RVO 是预期的，但不会发生

问题描述

我有以下连接池的基本实现。在非常高的层次上，它为客户端从池中获取打开的连接提供了一种安全的方式，或者如果所有连接当前都在使用，则打开一个新的连接。 Connection 结构是连接的低级实现。 ClientConnection 结构体是 Connection 的包装器，因此客户端只能有限地访问 Connection 对象，并为 Connection 对象提供了一种安全的方式使用 RAII 返回游泳池。调用 Pool::getNewConnection() 时，我希望 NRVO 发生，但它没有发生，而且我一生都无法弄清楚原因。我希望只对析构函数进行一次调用，因为我在 ClientConnection 中创建了 main()，然后它会被销毁。然而，发生的情况是在 Pool::getNewConnection() 中创建了一个实例，然后在 main() 中移动到对象的内存中，因此调用了 2 个析构函数。

使用 g++ -std=c++17 -O0 -g -o main main2.cpp 和 g++ -std=c++17 -O3 -g -o main main2.cpp 构建

输出为：

$ ./main 
opening connection
Pool::getNewConnection() -- Address of newClientConnection: 0x7ffe3d29c290
ClientConnection(ClientConnection&&) -- old: 0x7ffe3d29c290 new: 0x7ffe3d29c320
ClientConnection::~ClientConnection() -- Returning this connection back to pool: 0x7ffe3d29c290
Pool::returnConnection() -- Returned connection: 0x7ffe3d29c290
Writing
ClientConnection::~ClientConnection() -- Returning this connection back to pool: 0x7ffe3d29c320
Pool::returnConnection() -- Returned connection: 0x7ffe3d29c320

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

struct Connection {
  bool open(std::string /* ip */,std::uint16_t /* port */) {
    std::cout << "opening connection\n";
    return true;
  }
  bool close() {
    std::cout << "Closing connection\n";
    return true;
  }
  bool write(const char*) { return true; }
  bool read(const char*) { return true; }
};

struct ClientConnection;

struct Pool {
  ClientConnection getNewConnection();
  void returnConnection(ClientConnection& clientConnection);

private:
  friend class ClientConnection;

  std::vector<std::unique_ptr<Connection>> m_connections;
};

struct ClientConnection {
  ClientConnection()                        = delete;
  ClientConnection(const ClientConnection&) = delete;
  ClientConnection(ClientConnection&& rhs) noexcept : m_pool(rhs.m_pool),m_connection(std::move(rhs.m_connection)) {
    std::cout << "ClientConnection(ClientConnection&&) -- old: " << std::addressof(rhs) << " new: " << this
              << std::endl;
  }

  ClientConnection& operator=(const ClientConnection&) = delete;
  ClientConnection& operator=(ClientConnection&&) = delete;
  ~ClientConnection() noexcept {
    std::cout << "ClientConnection::~ClientConnection() -- Returning this connection back to pool: " << this
              << std::endl;
    m_pool.returnConnection(*this);
  }

  bool write(const char*) {
    std::cout << "Writing\n";
    if (!m_connection->write(nullptr)) {
      m_connectionValid = false;
    }
    return m_connectionValid;
  }

  bool read(const char*) {
    if (!m_connection->read(nullptr)) {
      m_connectionValid = false;
    }
    return m_connectionValid;
  }

private:
  friend class Pool;

  ClientConnection(Pool& pool,std::unique_ptr<Connection> connection)
      : m_pool(pool),m_connection(std::move(connection)) {}

private:
  bool m_connectionValid = true;
  Pool& m_pool;
  std::unique_ptr<Connection> m_connection;
};

ClientConnection Pool::getNewConnection() {
  if (m_connections.empty()) {
    auto conn = std::make_unique<Connection>();
    conn->open("",0);
    auto newClientConnection = ClientConnection(*this,std::move(conn));
    std::cout << "Pool::getNewConnection() -- Address of newClientConnection: " << std::addressof(newClientConnection)
              << std::endl;
    return newClientConnection;
  } else {
    auto newClientConnection = ClientConnection(*this,std::move(m_connections.back()));
    m_connections.erase(std::prev(m_connections.end(),1));
    return newClientConnection;
  }
}

void Pool::returnConnection(ClientConnection& clientConnection) {
  if (clientConnection.m_connectionValid) {
    m_connections.push_back(std::move(clientConnection.m_connection));
    std::cout << "Pool::returnConnection() -- Returned connection: " << std::addressof(clientConnection) << std::endl;
  } else {
    clientConnection.m_connection->close();
  }
}

int main() {
  Pool p;
  auto conn = p.getNewConnection();
  conn.write(nullptr);
  return 0;
}

解决方法

无法保证 NRVO。编译器可能会错过使用它的机会。

确保复制省略的替代方法是使用保证的替代形式。

类似于以下内容：

CREATE TRIGGER updateExamBacklogTrigger 
   ON  employees
   AFTER  UPDATE
AS 
BEGIN

IF(UPDATE(exam_results) OR UPDATE(exam_date ))

    INSERT INTO exams_backlog SELECT e.employee_id,e.exam_results,e.exam_date FROM employees e INNER JOIN inserted i ON i.employee_id = e.employee_id

SET NOCOUNT ON;

END
GO