Weak Pointers and Circular References in C++ 11: Listing 2

How to *not* deal with circular references in C++11.

// person.h – Buggy version (see Figure 1)
#ifndef PERSON_H_
#define PERSON_H_

#include <memory>
#include <string>

using namespace std;

class person {
  friend void get_couple_married(shared_ptr<person>, shared_ptr<person>);
public:
  person() = delete;
  person(const string);
  ~person();

  string get_name() const;
  shared_ptr<person> get_spouse() const;
private:
  string name_;
  typedef shared_ptr<person> spouse_ptr;
  spouse_ptr spouse_;

  void set_spouse(const shared_ptr<person>);
};

#endif /* PERSON_H_ */
/* ---------------------------------- */
// person.cpp
#include "person.h"
#include <iostream>
#include <stdexcept>

using namespace std;

person::person(const string name) : name_{name} {
  cout << name_ << " instance created." << endl;
}

person::~person() {
  cout << name_ << " instance to be disposed." << endl;
}

string person::get_name() const { return name_; }

shared_ptr<person> person::get_spouse() const { return spouse_; } 

void person::set_spouse(const shared_ptr<person> sp) {
  this->spouse_ = sp;
}

void get_couple_married(shared_ptr<person> husband, shared_ptr<person> wife) {
  // checks arguments aren’t null
  if ((husband==nullptr)||(wife==nullptr))
    throw invalid_argument("get_couple_married(husband, wife): can't get nullptr as couple member");

  // checks arguments aren’t the same instance
  if (husband.get()==wife.get())
    throw invalid_argument("get_couple_married(husband, wife): husband and wife can't be the same person");

  // Here’s where the issue is produced. Both husband and wife get each other’s shared_ptr
  // targeting them
  husband->set_spouse(wife);
  wife->set_spouse(husband);
}
/* ---------------------------------- */
// main.cpp
#include <iostream>
#include "person.h"

using namespace std;

void get_couple_married() {
  shared_ptr<person> husband = make_shared<person>("John"), wife = make_shared<person>("Pocahontas");
  get_couple_married(husband, wife);
  cout << husband->get_name() << "'s wife is " << husband->get_spouse()->get_name() << "." << endl;
  cout << wife->get_name() << "'s husband is " << wife->get_spouse()->get_name() << "." << endl;
}

int main() {
  get_couple_married();
  return 0;
}

About the Author

Diego Dagum is a software architect and developer with more than 20 years of experience. He can be reached at email@diegodagum.com.