#include <vector>
#include <queue>
#include <mutex>
#include <condition_variable>

class ConnectionPool {
public:
   ConnectionPool(size_t poolSize) : maxPoolSize(poolSize) {
       for (size_t i = 0; i < maxPoolSize; ++i) {
           connections.push(new Database());
       }
   }

   ~ConnectionPool() {
       while (!connections.empty()) {
           delete connections.front();
           connections.pop();
       }
   }

   Database* getConnection() {
       std::unique_lock<std::mutex> lock(mutex);
       while (connections.empty()) {
           condition.wait(lock);
       }
       Database* conn = connections.front();
       connections.pop();
       std::cout << "Pool " << __func__ << " ok" << std::endl;
       return conn;
   }

   void returnConnection(Database* conn) {
       std::unique_lock<std::mutex> lock(mutex);
       connections.push(conn);
       condition.notify_one();
       std::cout << "Pool " << __func__ << " ok" << std::endl;
   }

private:
   std::queue<Database*> connections; // 连接队列
   size_t maxPoolSize; // 最大连接池大小
   std::mutex mutex; // 互斥锁
   std::condition_variable condition; // 条件变量
};