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Sealing Classes in C++

, 2 Sep 2009 MIT 31.6K 17
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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. Penguinis 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 BigZshould 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 BigZderives from Penguinand 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? BigZderives from Sealedjust like Penguindoes, which means that it now has access to Sealed's constructor. And since Sealedis inherited virtually by both Penguinand BigZ, there is only one copy of it - which is now also accessible to BigZ. Bummer. We need to have a mechanism by which BigZis 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 Sealedis 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? SealedBaseis 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 Sealedis derived from, it generates a new SealedBaseclass using the incremental number which __COUNTER__expands to.

Now let's go back to our BigZclass which derives from both Penguinand Sealed:

class BigZ : Penguin, Sealed
{
};

BigZ bigZ; // error C2248

This time around though, BigZcan't escape from the compiler. Penguinderives from SealedBase<number1> and BigZderives from SealedBase<number2>, where number1and number2are two non-identical integers. So now BigZhas 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 Sealedmacro. This enables the Sealedmacro 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

 
GeneralThe soluion does not seem simple Pin
Mukit, Ataul20-Feb-11 7:02
memberMukit, Ataul20-Feb-11 7:02 

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