Upload observer design pattern example

Author: XuhuaHuang

DesignPattern/observer.cpp

@@ -0,0 +1,105 @@
+/**
+ * @file observer.cpp
+ * @author Xuhua Huang
+ * @brief
+ * @version 0.1
+ * @date 2026-04-12
+ *
+ * @copyright Copyright (c) 2026
+ *
+ */
+
+#include <algorithm>
+#include <iostream>
+#include <memory>
+#include <string>
+#include <vector>
+
+class spreadsheet_t;
+
+class graph_t {
+public:
+  virtual ~graph_t() = default;
+
+  virtual void update(const spreadsheet_t& sheet) = 0;
+};
+
+class spreadsheet_t {
+public:
+  using observer_ptr_t = std::shared_ptr<graph_t>;
+
+  void attach(observer_ptr_t observer) { observers.push_back(std::move(observer)); }
+
+  void detach(const observer_ptr_t& observer) {
+    std::erase_if(observers, [&](const observer_ptr_t& o) { return o == observer; });
+  }
+
+  void notify() {
+    for (auto& observer : observers) {
+      observer->update(*this);
+    }
+  }
+
+  // Example dataset API (pull-based model)
+  const std::vector<int>& data() const { return values; }
+
+  void set_data(std::vector<int> new_data) {
+    values = std::move(new_data);
+    notify();
+  }
+
+private:
+  std::vector<observer_ptr_t> observers;
+
+  std::vector<int> values;
+};
+
+class pie_chart_t : public graph_t {
+public:
+  void update(const spreadsheet_t& sheet) override {
+    const auto& data = sheet.data();
+
+    std::cout << "[PieChart] updating with data size: " << data.size() << "\n";
+  }
+};
+
+class bar_chart_t : public graph_t {
+public:
+  void update(const spreadsheet_t& sheet) override {
+    const auto& data = sheet.data();
+
+    std::cout << "[BarChart] sum = ";
+
+    int sum = 0;
+    for (int v : data)
+      sum += v;
+
+    std::cout << sum << "\n";
+  }
+};
+
+class line_chart_t : public graph_t {
+public:
+  void update(const spreadsheet_t& sheet) override {
+    const auto& data = sheet.data();
+
+    std::cout << "[LineChart] last value = " << (data.empty() ? 0 : data.back()) << "\n";
+  }
+};
+
+int main() {
+  spreadsheet_t sheet;
+
+  auto pie  = std::make_shared<pie_chart_t>();
+  auto bar  = std::make_shared<bar_chart_t>();
+  auto line = std::make_shared<line_chart_t>();
+
+  sheet.attach(pie);
+  sheet.attach(bar);
+  sheet.attach(line);
+
+  sheet.set_data({1, 2, 3, 4, 5});
+  sheet.set_data({10, 20, 30});
+
+  return 0;
+}