Member Avatar for kedar_challa

Hi,

Can anybody explain me how to create a sealed class on c++.

thanks in advance,
kedar

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  • Latest Post Latest Post by Karty619

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Short answer: if you can target C++11 or later, use the built-in final specifier. It is explicit, portable and gives a clear compile-time error when someone tries to derive.

class Sealed final {
public:
    Sealed() = default;
};

// error: class Derived : public Sealed { }; // base 'Sealed' is 'final'

For pre-C++11 compilers there are a few workarounds. The most common portable trick is to make all constructors private and provide a static factory; that prevents normal derivation because derived classes cannot call the base constructor. Be careful with special-member functions (copy/move/assignment): declare or delete them explicitly so the compiler does not create an unexpected public constructor. Another option on some toolchains is a vendor-specific language extension, but those are non-portable.

A note on 's macro approach: it does not reliably "seal" a class. Making a virtual base with a protected constructor still leaves derived classes able to access that protected constructor through the inheritance chain, so further derivation can compile. Also the example uses obsolete headers and other non-portable bits. As suggested, consider first whether you really need to forbid inheritance — often composition or documenting intended extension points is a better design.

Practical guidance: prefer final when available; if you must support older compilers, use the private-constructor + factory pattern and clearly document the reason. If only specific virtual functions should be closed for override, mark those functions final (or make them non-virtual) rather than sealing the entire class.

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Member Avatar for kedar_challa

A start?

i know this solution using private constructor.

So, the user has to call the static method to create the new object. Is there anything better than this.

Member Avatar for Narue

>Is there anything better than this.
No, not really. The question you should be asking yourself is why do you need this feature in the first place? Clicky.

Member Avatar for Karty619 
#include <iostream.h>
#include <conio.h>

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

#define Sealed private virtual SealedBase 

class Penguin : Sealed
{
};

class BigZ : Penguin
{
};

void main()
 {
BigZ bigZ;//cannot create obj beacuse penguin is sealed
  clrscr();
  cout<<"Sample for selaed":
  getch();
  }
Edited by Narue because: Added code tags
commented: 6 year bump, no code tags, void main rubbish for TurboC -4
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