QPointer is a Qt class to hold a pointer. Such a class is called a smart pointer, yet -in my humble opinion- QPointer behaves unexpectedly.

Prefer to use smart pointers over normal pointers [1].


#include <cassert>

#include <iostream>

#include <QPointer>



//QPointer can only hold classes derived from QObject

struct Test : public QObject

{

  Test(const int x) : m_x(x) { std::cout << "~Test\n"; }

  ~Test()

  {

    std::clog << "~Test\n";

  }

  const int m_x;

};



int main()

{

  {

    QPointer<Test> p;

    assert(!p);

    assert(!p.data());

    assert(p.isNull());

  }

  {

    QPointer<Test> p(new Test(42));

    assert(p);

    assert(p.data());

    assert(!p.isNull());

    std::clog << p->m_x << '\n';

    std::clog << "~Test will be called after this\n";

  }

  std::clog << "~Test should have been called before this\n";

}

Screen output:


~Test

42

~Test will be called after this

~Test should have been called before this

Expected screen output:


42

~Test will be called after this

~Test

~Test should have been called before this

References

Herb Sutter, Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. 2005. ISBN: 0-32-111358-6. Chapter 13: 'Ensure resources are owned by objects. Use explicit RAII and smart pointers.

Go back to Richel Bilderbeek's C++ page.

Go back to Richel Bilderbeek's homepage.

(C++) QPointer

References