How can I call pure virtual method? 2007-12-5(Wed)

• Sometimes my program aborted with a message, ''pure virtual method called,'' but how it is possible? If I did not implement the pure virtual method in a derived class, I can not instantiate it.
• But, it is possible to call a pure virtual method. It is a bug.
• This is a point that a constructor is not an initializer, but only an constructer of the object. So, some of the libraries put initialize() method separately at many classes. See the following code:

  //
  // How to call a pure virtual method.
  // Hitoshi Yamauchi 2007-12-5(Wed)
  //
  // bash % g++ -g 10purevirtual_is_called.cc
  // bash % ./a.out
  // pure virtual method called
  // terminate called without an active exception
  // Aborted
  // (You need usually -g option to see this message.)
  //
  #include <iostream>
  
  /// Base class
  class Base {
  public:
    /// constructor
    Base(){
      this->use_pure_virtual_method();
    }
    /// A pure virtual method.
    virtual void output_classname() = 0;
    /// calling the pure virtual method in this method.
    void use_pure_virtual_method(){
      this->output_classname(); // implemented in the derived class.
    }
  };
  
  /// Derived class
  class Derived : public Base {
  public:
    /// constructor
    Derived() : Base() {
      // empty
    }
    /// implementation of the output_classname.
    virtual void output_classname(){
      std::cout << "I am [Derived]" << std::endl;
    }
  };
  
  /// main
  int main(){
    Derived * p_derived = new Derived;
    return 0;
  }
  

• The trick is call the pure virtual method from the base class's constructor indirectory. (Direct call can be detected by compiler and linker). During the construction of the derived class, there is a time only the base class is constructed and the derived class has not yet existed. But only the virtual method pointer table is there. Then you can call the not-yet-existed method. This shows a case, a constructor is not an initializer, but just an constructor.
• Using initialize() method is one solution, but in that case, sometimes you can forget to call it. Even so, I prefer to use initialize() method. Since there is a case, initialization is depends on the derived class. In such initialization, I sometimes want to use the template pattern (Design pattern), thus there are sometimes pure virtual methods.