List View Mode, SetForegroundWindow, and Class Protection
  2. Vladislav Lazarenko: "[boost] Sealed C++ class"

History

  • 2nd September, 2009: Initial post

License

This article, along with any associated source code and files, is licensed under The MIT License

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About the Author

Francis Xavier Pulikotil
Software Developer
United States United States
Besides loving spending time with family, Francis Xavier likes to watch sci-fi/fantasy/action/drama movies, listen to music, and play video-games. After being exposed to a few video-games, he developed an interest in computer programming. He currently holds a Bachelor's degree in Computer Applications.
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Comments and Discussions

 
QuestionThanks Francis Pinmemberconsthab210-Apr-13 4:44 
BugPortable solution flaw [modified] PinmemberYaroslav Lobachevski6-Dec-12 13:52 
GeneralThe soluion does not seem simple PinmemberMukit, Ataul20-Feb-11 8:02 
GeneralSealed class need does exist PinmemberBhushan198023-Feb-10 20:27 
GeneralSimplier way to make a class sealed Pinmembersergey nazarov8-Sep-09 2:37 
GeneralRe: Simplier way to make a class sealed Pinmemberemilio_grv9-Sep-09 23:26 
GeneralRe: Simplier way to make a class sealed PinmemberBhushan198023-Feb-10 20:05 
Not if there is a case of multiple inheritance as in case of Penguin and SealedBase in this example. I mean if there is a multiple inheritance only one constructor is invoked and that is Base class constructor. Also, by making all the constructors private, you might make a class sealed, but it is useless. By using this method, a developer can still access the base class methods and other classes in the hierarchy without inheriting the sealed class any further. If the constructor was private, there would not be any access to the base member functions by the way of sealed classes' instance created, which I believe is the sole purpose of the sealed class...
QuestionWhat is the purpose of a Sealed Class? Pinmemberhector santos2-Sep-09 19:03 
AnswerRe: What is the purpose of a Sealed Class? PinmemberTim Craig2-Sep-09 19:25 
GeneralRe: What is the purpose of a Sealed Class? Pinmemberemilio_grv4-Sep-09 3:42 
GeneralRe: What is the purpose of a Sealed Class? PinmemberTim Craig4-Sep-09 8:14 
GeneralRe: What is the purpose of a Sealed Class? PinmemberMJessick5-Sep-09 18:02 
GeneralRe: What is the purpose of a Sealed Class? Pinmemberryo-oh-ki7-Sep-09 10:49 
GeneralRe: What is the purpose of a Sealed Class? Pinmemberemilio_grv8-Sep-09 22:11 
GeneralRe: What is the purpose of a Sealed Class? PinmemberTim Craig9-Sep-09 20:22 
GeneralRe: What is the purpose of a Sealed Class? Pinmemberemilio_grv9-Sep-09 23:21 
Generalsealed PinmemberGoran Mitrovic2-Sep-09 9:08 
AnswerRe: sealed PinmemberRich Berlint7-Sep-09 21:06 
GeneralRe: sealed PinmemberFrancisXavier7-Sep-09 23:16 
GeneralInteresting... PinmemberAnandChavali2-Sep-09 4:27 

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