pt.cpp

#include "pt.h"
#include <iostream>
#include <vector>
#include <string>

using namespace std;

static int bid = 0;

double cost(int items, double price = 0.8);

double cost(int items, double price) { return items * price; }

Button::Button(int a, int b): id{bid++}, a{a}, b{b}{
  cout << "🍎 Creating object " << this->id << endl;
}

Button::Button() : Button(6, 6) {}
Button::Button(int x) : Button(x, x) {}

Button::~Button() { cout << "🗑 Deleting object " << this->id << endl; }

Button::Button(const Button &old): id{bid++}, a{old.a}, b{old.b}{
  cout << "⎘ Copying object " << old.id << " to " << this->id << endl;
}

Button::Button(Button &&old) : id{old.id}, a{old.a}, b{old.b} {
  cout << "✂ Moving object " << old.id << endl;
}

int Button::getArea() const{ return a * b; }

void Button::setA(int a) { this->a = a; }

void Button::setB(int b) { this->b = b; }

ostream &operator<<(ostream &out, Button button) {
  out << "🔘 " << button.id << ": a=" << button.a << ", b=" << button.b << ", area=" << button.getArea();
  return out;
}

ostream &operator<<(ostream &out, Button *button) {
  out << "🔘 " << button->id << ": a=" << button->a << ", b=" << button->b << ", area=" << button->getArea();
  return out;
}

int main() {
  // Reference is an alias to a variable
  int a = 8;
  int &b = a;
  b = 9;
  cout << "a: " << a << " b: " << b << endl;

  ////////////////////////////////////////////////////////////////////////////////
  // Store pointers in vector
  // This won't work.
  vector<Button *> bs0;
  cout << string(80, '=') << endl;
  cout << "Testing bs0, objects with references" << endl;
  for (auto i = 0; i < 8; i++) {
    // DON'T do this
    // Allocated on stack
    Button button {i};
    // Pass address to an object on stack
    bs0.push_back(&button);
    // button object deleted!
  }

  cout << "bs0: " << endl;
  for (auto button : bs0) {
    cout << button << endl;
  }
  cout << string(80, '=') << endl;

  ////////////////////////////////////////////////////////////////////////////////
  // Store objects in vector
  // This works, but creates tooo many objects
  cout << "Testing bs1, objects" << endl;
  vector<Button> bs1; // vector will take care of garbage collection
  for (auto i = 0; i < 8; i++) {
    Button button {i, i}; // will be allocated on stack and deleted at the end of current loop
    // when adding objects, vector makes a copy
    // bs1.push_back(button);
    // explicitly move instead of copy
    bs1.push_back(move(button));
    // or better
    // bs1.push_back(Button{i, i});
  }

  cout << "bs1: " << endl;
  for (auto button : bs1) {
    cout << button << endl;
  }
  cout << string(80, '=') << endl;

  ////////////////////////////////////////////////////////////////////////////////
  // Store pointers in vector
  // This works, just make sure to delete all objects at the end
  cout << "Testing bs2, pointers" << endl;
  vector<Button *> bs2; // need to free memory of objects in the vector
  for (auto i = 0; i < 8; i++) {
    Button *button = new Button(i);
    bs2.push_back(button);
  }

  cout << "bs2: " << endl;
  for (auto button : bs2) {
    cout << button << endl;
  }
  cout << string(80, '=') << endl;
  cout << string(80, '=') << endl;


  // Using vector of objects involves object copying
  cout << "bs1 before direct modification: " << endl;
  for (auto button: bs1){
    button.setA(99);
  }

  cout << "bs1 after direct modification: " << endl;
  for (auto button : bs1) { // implicit copying occurs
    cout << button << endl;
  }
  cout << string(80, '=') << endl;

  cout << "bs1 before modification with reference: " << endl;
  for (auto &button : bs1) {
    button.setA(99);
  }

  cout << "bs1 after modification with reference: " << endl;
  // You can use `const` qualifier to prevent changing object
  for (auto const &button : bs1) {
    cout << button << endl;
  }
  cout << string(80, '=') << endl;

  // free memory for vector of pointers
  for (auto p: bs2){
    delete p;
  }
  bs2.clear();
  return 0;
}

pt.h

#ifndef __pt__
#define __pt__
#include <iostream>

class Button{
  int id;
  int a;
  int b;
public:
  Button();
  Button(int x);
  Button(int a, int b);
  // copy constructor
  Button(const Button &old);
  // move constructor
  Button(Button &&old);
  ~Button();

  int getArea() const;
  void setA(int a);
  void setB(int b);

  // object to string operators
  friend std::ostream &operator<<(std::ostream &out, Button button);
  friend std::ostream &operator<<(std::ostream &out, Button *button);
};
#endif