Question

我今天正在对boost::asio项目进行一些修改，并且在尝试将绑定函数传递给异步调用时遇到了令人困惑的编译错误。

我花了几个小时摆弄这段代码，所以我怀疑我错过了一些愚蠢的东西。我很感激任何能指出我出错的人。

error: no match for call to ‘(boost::_mfi::mf3<void, ClientServerManager, 
    const MCServer&, boost::shared_ptr<Connection>, 
    const boost::system::error_code&>) (ClientServerManager*&, 
    MCServer&, boost::shared_ptr<Connection>&, 
    const MCServer&)’

请注意const MCServer&出现两次？这不是我的绑定语句的写法：

void ClientServerManager::onConnect(const MCServer& server_info, 
    boost::shared_ptr<Connection> con,
    const boost::system::error_code& err,
    tcp::resolver::iterator i) {

    // Instruct utility to do some work, and call our function when complete.
    con->statusPing(
        boost::bind( // <-- OFFENDING CODE
            &ClientServerManager::onPingCompletion,
            this,
            server_info,
            con,  // shared_ptr to connection (maintains scope)
            boost::asio::placeholders::error // in case of a downstream failure
        )
    );
}

在Connection标头statusPing(...)中定义如下：

void statusPing(boost::function<
    void(
        MCServer&, boost::shared_ptr<Connection>, 
        const boost::system::error_code&
    )>);

最终要调用处理程序：

void ClientServerManager::onPingCompletion(
    const MCServer& server_info, 
    boost::shared_ptr<Connection>,     
    const boost::system::error_code& err){
    // Use the results to drive some function...
}

背景

我有一个类，它遍历服务器列表并打开与他们短暂的联系，以确保他们正在运作正常。
该类使用继承的实用程序Connection boost::enable_shared_from_this.因此，我调用一个函数Connection::create(..)来抓取boost::shared_ptr开始使用。进行任何异步调用时，必须传递此指针以保持连接的范围。
在此实例中，在成功建立连接之后，我想将执行传递给Connection，以便它可以执行某些任务，并且我希望它在完成这些任务后调用句柄。 在这种情况下，我无法让boost :: bind正确编译。
现在我遇到的问题是boost::bind可能与使用boost::asio::placeholders::error模板有关。

