Reading and rereading a file with QXmlStreamReader

[lni]

It doesn't matter as typically you are subclassing QXmlStreamReader to make your own reader supposed to read your own specific XML files, ao you add some methods which use QXmlStreamReader's methods.

What I have done is by following qt designer, by making various Dom objects and let QXmlStreamReader/QXmlStreamWriter read/write the data.

I don't think Qt people would make such mistake by not even providing a virtual destructor if they think people could use it as a based class, of course C++ doesn't stop people from doing it...unlike the keyword "final" in java...

[wysota]

I don't think Qt people would make such mistake by not even providing a virtual destructor if they think people could use it as a based class, of course C++ doesn't stop people from doing it...unlike the keyword "final" in java...

Correct me if I'm wrong but virtual destructor is needed only if you have some other methods declared as virtual. Lack of virtual methods doesn't prevent you from adding new methods.

[lni]

Correct me if I'm wrong but virtual destructor is needed only if you have some other methods declared as virtual. Lack of virtual methods doesn't prevent you from adding new methods.

virtual destructor is necessary if the class is intended to be a based class regardless the class itself has virtual methods or not. Otherwise they could be memory leak.

For instance, in the following code, B's dtor will not be called because A's dtor is not virtual. If you make A's dtor virtual, then the memory will be properly freed.

In java, you could add "final" keyword so no one can derive from A, but in C++ there is no such keyword...I guess this is one of C++'s weakness...

#include <iostream>
 
class A {
public:
  ~A() {
    std::cout << "A::~A called" << std::endl;
  }
 
};
 
class B : public A {
public:
 
  ~B() {
    std::cout << "B::~B called" << std::endl;
  }
 
};
 
int main()
{
  A* a = new B;
  delete a;
 
}

#include <iostream>

class A {
public:
  ~A() {
    std::cout << "A::~A called" << std::endl;
  }

};

class B : public A {
public:

  ~B() {
    std::cout << "B::~B called" << std::endl;
  }

};

int main()
{
  A* a = new B;
  delete a;

}

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[wysota]

Who said you had to use the class this way? If you cast the object to the base class then you have a point in doing so - you expect some method to be virtual. If you just use the object of the subclass as an extension of the superclass then both destructors will be called. If you have a stack-based object (and we're dealing with one here) then everything will be alright.

int main(){
  B b;
  return 0;
}

int main(){
  B b;
  return 0;
}

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[talk2amulya]

In java, you could add "final" keyword so no one can derive from A, but in C++ there is no such keyword...I guess this is one of C++'s weakness...

if you keep a destructor of a class as private, most compilers would not let that class be subclassed..at least thats how Stroustrup intended it, some compilers seem to have ignored that..but all GNU compilers would not let you subclass a class with private destructor..so its no weakness of C++..anything that java can do, c++ can do it on one leg with high heels..with less memory consumption

[wysota]

anything that java can do, c++ can do it on one leg with high heels..with less memory consumption

Except for proper exception handling

[lni]

if you keep a destructor of a class as private, most compilers would not let that class be subclassed..at least thats how Stroustrup intended it, some compilers seem to have ignored that..but all GNU compilers would not let you subclass a class with private destructor..so its no weakness of C++..anything that java can do, c++ can do it on one leg with high heels..with less memory consumption

How do you delete the object if the dtor is private? You will have to provide a public "release" method to call when you want to delete it, and you won't be able to create an object on stack with private dtor...

For a class without virtual dtor, the object will have no vtable, you won't gain anything to use "IsA" relationship. I would use "hasA" relationship to be more intuitive and avoid possible memory leak. Again, C++ don't stop you from deriving from it....

[spam]

void QXmlStreamReader::setDevice(QIODevice * device)

void QXmlStreamReader::setDevice(QIODevice * device)

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Sets the current device to device. Setting the device resets the stream to its initial state.

See also device() and clear().

http://doc.qt.io/qt-4.8/qxmlstreamre...l#namespaceUri