Click here to Skip to main content
Click here to Skip to main content

Sealing Classes in C++

, 2 Sep 2009
Rate this:
Please Sign up or sign in to vote.
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

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.
Follow on   LinkedIn

Comments and Discussions

 
GeneralRe: What is the purpose of a Sealed Class? PinmemberTim Craig4-Sep-09 7:14 
emilio_grv wrote:
there could be things that by design I don't want to do I can forget about later. By having a mechanism that forces me into a compile error makes me wonder abut what the problem is.

 
If you can forget such details, then maybe a comment or two might be appropriate. And in general, I've found that other people's code isn't nearly as obvious as they think it is anyhow.
 
emilio_grv wrote:
may be because the internal functionality of the class may change in the future, and I don't want to risk to create supportability problems, or because the "class" is just a language way to expose a functionality that's not internally a class (so whatever derivation mechanism will simply not work as intended by a language's users)

 
So you're basically saying that you can look into your crystal ball and tell me that you can see every foreseeable way I might use your class and you've got it all covered? I don't think so. And I think there was an article I saw go by recently although I didn't read it on how to break "sealed" so someone has run into the problem and given it some thought rather than just give up.
 
emilio_grv wrote:
But if you think to Java, C#, D and other languages where "sealed" is a keyword (so the compiler really knowns about it),

 
Another reason for me to continue my boycott of those languages.
 
emilio_grv wrote:
Hope this may give you a more wide perspective.

 
Yes, if I'm ever in a position to interview people for programming jobs again, I should ask them how they feel about sealed classes.
 
You measure democracy by the freedom it gives its dissidents, not the freedom it gives its assimilated conformists.

GeneralRe: What is the purpose of a Sealed Class? PinmemberMJessick5-Sep-09 17:02 
GeneralRe: What is the purpose of a Sealed Class? Pinmemberryo-oh-ki7-Sep-09 9:49 
GeneralRe: What is the purpose of a Sealed Class? Pinmemberemilio_grv8-Sep-09 21:11 
GeneralRe: What is the purpose of a Sealed Class? PinmemberTim Craig9-Sep-09 19:22 
GeneralRe: What is the purpose of a Sealed Class? Pinmemberemilio_grv9-Sep-09 22:21 
Generalsealed PinmemberGoran Mitrovic2-Sep-09 8:08 
AnswerRe: sealed PinmemberRich Berlint7-Sep-09 20:06 
GeneralRe: sealed PinmemberFrancisXavier7-Sep-09 22:16 
GeneralInteresting... PinmemberAnandChavali2-Sep-09 3:27 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.

| Advertise | Privacy | Mobile
Web01 | 2.8.140709.1 | Last Updated 2 Sep 2009
Article Copyright 2009 by Francis Xavier Pulikotil
Everything else Copyright © CodeProject, 1999-2014
Terms of Service
Layout: fixed | fluid