Virtual function 'BasicMidReader::~BasicMidReader()' conflicts with base class 'base 'TMemoryStream'

[tomek]

Can anybody explain why can I not get rid of the above message
in the following class definition? [I am using Borland C++ Builder 5.0]
Thank you in advance.


class BasicMidReader : public TMemoryStream
{
//some private date
public:
BasicMidReader(char* FN)
{// some code}
~BasicMidReader() {// some code}
};

 

[David Fisher]

Can anybody explain why can I not get rid of the above message
in the following class definition? [I am using Borland C++ Builder 5.0] ....
class BasicMidReader : public TMemoryStream
{
//some private date
public:
BasicMidReader(char* FN)
{// some code}
~BasicMidReader() {// some code}
};

I haven't used Borland C++, but just a guess:

"virtual function 'BasicMidReader::~BasicMidReader()' conflicts with base
class" sounds like TMemoryStream could have a virtual destructor, and you
haven't declared ~BasicMidReader() as virtual. If so, I would expect this to
be a warning rather than an error, since inherited virtual functions are
also virtual - even if they don't say so (but they should !)

(BTW: in general, all destructors should be virtual unless you are sure the
class will never be used as a base class - this even applies if the
destructor is empty (ie. there is nothing to explicitly delete)).

David F

 

[Victor Bazarov]

Can anybody explain why can I not get rid of the above message
in the following class definition? [I am using Borland C++ Builder 5.0] ...
class BasicMidReader : public TMemoryStream
{
//some private date
public:
BasicMidReader(char* FN)
{// some code}
~BasicMidReader() {// some code}
};

I haven't used Borland C++, but just a guess:

"virtual function 'BasicMidReader::~BasicMidReader()' conflicts with base
class" sounds like TMemoryStream could have a virtual destructor, and you
haven't declared ~BasicMidReader() as virtual. If so, I would expect this to
be a warning rather than an error, since inherited virtual functions are
also virtual - even if they don't say so (but they should !)

Why should they? If the language says that 'if any base class has
a virtual destructor, the class' destructor is virtual even if not
declared such', why declare it virtual? It's virtual already.

(BTW: in general, all destructors should be virtual unless you are sure the
class will never be used as a base class - this even applies if the
destructor is empty (ie. there is nothing to explicitly delete)).

That is nonsense. Even if you use some class as a base class for
something else, its destructor doesn't have to be virtual if you
will never delete a dynamically allocated object polymorphically.

Declaring functions virtual when there is no need, only leads to
performance degradation and wasted space.

Victor

 

[David Fisher]

Why should they? If the language says that 'if any base class has
a virtual destructor, the class' destructor is virtual even if not
declared such', why declare it virtual? It's virtual already.

Just to make it clearer to the reader.

That is nonsense. Even if you use some class as a base class for
something else, its destructor doesn't have to be virtual if you
will never delete a dynamically allocated object polymorphically.

Declaring functions virtual when there is no need, only leads to
performance degradation and wasted space.

Of course; I should have said, "unless you are sure the class will never be
used as a polymorphic base class." But assuming that the writer does not
have complete control over the future use of the class (which is most of the
time in a software company), you never know how the class is going to be
modified over time. It is also easy to forget to make a destructor virtual,
and hard to track this down if there is a problem (it happened to me !)

David F