First draft of behavioural design pattern state

Author: XuhuaHuang

DesignPattern/state.cpp

@@ -0,0 +1,126 @@
+#include <iostream>
+#include <memory>
+#include <string>
+
+// forward declaration
+class line_of_credit_t;
+
+class line_of_credit_state_t {
+public:
+  explicit line_of_credit_state_t(line_of_credit_t* loc)
+    : loc(loc) {}
+
+  virtual ~line_of_credit_state_t() = default;
+
+  virtual void apply(float) {}
+  virtual void approve() {}
+  virtual void withdraw(float) {}
+  virtual void make_payment(float) {}
+  virtual void cancel() {}
+
+  virtual std::string name() const = 0;
+
+protected:
+  line_of_credit_t* loc;
+};
+
+class line_of_credit_t {
+public:
+  line_of_credit_t();
+
+  void set_state(std::unique_ptr<line_of_credit_state_t> new_state) {
+    state = std::move(new_state);
+    std::cout << "State changed to: " << state->name() << "\n";
+  }
+
+  void apply(float amount) { state->apply(amount); }
+  void approve() { state->approve(); }
+  void withdraw(float amount) { state->withdraw(amount); }
+  void make_payment(float amt) { state->make_payment(amt); }
+  void cancel() { state->cancel(); }
+
+  float get_balance() const { return balance; }
+  void  set_balance(float amt) { balance = amt; }
+
+private:
+  std::unique_ptr<line_of_credit_state_t> state;
+
+  float balance = 0.0f;
+};
+
+class applied_state_t : public line_of_credit_state_t {
+public:
+  using line_of_credit_state_t::line_of_credit_state_t;
+
+  void apply(float amount) override {
+    std::cout << "Application submitted for $" << amount << "\n";
+    loc->set_balance(amount);
+  }
+
+  void approve() override;
+
+  std::string name() const override { return "Applied"; }
+};
+
+class approved_state_t : public line_of_credit_state_t {
+public:
+  using line_of_credit_state_t::line_of_credit_state_t;
+
+  void withdraw(float amount) override;
+
+  std::string name() const override { return "Approved"; }
+};
+
+class withdrawn_state_t : public line_of_credit_state_t {
+public:
+  using line_of_credit_state_t::line_of_credit_state_t;
+
+  void make_payment(float amount) override;
+
+  std::string name() const override { return "Withdrawn"; }
+};
+
+class closed_state_t : public line_of_credit_state_t {
+public:
+  using line_of_credit_state_t::line_of_credit_state_t;
+
+  std::string name() const override { return "Closed"; }
+};
+
+void applied_state_t::approve() {
+  std::cout << "Credit approved\n";
+  loc->set_state(std::make_unique<approved_state_t>(loc));
+}
+
+void approved_state_t::withdraw(float amount) {
+  std::cout << "Withdrawing $" << amount << "\n";
+  loc->set_balance(loc->get_balance() - amount);
+  loc->set_state(std::make_unique<withdrawn_state_t>(loc));
+}
+
+void withdrawn_state_t::make_payment(float amount) {
+  std::cout << "Payment of $" << amount << "\n";
+
+  float new_balance = loc->get_balance() + amount;
+  loc->set_balance(new_balance);
+
+  if (new_balance >= 0) {
+    std::cout << "Loan fully repaid\n";
+    loc->set_state(std::make_unique<closed_state_t>(loc));
+  }
+}
+
+line_of_credit_t::line_of_credit_t() {
+  state = std::make_unique<applied_state_t>(this);
+}
+
+int main() {
+  line_of_credit_t loc;
+
+  loc.apply(1000);       // Applied
+  loc.approve();         // -> Approved
+  loc.withdraw(500);     // -> Withdrawn
+  loc.make_payment(600); // -> Closed
+
+  return 0;
+}