没有匹配的构造函数错误Boost Beast，C++

问题描述

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);
}

我收到错误：

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_ 类型的私有成员变量 ssl_context_type，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),ssl_context_) {
}

解决方法

问题在于 boost::asio::ssl::stream 类构造函数（以及通过归纳法，beast::websocket::stream<beast::ssl_stream<beast::tcp_stream>> 构造函数）需要 l-value 的 boost::asio::ssl::context 作为第二个参数，并且 {{1} } 返回一个 r-value。换句话说，WebSocketClient::get_ssl_context 返回一个临时值，这对于 WebSocketClient::get_ssl_context 来说是不可接受的。

beast::websocket::stream<beast::ssl_stream<beast::tcp_stream>> 需要在 TLS 流的整个持续时间内保持不变，因为流将在其操作期间使用上下文。因此，正确的解决方案是创建一个 ssl::context 实例作为 ssl::context before WebSocketClient 的数据成员，并提供对 ws_ 的引用构造函数。