Passing reference to object in callback function to std::thread [duplicate] - c++

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C++ Thread taking reference argument failed compile
(2 answers)
Closed 5 years ago.
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?

Answer 1

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().