"candidate template ignored: substitution failure:" compiler error?

Question

I'm currently trying to write some code with boost::asio, compiling with clang 3.1.

I have a simple function object:

struct tcp_socket_match_condition
{
    template <typename TIter>
    std::pair<TIter, bool> operator()(TIter begin, TIter end) const
    {
        auto result(std::find(begin, end, '\n'));
        const bool found(result != end);
        return std::make_pair(found ? ++result : end, found);
    }
}; 

I attempt to pass this to the boost::asio::read_until function like so:

boost::asio::read_until(socket, stream, match_condition_, error); 

The compiler error generated looks to point out that it can't find the correct function overload. Any ideas why this isn't working?

I've provided the full class and compiler error.

In file included from src/network/admin_socket.cpp:1:
In file included from include/bytes42/arthur/network/admin_socket.hpp:4:
include/bytes42/arthur/network/tcp_socket.hpp:95:21: error: no matching function for call to 'read_until'
                    boost::asio::read_until(socket, stream, match_condition_, error);
                    ^~~~~~~~~~~~~~~~~~~~~~~
src/network/admin_socket.cpp:81:10: note: in instantiation of member function
      'bytes42::arthur::network::tcp_socket<bytes42::arthur::network::detail::tcp_socket_match_condition>::listen' requested here
        socket_.listen();
                ^
/usr/local/include/boost/asio/impl/read_until.hpp:47:13: note: candidate function [with SyncReadStream =
      boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> >, Allocator =
      std::__1::allocator<char>] not viable: no known conversion from 'bytes42::arthur::network::detail::tcp_socket_match_condition' to 'char' for 3rd
      argument;
std::size_t read_until(SyncReadStream& s,
            ^
/usr/local/include/boost/asio/impl/read_until.hpp:138:13: note: candidate function [with SyncReadStream =
      boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> >, Allocator =
      std::__1::allocator<char>] not viable: no known conversion from 'bytes42::arthur::network::detail::tcp_socket_match_condition' to
      'const std::string' (aka 'const basic_string<char, char_traits<char>, allocator<char> >') for 3rd argument;
std::size_t read_until(SyncReadStream& s,
            ^
/usr/local/include/boost/asio/impl/read_until.hpp:203:13: note: candidate function [with SyncReadStream =
      boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> >, Allocator =
      std::__1::allocator<char>] not viable: no known conversion from 'bytes42::arthur::network::detail::tcp_socket_match_condition' to
      'const boost::regex' (aka 'const basic_regex<char, regex_traits<char> >') for 3rd argument;
std::size_t read_until(SyncReadStream& s,
            ^
/usr/local/include/boost/asio/impl/read_until.hpp:260:13: note: candidate template ignored: substitution failure [with SyncReadStream =
      boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> >, Allocator =
      std::__1::allocator<char>, MatchCondition = bytes42::arthur::network::detail::tcp_socket_match_condition]
std::size_t read_until(SyncReadStream& s,
            ^
/usr/local/include/boost/asio/impl/read_until.hpp:312:20: note: candidate template ignored: substitution failure [with SyncReadStream =
      boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> >, Allocator =
      std::__1::allocator<char>, MatchCondition = bytes42::arthur::network::detail::tcp_socket_match_condition]
inline std::size_t read_until(SyncReadStream& s,
                   ^
/usr/local/include/boost/asio/impl/read_until.hpp:37:20: note: candidate function template not viable: requires 3 arguments, but 4 were provided
inline std::size_t read_until(SyncReadStream& s,
                   ^
/usr/local/include/boost/asio/impl/read_until.hpp:93:20: note: candidate function template not viable: requires 3 arguments, but 4 were provided
inline std::size_t read_until(SyncReadStream& s,
                   ^
/usr/local/include/boost/asio/impl/read_until.hpp:193:20: note: candidate function template not viable: requires 3 arguments, but 4 were provided
inline std::size_t read_until(SyncReadStream& s,
                   ^
1 error generated.
make: *** [build/src/network/admin_socket.o] Error 1

Class:

    template <typename TMatchCondition>
    class tcp_socket
    {
        public:
            typedef std::function<std::string(const std::string&)> data_callback;

        public:
            tcp_socket(
                const unsigned short port,
                TMatchCondition match_condition,
                data_callback callback);

            void listen();
            void stop();

        private:
            tcp_socket(const tcp_socket&)   = delete;
            tcp_socket(tcp_socket&&)        = delete;

            tcp_socket& operator=(const tcp_socket&) = delete;
            tcp_socket& operator=(tcp_socket&&)      = delete;

        private:
            const utils::entry_exit         entry_exit_;
            boost::asio::io_service         service_;
            boost::asio::ip::tcp::acceptor  acceptor_;
            TMatchCondition                 match_condition_;
            data_callback                   callback_;
    };


    template <typename TMatchCondition>
    tcp_socket<TMatchCondition>::tcp_socket(
        const unsigned short port,
        TMatchCondition match_condition,
        data_callback callback)
        : entry_exit_("tcp_socket:" + std::to_string(port))
        , acceptor_(service_, boost::asio::ip::tcp::endpoint(boost::asio::ip::tcp::v4(), port))
        , match_condition_(match_condition)
        , callback_(callback) {}

    template <typename TMatchCondition>
    void tcp_socket<TMatchCondition>::listen()
    {
        const auto port(acceptor_.local_endpoint().port());
        const std::string port_str(std::to_string(port));

        while(acceptor_.is_open())
        {
            boost::system::error_code error;

            utils::entry_exit ee("Listening on port " + port_str);

            boost::asio::ip::tcp::socket socket(service_);
            acceptor_.accept(socket, error);

            if(error)
            {
                if(error != boost::asio::error::bad_descriptor)
                {
                    LOG(ERROR)
                        << "An error occured while trying to accept a client connection; error="
                        << error.message();

                    sleep(1);  // don't want to flood logs
                }
            }
            else
            {
                while(socket.is_open())
                {
                    boost::asio::streambuf stream;
                    boost::asio::read_until(socket, stream, match_condition_, error);

                    const std::string msg(
                        (std::istreambuf_iterator<char>(&stream)),
                        std::istreambuf_iterator<char>());

                    LOG(INFO) << "Received message: " << msg;

                    boost::asio::write(
                        socket,
                        boost::asio::buffer(callback_(msg)),
                        error);

                    if(error)
                    {
                        if(error != boost::asio::error::broken_pipe)
                        {
                            LOG(ERROR)
                                << "Error whilst writing response, closing client connection: "
                                << error.message();
                        }

                        socket.close();

                        sleep(1);  // don't want to flood logs
                    }
                }
            }
        }
    }


    template <typename TMatchCondition>
    void tcp_socket<TMatchCondition>::stop()
    {
        boost::system::error_code error;
        acceptor_.close(error);

        if(error)
        {
            LOG(ERROR) << "Error whilst stopping TCP socket; error=" << error.message();
        }
    }                                

Answer 1

Ok; so clang is being (kinda) helpful here.

it's telling you that there are eight specializations of read_until, and that it couldn't make any of them work.

The first three take a char, char, and std::string for the third parameter. That won't work.

The last three have three parameters - you're passing four. Those worn't work either.

That leaves the two in the middle. For some reason, your TMatchCondition is generating a substitution failure.

As Dietmar just pointed out, you should definitely check to see if boost::is_match_condition<TMatchCondition>::value is true - because it won't work if it is not.

Answer 2

I don't think you posted the full code but the problems seems to be the match condition: Did you specify using boost::is_match_condition that your tcp_socket_match_condition is a match condition?