Passing reference to object in callback function to std::thread

Question

I'm trying to do something like this:

void commands_conn_handler(int socket, RPiServer& server) {
  // Not important code about handling connection
}

class RPiServer {
public:
  void Accept(void Accept(void (*acceped_conn_handler)(int, RPiServer&)) {
    // (...)
    int remote_socket = 0; // Doesn't matter - example.
    std::thread conn_handler_thread(acceped_conn_handler, remote_socket, *this);
    conn_handler_thread.join();
  }
};

int main() {
  RPiServer commands_server();
  commands_server.Accept(commands_conn_handler);
}

And when I try to build this, there are some errors:

In file included from /usr/include/c++/6/thread:39:0, from src/rpi_server/rpiserver.cpp:11: /usr/include/c++/6/functional: In instantiation of ‘struct std::_Bind_simple’: /usr/include/c++/6/thread:138:26: required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (*&)(int, RPiServer&); _Args = {int&, RPiServer&}]’ src/rpi_server/rpiserver.cpp:89:79: required from here /usr/include/c++/6/functional:1365:61: error: no type named ‘type’ in ‘class std::result_of’ typedef typename result_of<_Callable(_Args...)>::type result_type; ^~~~~~~~~~~ /usr/include/c++/6/functional:1386:9: error: no type named ‘type’ in ‘class std::result_of’ _M_invoke(_Index_tuple<_Indices...>) ^~~~~~~~~ Makefile:29: recipe for target 'build/rpi_server/rpiserver.o' failed

When I change thread function in following way (remove reference to object):

void commands_conn_handler(int socket) {
  // Not important code about handling connection
}

class RPiServer {
public:
  void Accept(void (*acceped_conn_handler)(int)) {
    // (...)
    int remote_socket = 0; // Doesn't matter - example.
    std::thread conn_handler_thread(acceped_conn_handler, remote_socket);
    conn_handler_thread.join();
  }
};

int main() {
  RPiServer commands_server();
  commands_server.Accept(commands_conn_handler);
}

Everything builds perfectly fine. What I'm doing wrong when I'm passing reference as parameter to thread function?

score 3 · Accepted Answer · edited Jan 22 '18 at 18:41

So here you have a working example:

#include <thread>
#include <functional>
class RPiServer;

void commands_conn_handler(int socket, RPiServer &server) {
    // Not important code about handling connection
}

class RPiServer {
public:
    void Accept(void (*acceped_conn_handler)(int, RPiServer&)) {
        // (...)
        int remote_socket = 0; // Doesn't matter - example.
        std::thread conn_handler_thread(acceped_conn_handler, remote_socket, std::ref(*this));
        conn_handler_thread.join();
    }
};

int main() {
    RPiServer commands_server;
    commands_server.Accept(commands_conn_handler);
}

The error you was getting was because you were not providing correct type for the constructor of conn_handler_thread. To explicitly get a reference to an object (which you need to do here), use a std::ref() function.

P.S.: Also you copy pasted your code example wrong, duplicating the void Accept part. You also had a most vexing parse error in main().

The compiler is deducing the arguments of the `template< class Function, class... Args > explicit thread( Function&& f, Args&&... args )` constructor as `void(*)(int,RPiServer&),int,RPiServer)`, which will not work as you cannot bind the RPiServer value to the non-const lvalue reference. You cannot pass template parameters to a constructor, so you need to explicitly specify `std::ref` on `*this`. Alternatively, making the `RPiServer&` argument const compiles as well, but you might create a temporary. — iboisver, Jan 22 '18 at 19:08

Passing reference to object in callback function to std::thread

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