C++中的连接池如何工作？

#include <iostream>
#include <vector>
#include <memory>
#include <mutex>

class Connection {
public:
    void connect() { std::cout << "Connecting to database..." << std::endl; }
    void disconnect() { std::cout << "Disconnecting from database..." << std::endl; }
};

class ConnectionPool {
private:
    std::vector<std::shared_ptr<Connection>> connections;
    std::mutex mtx;

public:
    ConnectionPool(int size) {
        for (int i = 0; i < size; ++i) {
            connections.push_back(std::make_shared<Connection>());
        }
    }

    std::shared_ptr<Connection> getConnection() {
        std::lock_guard<std::mutex> lock(mtx);
        if (!connections.empty()) {
            auto conn = connections.back();
            connections.pop_back();
            conn->connect();
            return conn;
        }
        return nullptr;
    }

    void releaseConnection(std::shared_ptr<Connection> conn) {
        std::lock_guard<std::mutex> lock(mtx);
        conn->disconnect();
        connections.push_back(conn);
    }
};

int main() {
    ConnectionPool pool(5);
    auto conn1 = pool.getConnection();
    auto conn2 = pool.getConnection();

    // 使用连接...

    pool.releaseConnection(conn1);
    pool.releaseConnection(conn2);

    return 0;
}