没有匹配的构造函数错误（Boost Beast，C++）

Question

这是我的 WebSocket 客户端头文件：

namespace beast = boost::beast;
namespace asio = boost::asio;

class WebSocketClient {
 public:
  explicit WebSocketClient(asio::io_context &ioc);

 private:
  using stream_type = beast::websocket::stream<beast::ssl_stream<beast::tcp_stream>>;
  using ssl_context_type = asio::ssl::context;

  asio::ip::tcp::resolver resolver_;
  stream_type ws_;

  ssl_context_type get_ssl_context();
};

这是我的定义：

WebSocketClient::WebSocketClient(asio::io_context &ioc)
    : resolver_(asio::make_strand(ioc)),
      ws_(asio::make_strand(ioc), get_ssl_context()) {
}

asio::ssl::context WebSocketClient::get_ssl_context() {
  return WebSocketClient::ssl_context_type(asio::ssl::context_base::tlsv12_client);
}

我遇到错误：

In template: no matching constructor for initialization of 'boost::beast::ssl_stream<boost::beast::basic_stream<boost::asio::ip::tcp, boost::asio::execution::any_executor<boost::asio::execution::context_as_t<boost::asio::execution_context &>, boost::asio::execution::detail::blocking::never_t<0>, boost::asio::execution::prefer_only<boost::asio::execution::detail::blocking::possibly_t<0>>, boost::asio::execution::prefer_only<boost::asio::execution::detail::outstanding_work::tracked_t<0>>, boost::asio::execution::prefer_only<boost::asio::execution::detail::outstanding_work::untracked_t<0>>, boost::asio::execution::prefer_only<boost::asio::execution::detail::relationship::fork_t<0>>, boost::asio::execution::prefer_only<boost::asio::execution::detail::relationship::continuation_t<0>>>, boost::beast::unlimited_rate_policy>>' error occurred here in instantiation of function template specialization 'boost::empty_::empty_value<boost::beast::ssl_stream<boost::beast::basic_stream<boost::asio::ip::tcp, boost::asio::execution::any_executor<boost::asio::execution::context_as_t<boost::... in instantiation of function template specialization 'boost::beast::websocket::stream<boost::beast::ssl_stream<boost::beast::basic_stream<boost::asio::ip::tcp, boost::asio::execution::any_executor<boost::asio::execution::context_as_t<bo... in instantiation of function template specialization 'boost::make_shared<boost::beast::websocket::stream<boost::beast::ssl_stream<boost::beast::basic_stream<boost::asio::ip::tcp, boost::asio::execution::any_executor<boost::asio::executi... in instantiation of function template specialization 'boost::beast::websocket::stream<boost::beast::ssl_stream<boost::beast::basic_stream<boost::asio::ip::tcp, boost::asio::execution::any_executor<boost::asio::execution::context_as_t<bo... candidate constructor template not viable: expects an l-value for 2nd argument candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 2 were provided candidate constructor (the implicit move constructor) not viable: requires 1 argument, but 2 were provided

如果有人能解释为什么我会收到此错误，我将不胜感激？如果我改为使用下面的代码，并添加另一个 ssl_context_type 类型的私有成员变量 ssl_context_，错误就会消失，但我不确定为什么......我对 C++ 很陌生.

WebSocketClient::WebSocketClient(asio::io_context &ioc)
    : resolver_(asio::make_strand(ioc)),
      ssl_context_(asio::ssl::context_base::tlsv12_client),
      ws_(asio::make_strand(ioc), ssl_context_) {
}

Answer 1

问题是 boost::asio::ssl::stream class 构造函数（通过归纳，beast::websocket::stream<beast::ssl_stream<beast::tcp_stream>> 构造函数）需要一个 l-value of boost::asio::ssl::context as the second parameter, and WebSocketClient::get_ssl_context returns an r-value。换句话说，WebSocketClient::get_ssl_context returns 是临时的，beast::websocket::stream<beast::ssl_stream<beast::tcp_stream>>.

是不可接受的

ssl::context 需要在 TLS 流的整个持续时间内持续存在，因为流将在其操作期间使用上下文。所以正确的解决方案是在 ws_之前创建一个ssl::context的实例作为WebSocketClient的数据成员，并提供对它的引用给ws_构造函数。

没有匹配的构造函数错误（Boost Beast，C++）

No matching constructor error (Boost Beast, C++)

c++

boost

boost-asio

initializer-list

boost-beast