Clarification on the use of `boost::bind` and `this` - c++

Question

I was making some modifications to a boost::asio project today and I ran into a confusing compilation error when attempting to pass a bound function into an async call.
I have spent hours fiddling with this code, so I suspect I am missing something stupid. I would appreciate anyone who can point out where I am going wrong.
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&)’
Notice that const MCServer& appears twice? That isn't how my bind statement is written:
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
)
);
}
In the Connection header statusPing(...) is defined as follows:
void statusPing(boost::function<
void(
MCServer&, boost::shared_ptr<Connection>,
const boost::system::error_code&
)>);
The handler eventually to be called:
void ClientServerManager::onPingCompletion(
const MCServer& server_info,
boost::shared_ptr<Connection>,
const boost::system::error_code& err){
// Use the results to drive some function...
}
Background:
I have a class which iterates through a list of servers and opens
short-lived connections with them to make sure they are operating
properly.
The class uses a utility, Connection which inherits
boost::enable_shared_from_this. Therefore, I call a function, Connection::create(..) to grab a boost::shared_ptr to start work with. When any async calls are made, this pointer has to be passed to keep the connection in scope.
In this instance after the connection is successfully established, I want to pass execution to Connection such that it can execute some tasks, and I want it to call a handle once those tasks are complete. I cannot get boost::bind to compile properly in this case.
Now the issue I am having is that the boost::bind might be linked to the use of the boost::asio::placeholders::error template.
The full output follows:
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&’
Edit: (boost::function<void(...)> -> boost::function<void()>)
Per suggestion, I changed the Connection function to read void statusPing(boost::function<void()>); as the parameters are bound in. This change yields a different error:
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
A simplified example:
#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 is just like using boost::placeholders::_1 with boost::bind, and when you pass a bind expression containing that to an asio completion handler function, the function will invoke your handler with the boost::system::error_code result from the operation it performed, thus allowing your handler access to the error code.
In your second example, the boost::function signature for the following bind expression
boost::bind(
&someHandle,
ptr,
boost::asio::placeholders::error
)
is
boost::function<void(boost::system::error_code const&)>
To get your code to compile, change the statusPing() member function to
void statusPing(boost::function<void(boost::system::error_code const&)> f){
// Do work.
boost::system::error_code err;
f(err); // Call handle
}