Question

Hi I was working on a bit of fun, making an interface to run gnuplot from within c++, and for some reason the my linked list implementation fails.

The code below fails on the line plots->append(&plot). Stepping through the code I discovered that for some reason the destructor ~John() is called immediately after the constructor John(), and I cannot seem to figure out why.

The code included below is a stripped down version operating only on Plot*. Originally I made the linked list as a template class. And it worked fine as ll<int> and ll<char*> but for some reason it fails as ll<Plot*>.

Could youp please help me figure out why it fails? and perhaps help me understand how to make it work?

In advance: Thanks a heap!

//B2S

#include <string.h>

class Plot{
  char title[80];
public:
  Plot(){  }
};

class Link{
  Plot* element;
  Link* next;
  Link* prev;
  friend class ll;
};

class ll{
  Link* head;
  Link* tail;
public:
  ll(){
    head = tail = new Link();
    head->prev = tail->prev = head->next = tail->next = head;
  }
  ~ll(){
    while (head!=tail){
      tail = tail->prev;
      delete tail->next;
    }
    delete head;
  }
  void append(Plot* element){
    tail->element = element;
    tail->next = new Link();
    tail->next->prev = tail;
    tail->next = tail;
  }
};

class John{
  ll* plots;
public:
  John(){
   plots= new ll();
  }
  ~John(){
    delete plots;
  }
  John(Plot* plot){
    John();
    plots->append(plot);
  }
};   

int main(){
  Plot p;
  John k(&p);
}

Answer 1

The statement:

 John();

Just constructs a new nameless John which is created and immediately destroyed. There is no way (in the current version of C++) to call a constructor from another constructor or on an already constructed object.

This means that the plots member of your John called k is never being initalized, hence the crash when you call append from the John constructor.

[As Neil Butterworth comments, if you had actually constructed an ll instance there are other issues with it, this is just the most immediate problem.]