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