完整输出如下：

   g++ -std=c++0x -DBOOST_LOG_DYN_LINK -I/home/donald/Desktop/workspace/MCProxy/Debug/ -O3 -march=corei7-avx -g1 -w -c -fmessage-length=0 -MMD -MP -MF"src/ClientServerManager.d" -MT"src/ClientServerManager.d" -o "src/ClientServerManager.o" "../src/ClientServerManager.cpp"
In file included from ....
/boost/bind/bind.hpp: In instantiation of ‘void boost::_bi::list4<A1, A2, A3, A4>::operator()(boost::_bi::type<void>, F&, A&, int) [with F = boost::_mfi::mf3<void, ClientServerManager, const MCServer&, boost::shared_ptr<Connection>, const boost::system::error_code&>; A = boost::_bi::list3<const MCServer&, boost::shared_ptr<Connection>&, const boost::system::error_code&>; A1 = boost::_bi::value<ClientServerManager*>; A2 = boost::_bi::value<MCServer>; A3 = boost::_bi::value<boost::shared_ptr<Connection> >; A4 = boost::arg<1> (*)()]’:
/boost/bind/bind_template.hpp:116:59:   required from ‘boost::_bi::bind_t<R, F, L>::result_type boost::_bi::bind_t<R, F, L>::operator()(A1&, A2&, A3&) [with A1 = const MCServer; A2 = boost::shared_ptr<Connection>; A3 = const boost::system::error_code; R = void; F = boost::_mfi::mf3<void, ClientServerManager, const MCServer&, boost::shared_ptr<Connection>, const boost::system::error_code&>; L = boost::_bi::list4<boost::_bi::value<ClientServerManager*>, boost::_bi::value<MCServer>, boost::_bi::value<boost::shared_ptr<Connection> >, boost::arg<1> (*)()>; boost::_bi::bind_t<R, F, L>::result_type = void]’
/boost/function/function_template.hpp:153:11:   required from ‘static void boost::detail::function::void_function_obj_invoker3<FunctionObj, R, T0, T1, T2>::invoke(boost::detail::function::function_buffer&, T0, T1, T2) [with FunctionObj = boost::_bi::bind_t<void, boost::_mfi::mf3<void, ClientServerManager, const MCServer&, boost::shared_ptr<Connection>, const boost::system::error_code&>, boost::_bi::list4<boost::_bi::value<ClientServerManager*>, boost::_bi::value<MCServer>, boost::_bi::value<boost::shared_ptr<Connection> >, boost::arg<1> (*)()> >; R = void; T0 = const MCServer&; T1 = boost::shared_ptr<Connection>; T2 = const boost::system::error_code&]’
/boost/function/function_template.hpp:934:60:   required from ‘void boost::function3<R, T1, T2, T3>::assign_to(Functor) [with Functor = boost::_bi::bind_t<void, boost::_mfi::mf3<void, ClientServerManager, const MCServer&, boost::shared_ptr<Connection>, const boost::system::error_code&>, boost::_bi::list4<boost::_bi::value<ClientServerManager*>, boost::_bi::value<MCServer>, boost::_bi::value<boost::shared_ptr<Connection> >, boost::arg<1> (*)()> >; R = void; T0 = const MCServer&; T1 = boost::shared_ptr<Connection>; T2 = const boost::system::error_code&]’
/boost/function/function_template.hpp:722:7:   required from ‘boost::function3<R, T1, T2, T3>::function3(Functor, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type) [with Functor = boost::_bi::bind_t<void, boost::_mfi::mf3<void, ClientServerManager, const MCServer&, boost::shared_ptr<Connection>, const boost::system::error_code&>, boost::_bi::list4<boost::_bi::value<ClientServerManager*>, boost::_bi::value<MCServer>, boost::_bi::value<boost::shared_ptr<Connection> >, boost::arg<1> (*)()> >; R = void; T0 = const MCServer&; T1 = boost::shared_ptr<Connection>; T2 = const boost::system::error_code&; typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type = int]’
/boost/function/function_template.hpp:1071:16:   required from ‘boost::function<R(T0, T1, T2)>::function(Functor, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type) [with Functor = boost::_bi::bind_t<void, boost::_mfi::mf3<void, ClientServerManager, const MCServer&, boost::shared_ptr<Connection>, const boost::system::error_code&>, boost::_bi::list4<boost::_bi::value<ClientServerManager*>, boost::_bi::value<MCServer>, boost::_bi::value<boost::shared_ptr<Connection> >, boost::arg<1> (*)()> >; R = void; T0 = const MCServer&; T1 = boost::shared_ptr<Connection>; T2 = const boost::system::error_code&; typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type = int]’
../src/ClientServerManager.cpp:90:3:   required from here
/boost/bind/bind.hpp:457:9: error: no match for call to ‘(boost::_mfi::mf3<void, ClientServerManager, const MCServer&, boost::shared_ptr<Connection>, const boost::system::error_code&>) (ClientServerManager*&, MCServer&, boost::shared_ptr<Connection>&, const MCServer&)’
In file included ....
/boost/bind/mem_fn_template.hpp:366:85: note: candidates are:
/boost/bind/mem_fn_template.hpp:391:7: note: R boost::_mfi::mf3<R, T, A1, A2, A3>::operator()(T*, A1, A2, A3) const [with R = void; T = ClientServerManager; A1 = const MCServer&; A2 = boost::shared_ptr<Connection>; A3 = const boost::system::error_code&]
/boost/bind/mem_fn_template.hpp:391:7: note:   no known conversion for argument 4 from ‘const MCServer’ to ‘const boost::system::error_code&’
/boost/bind/mem_fn_template.hpp:396:25: note: template<class U> R boost::_mfi::mf3::operator()(U&, A1, A2, A3) const [with U = U; R = void; T = ClientServerManager; A1 = const MCServer&; A2 = boost::shared_ptr<Connection>; A3 = const boost::system::error_code&]
/boost/bind/mem_fn_template.hpp:396:25: note:   template argument deduction/substitution failed:
In file included ....
/boost/bind/bind.hpp:457:9: note:   cannot convert ‘(& a)->boost::_bi::list3<A1, A2, A3>::operator[]<const MCServer&, boost::shared_ptr<Connection>&, const boost::system::error_code&>(boost::_bi::storage4<A1, A2, A3, boost::arg<I> (*)()>::a4_<boost::_bi::value<ClientServerManager*>, boost::_bi::value<MCServer>, boost::_bi::value<boost::shared_ptr<Connection> >, 1>)’ (type ‘const MCServer’) to type ‘const boost::system::error_code&’
In file included ....
/boost/bind/mem_fn_template.hpp:404:25: note: template<class U> R boost::_mfi::mf3::operator()(const U&, A1, A2, A3) const [with U = U; R = void; T = ClientServerManager; A1 = const MCServer&; A2 = boost::shared_ptr<Connection>; A3 = const boost::system::error_code&]
/boost/bind/mem_fn_template.hpp:404:25: note:   template argument deduction/substitution failed:
In file included ....
/boost/bind/bind.hpp:457:9: note:   cannot convert ‘(& a)->boost::_bi::list3<A1, A2, A3>::operator[]<const MCServer&, boost::shared_ptr<Connection>&, const boost::system::error_code&>(boost::_bi::storage4<A1, A2, A3, boost::arg<I> (*)()>::a4_<boost::_bi::value<ClientServerManager*>, boost::_bi::value<MCServer>, boost::_bi::value<boost::shared_ptr<Connection> >, 1>)’ (type ‘const MCServer’) to type ‘const boost::system::error_code&’
In file included from ....
/boost/bind/mem_fn_template.hpp:412:7: note: R boost::_mfi::mf3<R, T, A1, A2, A3>::operator()(T&, A1, A2, A3) const [with R = void; T = ClientServerManager; A1 = const MCServer&; A2 = boost::shared_ptr<Connection>; A3 = const boost::system::error_code&]
/boost/bind/mem_fn_template.hpp:412:7: note:   no known conversion for argument 1 from ‘ClientServerManager*’ to ‘ClientServerManager&’

