55: Implement a Basic Object System

This exercise is pending. Quick notes about this exercise:

• This will effectively use functions on structs to recreate everything in OOP except inheritance

The Code

First, using functions that live outside of the struct, kind of C-style OOP:

View Source file ex55a.cpp Only

#include <fmt/core.h>
#include <chrono>
#include <thread>
#include <string>
#include <functional>
#include <memory>

using std::shared_ptr, std::make_shared, std::string;

struct Person {
  std::string name;
  int age;
};

shared_ptr<Person> Person_new(const string& name, int age) {
  auto self = make_shared<Person>(name, age);
  return self;
}

void Person_talk(shared_ptr<Person> self) {
  fmt::println("I am {} and I am {} years old.",
      self->name, self->age);
}

int main(int argc, char* argv[]) {
  auto zed = Person_new("Zed", 51);
  auto mary = Person_new("Mary", 28);

  Person_talk(zed);
  Person_talk(mary);

  return 0;
}

Then using a lambda inside struct Person since they know that you can assign a lambda to a variable, and you can put a variable in a struct, so it follows:

View Source file ex55b.cpp Only

#include <fmt/core.h>
#include <chrono>
#include <thread>
#include <string>
#include <functional>
#include <memory>

using std::shared_ptr, std::make_shared, std::string;

struct Person {
  std::string name;
  int age;

  std::function<void()> talk = nullptr;
};

shared_ptr<Person> Person_new(const string& name, int age) {
  auto self = make_shared<Person>(name, age);

  self->talk = [self]() {
    fmt::println("I am {} and I am {} years old.",
        self->name, self->age);
  };

  return self;
}

int main(int argc, char* argv[]) {
  auto zed = Person_new("Zed", 51);
  auto mary = Person_new("Mary", 28);

  zed->talk();
  mary->talk();

  return 0;
}

Finally, using this in the combat engine so that you can tell two combatants to fight:

View Source file ex55.cpp Only

#include <fmt/core.h>
#include <string>
#include <random>
#include <memory>
#include <algorithm>
#include <unordered_map>
#include <vector>
#include <ranges>
#include <functional>

using std::shared_ptr, std::make_shared, std::string;

std::random_device RNG;
std::mt19937 GENERATOR(RNG());

struct Combatant {
  string name;
  int damage{0};
  int hp{0};
  bool dead{false};

  std::function<void(shared_ptr<Combatant>)> fight = nullptr;
};

using ArenaID = int;
using Arena = std::unordered_map<ArenaID, shared_ptr<Combatant>>;
using CombatQueue = std::vector<size_t>;

int random_number(int from, int to) {
  std::uniform_int_distribution rand(from, to);
  return rand(GENERATOR);
}

void fight(shared_ptr<Combatant> attacker, shared_ptr<Combatant> defender) {
  int damage = random_number(0, attacker->damage);

  if(damage > 0) {
    defender->hp -= damage;
    fmt::println("{} hit {} for {}. {} now has {} hp",
        attacker->name, defender->name, damage,
        defender->name, defender->hp);
  } else {
    fmt::println("{} missed {}!",
        attacker->name, defender->name);
  }

  if(defender->hp <= 0) {
    fmt::println("{} died. {} wins! {} has {} HP",
        defender->name, attacker->name,
        attacker->name, attacker->hp);
  }
}

shared_ptr<Combatant> Combatant_new(const string& name, int damage, int hp) {
  auto self = make_shared<Combatant>(name, damage, hp);

  self->fight = [self](shared_ptr<Combatant> target) {
    fight(self, target);
  };

  return self;
}

void fill_arena(Arena& arena, int combatants) {
  using namespace std::ranges;

  for(auto arena_id : views::iota(0, combatants)) {
    string name = fmt::format("Giant Rat #{}", arena_id);
    int hp = random_number(5,20);
    int damage = random_number(2,10);
    auto fighter = Combatant_new(name, damage, hp);
    arena.try_emplace(arena_id, fighter);
  }
}

bool combat_active(Arena& arena, shared_ptr<Combatant> player) {
  return !arena.empty() && player->hp > 0;
}

CombatQueue queue_combat(Arena& arena) {
  using namespace std::ranges;

  auto queue = arena | views::keys | to<CombatQueue>();

  shuffle(queue, GENERATOR);

  return queue;
}

void run_combat(shared_ptr<Combatant> player, shared_ptr<Combatant> who) {
  bool player_turn = random_number(0,1);

  if(player_turn) {
    who->fight(player);
  } else {
    player->fight(who);
  }

  // now how to remove when dead
  if(who->hp <= 0) {
    who->dead = true;
  }
}

void cull_the_dead(Arena& arena) {
  using namespace std::ranges;

  auto is_dead = [&](auto& tuple) -> bool { return tuple.second->dead; };

  auto dead = arena | views::filter(is_dead) | views::elements<0> | to<CombatQueue>();

  for(auto id : dead) {
    arena.erase(id);
  }
}

int main(int argc, char* argv[]) {
  using namespace std::ranges;

  Arena arena;
  auto player = Combatant_new("Player", 20, 50);

  fill_arena(arena, 10);

  // now with multiple combatants we have to keep going until the player dies
  while(combat_active(arena, player)) {
    auto whos_next = queue_combat(arena);

    for_each(whos_next, [&](auto enemy_id) {
      run_combat(player, arena[enemy_id]);
    });

    cull_the_dead(arena);
  }

  if(arena.empty()) {
    fmt::println("The Player Wins! You defeated all of the rats.");
  } else {
    fmt::println("You Died! The Rats Win.");
  }
}

The Breakdown

line of code

Description.

The Discussion

Blah blah.

Further Study

• Do this next.