如何使用 C++ ->() 运算符获取基类私有成员？这样我就可以访问派生？

Question

我试图在通过基础class 工厂方法

接收派生的 class TMAConnection 后访问它

template <>
class cpool::ConnectionPoolFactory<TMAConnection> {
public:
    static std::unique_ptr<cpool::ConnectionPool> create( const std::uint16_t num_connections, const char* tma_gw_host, const int tma_gw_port) {
        std::vector< std::unique_ptr<cpool::Connection> > connections;

        for ( std::uint16_t k = 0; k < num_connections; ++k ) {
            // cannot use std::make_unique, because constructor is hidden
            connections.emplace_back( std::unique_ptr<TMAConnection>( new TMAConnection{tma_gw_host, tma_gw_port} ) );
        }

        return std::unique_ptr<cpool::ConnectionPool>( new cpool::ConnectionPool{std::move( connections )} );
    }
};

我的麻烦在于尝试访问派生的 classes 函数和成员.. 例如 TestCPPClient 成员或 querySymbols 函数...

class TMAConnection final : public cpool::Connection {
public:
    TMAConnection(const char* hostname, const int port){
      this->gateway_host=hostname;
      this->gateway_port=port;

    }
    bool heart_beat() override { return connected; }

    bool is_healthy() override { return connected; }

    bool connect() override {
        connected = true;
        return connected;
    }

    void disconnect() override { 
      connected = false; 
    }

    void querySymbols(std::vector<tmaapi::CD> & empty_contract_vect, std::vector<std::string> & queries_list) {
      this->tma_client.query_matching_tickers(empty_contract_vect, queries_list);
      std::vector<std::string> empty_vec;
    }

    TestCppClient tma_client;

private:
    TMAConnection(){}
    friend cpool::ConnectionPoolFactory<TMAConnection>;
    bool connected = false;
    
    const char* gateway_host;
    int gateway_port;
    uint16_t tma_client_id;
};

当我查看 ConnectionProxy 代码时.. 似乎我可以使用运算符 -> 获取成员或派生 class 但无济于事到目前为止

https://github.com/malikkirchner/connection-pool/blob/master/src/pool.cpp#L61-L63

Connection* ConnectionPool::ConnectionProxy::operator->() { return m_connection; }

Connection& ConnectionPool::ConnectionProxy::operator*() { return *m_connection; }

这里是我尝试调用 create 以建立连接的地方..但随后无法获得派生的 class 指针

cpool::ConnectionPool::ConnectionProxy proxy_conn = this->tma_conn_pool->get_connection();

proxy_conn->is_healthy(); // this works fine if i don't cast.. since it's in the base class
proxy_conn->querySymbols(empty_contract_vect, sub_batch_vector);  // this fails

我尝试添加动态转换.. 似乎不起作用

    cpool::ConnectionPool::ConnectionProxy proxy_conn = this->tma_conn_pool->get_connection();
    TMAConnection* x = dynamic_cast<TMAConnection*>(&proxy_conn));

    x->is_healthy();
    x->querySymbols(empty_contract_vect, sub_batch_vector); 

/home/server/src/server.cpp:48:64: error: cannot dynamic_cast ‘& proxy_conn’ (of type ‘class cpool::ConnectionPool::ConnectionProxy*’) to type ‘class TMAConnection*’ (source type is not polymorphic)
   48 |     TMAConnection* x = dynamic_cast<TMAConnection*>(&proxy_conn));

如何获得指向派生 class 的指针？我的工厂错了吗？谢谢

编辑：我试过@typewriters 建议使用 auto.. 还是没用

auto proxy_conn = this->tma_conn_pool->get_connection();
TMAConnection* x = dynamic_cast<TMAConnection*>(&proxy_conn));

error: cannot dynamic_cast ‘& proxy_conn’ (of type ‘class cpool::ConnectionPool::ConnectionProxy*’) to type ‘class TMAConnection*’ (source type is not polymorphic)

Answer 1

ConnectionPool::get_connection() returns 一个 ConnectionProxy 其中不是一个 Connection 直接对象，而是包含一个：

ConnectionProxy 允许您访问底层连接对象。

// create a pool
auto pool = ...
// get a ConnectionProxy
auto proxy = pool->get_connection();

// cast to your type
if(auto conn = dynamic_cast<TMAConnection*>(proxy.operator->()))
{
    // success!
}

如何使用 C++ ->() 运算符获取基类私有成员？这样我就可以访问派生？

how to use c++ ->() operator to get a baseclasses private member? so I can access derived?

c++

factory