如何解决RVO 是预期的,但不会发生
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
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