Re: [C++]: Multiple inheritage/Conflicts

[denilsson31 <denilsson31 at hotmail dot fr>]

First of all, thank you, Axel for answering.

Yes, this was my first guess regarding the compiler's message.

But, see again my trivial example. It's exactly the same design. Try
compiling. And it's troublesome.


Axel Freyn wrote:
> 
> Hi,
> 
> I think your code is no valid C++:
> On Sun, Nov 07, 2010 at 09:17:56AM -0800, denilsson31 wrote:
>> Hi,
>> 
>> I'm developping in Qt and I'm experiencing some compilation errors I
>> can't
>> correct.
>> I define a class, TemporaryDbFile that is a QFile's and dbFile's
>> derivate.
>> 
>> QFile is defined like this:
> (I corrected some typos in your code)
>> 
>> class QFile {
>> public:
>> virtual void close();
>> };
>> class dbFile {
>> public:
>> virtual int close() = 0;
>> };
>> 
>> So, TemporaryDbFile has the duty to implement dbFile's interface with
>> QTemporaryFile (derivated from QFile) :
>> 
>> class TemporaryDbFile :public QFile, public dbFile
>> {
>> int close() { }
>> };
>> 
>> But in this nice picture, there is a sand grain : the gcc compiler 4.4.0.
>> He tells, mockingly :
>> 
>> TemporaryDbFile.h:42:error: conflicting return type specified for
>> 'virtual
>> int TemporaryDbFile::close()'
>> C:\Qt\2010.03\qt\include\QtCore\..\..\src\corelib\io\qfile.h:148:error:  
>> overriding 'virtual void QFile::close()'
> The critical point is: The base-class "QFile" defines "close" to be a
> VIRTUAL function! So, when you define "int close()" in the derived
> class, C++ believes you want to override the virtual function defined in
> the base classes: those defined in QFile AND in dbFile. However,
> "close()" in QFile has a different return type as the one defined in
> TemporaryDbFile ==> Thus the error message from the compiler.
>> 
>> Nevertheless, this minimalist sample, representative of my concern is
>> accepted by the compiler:
>> 
>> class A {
>>     public:
>>     void close() {}
>> };
>> 
>> class B {
>>     public:
>>     virtual int close() = 0;
>> };
>> 
>> 
>> class C :public A, public B
>> {
>> public:
>> int close()
>> {
>>     A::close();
>>     return 0;
>> }
>> };
> In this example, you ommitted the "virtual" in class A, thus defining in
> class A a non-virtual function. If you define now "int close()" in class
> C, this new function will override the non-virtual function "void
> close()" from class A (which is perfectly fine).
> 
> So, to state it differently:
> 1) If you create a "TemporaryDbFile" and access it as QFile:
> QFile &q = TemporaryDbFile();
> q.close()
> Here, the caller expects that "close" returns "void"; however
> (QFile::close() beeing a virtual function) the code will call
> "TemporaryDbFile::close()" which returns an integer ==> that's why the
> compiler won't compile this code.
> 
> 2) if you create a "C" and access it as A:
> A& a = C();
> a.close();
> Here, as "A::close()" is NO virtual function, the code will just execute
> "A::close()" (which returns void), completely ignoring the "C::close()"
> returning an int ==> everything works fine and the code compiles.
> 
> So, I think you have to decide what the code shall do -- and then
> correct your code
> 
> (At least, thats what I think -- Maybe I'm wrong with my interpretation
> of the virtual functions mechanisms here -- I don't have a C++-Standard
> handy...)
> 
> 
> HTH,
> 
> Axel
> 
> 

-- 
View this message in context: http://old.nabble.com/-C%2B%2B-%3A-Multiple-inheritage-Conflicts-tp30152349p30155906.html
Sent from the gcc - Help mailing list archive at Nabble.com.