Weak Pointers and Circular References in C++ 11: Listing 3

Weak_ptr is an exhaust vent for circular references.

// person.h – Correct version (see Figure 2)
#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:
  // nested class that models a smart pointer that could be strong or weak
  class spouse_ptr {
  private:
    shared_ptr<person> strong_;
    weak_ptr<person> weak_;
    spouse_ptr& set(const shared_ptr<person>);
  public:
    spouse_ptr();
    spouse_ptr(const shared_ptr<person>);

    spouse_ptr& operator=(const shared_ptr<person>);

    shared_ptr<person> get() const;
  };
  string name_;
  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 this->spouse_.get(); } 

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

person::spouse_ptr::spouse_ptr() : strong_{nullptr}, weak_{} {}
person::spouse_ptr::spouse_ptr(const shared_ptr<person> sp) : strong_{nullptr}, weak_{} {
  set(sp);
}

person::spouse_ptr& person::spouse_ptr::operator=(const shared_ptr<person> sp) {
  return set(sp);
}

shared_ptr<person> person::spouse_ptr::get() const {
  if (strong_!=nullptr)
    return strong_;
  else
    return weak_.lock();
}

person::spouse_ptr& person::spouse_ptr::set(const shared_ptr<person> sp) {
  // the strong or weak reference is used depending on 
  if (sp->get_spouse()==nullptr) {
    strong_ = sp;
    weak_.reset();
  } else {
    strong_ = nullptr;
    weak_ = sp;
  }

  return *this;
}

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");

  // the husband strongly points to his wife, while she weakly points back to him
  // (see function person::spouse_ptr::set)
  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 protected]

comments powered by Disqus

Featured

  • Black White Wave IMage

    Neural Regression Using PyTorch: Training

    The goal of a regression problem is to predict a single numeric value, for example, predicting the annual revenue of a new restaurant based on variables such as menu prices, number of tables, location and so on.

  • Microsoft Ships Visual Studio 2019 v16.9 Servicing Baseline Release

    Microsoft is urging enterprises and professional coders to standardize on the new Visual Studio 2019 v16.9, a servicing baseline release that's guaranteed to receive official support for an extended period.

  • Microsoft Extends Low-Code Push with Power Fx Programming Language

    "Using formulas that are already familiar to hundreds of millions of users, Power Fx allows a broad range of people to bring skills they already know to low code solutions."

  • Microsoft's Tools to Fight Solorigate Attack Are Now Open Source

    Microsoft open sourced homegrown tools it used to check its systems for code related to the recent massive breach of supply chains that the company has named Solorigate.

Upcoming Events