1. Never though about enforcing derivation before, but yes, it looks as it may be useful. Especially if the utility class does not increase the size of the final objects. 2. The technique of using protected constructor and virtual inheritance to stop derivation does not look compiler-specific, limited to gcc 4.1.1. Just in case, I tested your codes under MinGW gcc 3.4.2 and under MSVC++ 8. Yes, the utility stops derivation. The following line is OK: Foo foo; but line Goo goo; does not compile. 3. If the final class is itself derived, using the utility involves multiple inheritances. Fortunately, it looks as in this context it is fine. Utility class does not have data members, and, any case, "rhombic" inheritance is impossible. 4. I still do not see why you need protected destructor: is not enough to have a protected constructor? Why is it bad to destroy an object of a final class by pointer? 5. It probably would be better to put the utility in a nested namespace under boost::. See, for example, noncopyable.hhp. Sincerely, Yura. On 3/3/07, Manuel Fiorelli <manuel.fiorelli@gmail.com> wrote:
Is there any interest in a small utility which let developers to define pseudo-final classes, meaning that it is not possible to instantiate objects belonging to derived classes.
Unfortunately, I could test my tool only on a Linux platform with the compiler gcc 4.1.1 and I am not sure the technique I used is portable to other compilers.
Usage --------------------------------------------------------------------// Suppose you want to define a final class Foo, then you have to write
class Foo : BOOST_NON_DERIVABLE { // .... };
Now, you try to subclass it
class Goo : public Foo { // ... };
When you use inheritance it is quite obvious, you want to instantiate objects of the classes involved in the hierarchy; thus, somewhere you will probably write
Goo goo; // a variant or a member of type Goo
But the compiler will produce an error like that
utility/nonderivable.hpp: In constructor 'Goo::Goo()': utility/nonderivable.hpp:23: error: 'boost::nonderivable_helper::nonderivable_helper()' is protected main.cpp:14: error: within this context utility/nonderivable.hpp:30: error: 'boost::nonderivable_helper::~nonderivable_helper()' is protected main.cpp:14: error: within this context main.cpp:14: error: no matching function for call to 'Foo::Foo()' main.cpp:9: note: candidates are: Foo::Foo(int) main.cpp:8: note: Foo::Foo(const Foo&) main.cpp: In function 'int main()': main.cpp:26: note: synthesized method 'Goo::Goo()' first required here utility/nonderivable.hpp: In destructor 'Goo::~Goo()': utility/nonderivable.hpp:30: error: 'boost::nonderivable_helper::~nonderivable_helper()' is protected main.cpp:14: error: within this context main.cpp: In function 'int main()': main.cpp:26: note: synthesized method 'Goo::~Goo()' first required here
Best regards, Manuel Fiorelli
_______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost