不能在构造函数中使用 boost.asio 解析器

Question

我有一个类，我定义的属性如下：

private:
    const std::string m_ip;
    bool is_connected;
    boost::asio::io_service m_io_service;
    tcp::resolver m_resolver;
    tcp::resolver::query m_query;
    tcp::resolver::iterator m_endpoint_iterator;
    tcp::socket m_socket;

以及我这样定义的构造函数实现：

Connection::Connection(const std::string ip) 
    : m_ip(ip)
    , is_connected(false)
    , m_resolver(tcp::resolver(&m_io_service))
    , m_query(tcp::resolver::query(m_ip, "connect_back"))
    , m_endpoint_iterator(m_resolver.resolve(m_query))
    , m_socket(tcp::socket(m_io_service))
{}

在这里，在构造函数中，我定义了几乎所有需要定义的属性。

但是我在完整的.cpp 文件中遇到了一个我从未遇到过的问题：当我在构造函数中定义解析器时，编译器说我有一个错误：

/home/User/C++ Projects/Client/Network.cpp:9: error: no matching function for call to ‘boost::asio::ip::basic_resolver<boost::asio::ip::tcp>::basic_resolver(boost::asio::io_service*)’
 Connection::Connection(const std::string ip) : m_ip(ip), is_connected(false), m_resolver(tcp::resolver(&m_io_service)), m_query(tcp::resolver::query(m_ip, "connect_back")), m_endpoint_iterator(m_resolver.resolve(m_query)), m_socket(tcp::socket(m_io_service)){
                                                                                                                ^

我不明白为什么，因为我使用的是常规的东西！

有什么想法吗？

PS： 如果我尝试从m_io_service 中删除&，我会得到更多错误：

/home/User/C++ Projects/Client/Network.cpp:9: error: use of deleted function ‘boost::asio::ip::basic_resolver<boost::asio::ip::tcp>::basic_resolver(boost::asio::ip::basic_resolver<boost::asio::ip::tcp>&&)’
Connection::Connection(const std::string ip) : m_ip(ip), 
is_connected(false), 
m_resolver(tcp::resolver(m_io_service)), 
m_query(tcp::resolver::query(m_ip, "connect_back")),
m_endpoint_iterator(m_resolver.resolve(m_query)),
m_socket(tcp::socket(m_io_service)){

与

/usr/include/boost/asio/ip/basic_resolver.hpp:44: error: ‘boost::asio::basic_io_object<IoObjectService, Movable>::basic_io_object(const boost::asio::basic_io_object<IoObjectService, Movable>&) [with IoObjectService = boost::asio::ip::resolver_service<boost::asio::ip::tcp>; bool Movable = false]’ is private within this context

Answer 1

添加到@Muscampester 的答案，看起来你的构造函数是多余的。

Connection::Connection(const std::string ip) 
    : m_ip(ip)
    , is_connected(false)
    , m_resolver(tcp::resolver(&m_io_service))
    , m_query(tcp::resolver::query(m_ip, "connect_back"))
    , m_endpoint_iterator(m_resolver.resolve(m_query))
    , m_socket(tcp::socket(m_io_service))
{}

应该这样写：

//I changed this to const& because, while Copy Ellision *might* optimize this, it's still 
//more semantically correct to pass by const reference.
Connection::Connection(const std::string & ip) 
    : m_ip(ip)
    , is_connected(false)
      //Note that we're no longer referring to the underlying type.
    , m_resolver(m_io_service)
    , m_query(m_ip, "connect_back")
    , m_endpoint_iterator(m_resolver.resolve(m_query))
    , m_socket(m_io_service)
{}

不同之处在于您的解析器、查询和套接字对象不再是“移动构造”的，而是直接构造的。根据您遇到的第二个错误，您似乎正在使用这些对象不可移动/可移动构造的 boost 版本（或者可能 boost 从未打算让这些类型可移动；不确定是哪个）。

如果您使用的是旧版本的 boost，请尝试更新到更高版本的库。不过，无论您是否愿意，您都需要更新代码，以便不再进行不必要的移动构建。

Answer 2

boost::asio::ip::basic_resolver 的constructor 通过引用接受参数：

basic_resolver(boost::asio::io_service& io_service)

你已经获取了变量的地址并传递了一个指针：

m_resolver(tcp::resolver(&m_io_service))

尝试删除&。