修改：（boost::function<void(...)> - ＆gt; boost::function<void()>）

根据建议，当参数绑定时，我将Connection函数更改为读取void statusPing(boost::function<void()>);。此更改会产生不同的错误：

g++ -std=c++0x -DBOOST_LOG_DYN_LINK -I/home/donald/Desktop/workspace/MCProxy/Debug/ -O3 -march=corei7-avx -g1 -w -c -fmessage-length=0 -MMD -MP -MF"src/ClientServerManager.d" -MT"src/ClientServerManager.d" -o "src/ClientServerManager.o" "../src/ClientServerManager.cpp"
In file included from ....
/boost/bind/bind.hpp: In instantiation of ‘void boost::_bi::list4<A1, A2, A3, A4>::operator()(boost::_bi::type<void>, F&, A&, int) [with F = boost::_mfi::mf3<void, ClientServerManager, const MCServer&, boost::shared_ptr<Connection>&, const boost::system::error_code&>; A = boost::_bi::list0; A1 = boost::_bi::value<ClientServerManager*>; A2 = boost::_bi::value<MCServer>; A3 = boost::_bi::value<boost::shared_ptr<Connection> >; A4 = boost::arg<1> (*)()]’:
/boost/bind/bind_template.hpp:20:59:   required from ‘boost::_bi::bind_t<R, F, L>::result_type boost::_bi::bind_t<R, F, L>::operator()() [with R = void; F = boost::_mfi::mf3<void, ClientServerManager, const MCServer&, boost::shared_ptr<Connection>&, const boost::system::error_code&>; L = boost::_bi::list4<boost::_bi::value<ClientServerManager*>, boost::_bi::value<MCServer>, boost::_bi::value<boost::shared_ptr<Connection> >, boost::arg<1> (*)()>; boost::_bi::bind_t<R, F, L>::result_type = void]’
/boost/function/function_template.hpp:153:11:   required from ‘static void boost::detail::function::void_function_obj_invoker0<FunctionObj, R>::invoke(boost::detail::function::function_buffer&) [with FunctionObj = boost::_bi::bind_t<void, boost::_mfi::mf3<void, ClientServerManager, const MCServer&, boost::shared_ptr<Connection>&, const boost::system::error_code&>, boost::_bi::list4<boost::_bi::value<ClientServerManager*>, boost::_bi::value<MCServer>, boost::_bi::value<boost::shared_ptr<Connection> >, boost::arg<1> (*)()> >; R = void]’
/boost/function/function_template.hpp:934:60:   required from ‘void boost::function0<R>::assign_to(Functor) [with Functor = boost::_bi::bind_t<void, boost::_mfi::mf3<void, ClientServerManager, const MCServer&, boost::shared_ptr<Connection>&, const boost::system::error_code&>, boost::_bi::list4<boost::_bi::value<ClientServerManager*>, boost::_bi::value<MCServer>, boost::_bi::value<boost::shared_ptr<Connection> >, boost::arg<1> (*)()> >; R = void]’
/boost/function/function_template.hpp:722:7:   required from ‘boost::function0<R>::function0(Functor, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type) [with Functor = boost::_bi::bind_t<void, boost::_mfi::mf3<void, ClientServerManager, const MCServer&, boost::shared_ptr<Connection>&, const boost::system::error_code&>, boost::_bi::list4<boost::_bi::value<ClientServerManager*>, boost::_bi::value<MCServer>, boost::_bi::value<boost::shared_ptr<Connection> >, boost::arg<1> (*)()> >; R = void; typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type = int]’
/boost/function/function_template.hpp:1071:16:   required from ‘boost::function<R()>::function(Functor, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type) [with Functor = boost::_bi::bind_t<void, boost::_mfi::mf3<void, ClientServerManager, const MCServer&, boost::shared_ptr<Connection>&, const boost::system::error_code&>, boost::_bi::list4<boost::_bi::value<ClientServerManager*>, boost::_bi::value<MCServer>, boost::_bi::value<boost::shared_ptr<Connection> >, boost::arg<1> (*)()> >; R = void; typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type = int]’
../src/ClientServerManager.cpp:90:2:   required from here
/boost/bind/bind.hpp:457:9: error: no match for ‘operator[]’ in ‘a[boost::_bi::storage4<A1, A2, A3, boost::arg<I> (*)()>::a4_<boost::_bi::value<ClientServerManager*>, boost::_bi::value<MCServer>, boost::_bi::value<boost::shared_ptr<Connection> >, 1>]’
/boost/bind/bind.hpp:457:9: note: candidates are:
In file included ....
/boost/bind/bind.hpp:170:27: note: template<class T> T& boost::_bi::list0::operator[](boost::_bi::value<T>&) const
/boost/bind/bind.hpp:170:27: note:   template argument deduction/substitution failed:
In file included ....
/boost/bind/bind.hpp:457:9: note:   mismatched types ‘boost::_bi::value<T>’ and ‘boost::arg<1>()’
In file included ....
/boost/bind/bind.hpp:172:33: note: template<class T> const T& boost::_bi::list0::operator[](const boost::_bi::value<T>&) const
/boost/bind/bind.hpp:172:33: note:   template argument deduction/substitution failed:
In file included ....
/boost/bind/bind.hpp:457:9: note:   mismatched types ‘const boost::_bi::value<T>’ and ‘boost::arg<1>()’
In file included ....
/boost/bind/bind.hpp:174:27: note: template<class T> T& boost::_bi::list0::operator[](const boost::reference_wrapper<T>&) const
/boost/bind/bind.hpp:174:27: note:   template argument deduction/substitution failed:
In file included from ....
/boost/bind/bind.hpp:457:9: note:   mismatched types ‘const boost::reference_wrapper<T>’ and ‘boost::arg<1>()’
In file included from ....
/boost/bind/bind.hpp:176:76: note: template<class R, class F, class L> typename boost::_bi::result_traits<R, F>::type boost::_bi::list0::operator[](boost::_bi::bind_t<R, F, L>&) const
/boost/bind/bind.hpp:176:76: note:   template argument deduction/substitution failed:
In file included from ....
/boost/bind/bind.hpp:457:9: note:   mismatched types ‘boost::_bi::bind_t<R, F, L>’ and ‘boost::arg<1>()’
/boost/bind/bind.hpp:178:76: note: template<class R, class F, class L> typename boost::_bi::result_traits<R, F>::type boost::_bi::list0::operator[](const boost::_bi::bind_t<R, F, L>&) const
/boost/bind/bind.hpp:178:76: note:   template argument deduction/substitution failed:
/boost/bind/bind.hpp:457:9: note:   mismatched types ‘const boost::_bi::bind_t<R, F, L>’ and ‘boost::arg<1>()’
make: *** [src/ClientServerManager.o] Error 1

