Error"pure virtual method called",when this method has been override - c++

Question

I'm trying to practice "Observer Design Pattern". When I thought a abstract's pure virtual method has been override by it's derived class, a error occurred.
There is a observer which is an abstract class in a independent file:
#ifndef DESIGN_PATTERNS_OBSERVER_H
#define DESIGN_PATTERNS_OBSERVER_H
#include "subject.h"
class Subject;
class Observer{
protected:
Observer();
public:
virtual ~Observer();
virtual void update(Subject *the_changed_subject) = 0;
};
Observer::Observer() {}
Observer::~Observer() {}
#endif //DESIGN_PATTERNS_OBSERVER_H
Observer defined a pure virtual method "update" which overrides as follow:
#ifndef DESIGN_PATTERNS_CONCRETE_OBSERVER_H
#define DESIGN_PATTERNS_CONCRETE_OBSERVER_H
#include <iostream>
#include "observer.h"
#include "concrete_subject.h"
class ConcreteObserver : public Observer{
public:
void update(Subject *the_changed_subject) override {
auto cs = dynamic_cast<ConcreteSubject *>(the_changed_subject);
std::cout << "status changed to " << cs->get_status() << std::endl;
}
};
#endif //DESIGN_PATTERNS_CONCRETE_OBSERVER_H
And also there is a subject which is an abstract class too.The error "pure virtual method called" happened in "notify" method where I had marked.
From debug, it seems "notify" uses Observer's "update" rather than ConcreteObserver's.
However,in main function the _observers should stored pointers of ConcreteObservers which override "update".
#ifndef DESIGN_PATTERNS_SUBJECT_H
#define DESIGN_PATTERNS_SUBJECT_H
#include <list>
#include "observer.h"
class Subject {
private:
std::list<Observer*> *_observers;
protected:
Subject();
public:
virtual ~Subject();
virtual void attach(Observer*);
virtual void detach(Observer*);
virtual void notify();
};
Subject::Subject() {
_observers = new std::list<Observer*>;
}
Subject::~Subject() {
delete _observers;
}
void Subject::attach(Observer *o) {
_observers->push_back(o);
}
void Subject::detach(Observer *o) {
_observers->remove(o);
}
void Subject::notify() {
for (Observer* observer : *_observers) {
//here is where error comes out, found by debug
observer->update(this);
}
}
#endif //DESIGN_PATTERNS_SUBJECT_H
And it has a derived class "ConcreteSubject":
#ifndef DESIGN_PATTERNS_CONCRETE_SUBJECT_H
#define DESIGN_PATTERNS_CONCRETE_SUBJECT_H
#include "subject.h"
class ConcreteSubject : public Subject {
private:
int status;
public:
ConcreteSubject() {
status = 0;
}
void set_status(int s) {
this->status = s;
Subject::notify();
}
int get_status() {
return status;
}
};
#endif //DESIGN_PATTERNS_CONCRETE_SUBJECT_H
The main function:
#include <iostream>
#include <vector>
#include "singleton.h"
#include "observer/concrete_subject.h"
#include "observer/concrete_observer.h"
void test2() {
ConcreteSubject concreteSubject;
std::vector<ConcreteObserver> observers;
for (int i = 0; i < 5; ++i) {
ConcreteObserver observer = ConcreteObserver();
concreteSubject.attach(&observer);
observers.push_back(observer);
}
concreteSubject.set_status(2);
}
int main() {
test2();
return 0;
}
As I mentioned before, the _observers of ConcreteSubject's super class Subject should stored pointers of ConcreteObservers which override "update" already.
I don't understand why Observer's "update" still called.
Here is another strange thing.I make a small test has almost the same relationship of classes I showed.But no error occured.
class ABaseA{
public:
virtual void do_some() = 0;
};
class MidA : public ABaseA{
public:
void do_some() override {
cout << "real do some" << endl;
}
};
class ABaseB{
private:
list<ABaseA*> *bases;
public:
ABaseB() {
bases = new list<ABaseA*>();
}
virtual ~ABaseB() = default;
virtual void add(ABaseA* item) {
bases->push_back(item);
}
virtual void do_active() {
for(ABaseA *p : *bases) {
p->do_some();
}
}
};
class MidB : public ABaseB{
public:
MidB() = default;
void active() {
ABaseB::do_active();
}
};
void test3() {
MidA midA;
MidB midB;
midB.add(&midA);
midB.active();
}
The only difference is this code is in one file.

Answer 1

In the file of Subject.h you should be transfer below code to Subject.cpp:
Subject::Subject() {
_observers = new std::list<Observer*>;
}
Subject::~Subject() {
delete _observers;
}
void Subject::attach(Observer *o) {
_observers->push_back(o);
}
void Subject::detach(Observer *o) {
_observers->remove(o);
}
void Subject::notify() {
for (Observer* observer : *_observers) {
//here is where error comes out, found by debug
observer->update(this);
}
}
Also you should be add class Observer; in top of Subject.h
#include <list>
#include "Observer.h"
class Observer; //you should be add this line
class Subject {
private:
std::list<Observer*> *_observers;
protected:
Subject();
public:
virtual ~Subject();
virtual void attach(Observer*);
virtual void detach(Observer*);
virtual void notify();
};