Boost Function Pointer Multithreaded Mystery

Question

I am trying to use a boost::function in my ReceiveRequest to run on its own thread but I must be sending the wrong parameters. (At least thats what I think the compiler is trying to tell me)

Here are the lines that is causing the issue:

//some variables for the function call
std::string something("");
asio::system_error e();                             
asio::thread daThread();
CurrentRequest.payload;

//attempts to call function
CurrentRequest.Callback(something, &e, CurrentRequest.payload); //line 184          
CurrentRequest.Callback(something, &e, &CurrentRequest.payload); //line 185

Here is what the compiler is telling me:

g++ -o ss -pthread -lrt StringSocket.cpp main.cpp -I ~/asio/include -I ~/boost/include ~/boost/lib/*.a
StringSocket.cpp: In member function ‘void StringSocket::ProcessReceive()’:
StringSocket.cpp:184: error: no match for call to ‘(ReceiveRequest::receiveCallback) (std::string&, asio::system_error (*)(), void*&)’
/home/jsander/boost/include/boost/function/function_template.hpp:761: note: candidates are: R boost::function3<R, T1, T2, T3>::operator()(T0, T1, T2) const [with R = void, T0 = std::string*, T1 = asio::system_error&, T2 = void*]
StringSocket.cpp:185: error: no match for call to ‘(ReceiveRequest::receiveCallback) (std::string&, asio::system_error (*)(), void**)’
/home/jsander/boost/include/boost/function/function_template.hpp:761: note: candidates are: R boost::function3<R, T1, T2, T3>::operator()(T0, T1, T2) const [with R = void, T0 = std::string*, T1 = asio::system_error&, T2 = void*]

Here is the ReceiveRequest class:

class ReceiveRequest
    {
        typedef boost::function<void (std::string *message, asio::system_error& e, void *payload) > receiveCallback;

        public:
        receiveCallback Callback;
        void* payload;

        ReceiveRequest(receiveCallback _Callback, void* _payload)
        {
            Callback = _Callback;
            payload = _payload;
        }

        ~ReceiveRequest() {  }

    };

These errors seem to be making a distinction between pointers and references to variables. I thought they could be used interchangeable as parameters. boost::function also appears to turn all of my local variables into references.

I am also confused that one of my parameters passes as "e" turns into "asio::system_error (*)()". Why is there a second pair of parenthesis added to my variable?

Answer 1

There are multiple issues here:

asio::system_error e();

This isn't doing what you want. Because of the way C++ syntax works, this is actually declaring a function e that takes no parameters and returns an asio::system_error. If you add a void in the parenthesis, this becomes easier to see. It should be declared as:

asio::system_error e;

Secondly, your typedef says your function should take a reference to a system_error: asio::system_error& e. However, when you pass the above in (assuming you fix the first problem), you're trying to pass a pointer:

CurrentRequest.Callback(..., &e, ....);  // Should just be 'e'