简化示例：

#include <boost/shared_ptr.hpp>
#include <boost/bind.hpp>
#include <boost/asio.hpp>
#include <boost/system/error_code.hpp>
#include <boost/function.hpp>

class ExampleWorker {
    public:
        void statusPing(boost::function<void()> f){
            // Do work.
            f(); // Call handle
        }
};

class SomeObject {};

void someHandle(boost::shared_ptr<SomeObject> obj,
        const boost::system::error_code& err){
    // ...
}

int main() {
    ExampleWorker* e = new ExampleWorker();
    boost::shared_ptr<SomeObject> ptr(new SomeObject());

    e->statusPing(
            boost::bind(
                &someHandle,
                ptr,
                boost::asio::placeholders::error
            )
    );
}

Answer 1

boost::asio::placeholders::error就像将boost::placeholders::_1与boost::bind一起使用，当您将包含该表达式的绑定表达式传递给asio完成处理函数时，该函数将调用您的处理程序使用它执行的操作的boost::system::error_code结果，从而允许您的处理程序访问错误代码。

在第二个示例中，以下boost::function表达式的bind签名

boost::bind(
    &someHandle,
    ptr,
    boost::asio::placeholders::error
)

是

boost::function<void(boost::system::error_code const&)>

要编译代码，请将statusPing()成员函数更改为

void statusPing(boost::function<void(boost::system::error_code const&)> f){
    // Do work.
    boost::system::error_code err;
    f(err); // Call handle
}

澄清使用`boost :: bind`和`this`

1 个答案: