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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

 
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 
Tim, the world is not so... boolean!
The purpose to prohibit things that are not supposed to happen has in general two main goals:
  1. Prevent mistakes
  2. Give chances to more optimizations
 
In this specific case, even if I'm writing for myself, 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.
 
And -in any case- I can public a class because I want you to create object instances but not new object types (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)
 
Also, If I know that a class will never be derived, letting the compiler to know can open to it more chances in optimizing the code (that's not the case of C++, the trick explained here give not evidence of that to the compiler),
But if you think to Java, C#, D and other languages where "sealed" is a keyword (so the compiler really knowns about it), that could mean to the compiler "you can discard any V-table mapping export into an import library since I grant nobody else will ever touch this". And that's are all implicit pointers that are not encoded and redirection that are skipped away.
 
Hope this may give you a more wide perspective.
 

2 bugs found.
> recompile ...
65534 bugs found.
D'Oh! | :doh:


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 

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