C++ error: reference to non-static member function must be called - c++

Question

I'm trying to create a class to abstract some basic behavior of libuv's networking functions.
#define TCP_BACKLOG 256
class _tcp {
uv_tcp_t* tcp = NULL;
public:
~_tcp() { delete tcp; }
void listen_uv_listen_uv_connection_cb(uv_stream_t* stream, int status) {
printf("NEW CONNECTION\n");
}
void listen(const char* host, int port) {
tcp = new uv_tcp_t();
uv_tcp_init(uv_default_loop(), tcp);
sockaddr_in* addr = new sockaddr_in();
uv_ip4_addr(host, port, addr);
uv_tcp_bind(tcp, (const sockaddr*)addr, 0);
delete addr;
uv_listen((uv_stream_t*)tcp, TCP_BACKLOG, listen_uv_listen_uv_connection_cb);
}
};
The problem with the previously shown code is that when I try to compile it I get the following error:
error: reference to non-static member function must be called
on: uv_listen((uv_stream_t*)tcp, TCP_BACKLOG, listen_uv_listen_uv_connection_cb);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
And it points to listen_uv_listen_uv_connection_cb as the culprit.
Can someone explain to me, why is that an error, and how am I supposed to fix it?
The uv_listen() and uv_connection_cb signatures are declared as follows
UV_EXTERN int uv_listen(uv_stream_t* stream, int backlog, uv_connection_cb cb);
typedef void (*uv_connection_cb)(uv_stream_t* server, int status);

Answer 1

You cannot convert non-static member function to a pointer to function even with the same signature, as technically member function has a hidden parameter called this. One of the solution is to make listen_uv_listen_uv_connection_cb static:
class _tcp {
uv_tcp_t* tcp = NULL;
public:
~_tcp() { delete tcp; }
static void listen_uv_listen_uv_connection_cb(uv_stream_t* stream, int status) {
printf("NEW CONNECTION\n");
}
void listen(const char* host, int port) {
tcp = new uv_tcp_t();
uv_tcp_init(uv_default_loop(), tcp);
sockaddr_in* addr = new sockaddr_in();
uv_ip4_addr(host, port, addr);
uv_tcp_bind(tcp, (const sockaddr*)addr, 0);
delete addr;
uv_listen((uv_stream_t*)tcp, TCP_BACKLOG,
&_tcp::listen_uv_listen_uv_connection_cb);
}
};
PS to be able to call a non-static method you would need a way to get a pointer to your _tcp instance from "uv_stream_t* stream" parameter. I would suggest to use "void* uv_handle_t.data" pointer from this doc http://docs.libuv.org/en/latest/handle.html#c.uv_handle_t
static void listen_uv_listen_uv_connection_cb(uv_stream_t* stream, int status) {
_tcp *tcp = static_cast<_tcp *>( stream->data );
tcp->regularMethod();
}
Of course you should assign this pointer to uv_handle_t.data when you initialize uv_tcp_t *:
void listen(const char* host, int port) {
tcp = new uv_tcp_t();
uv_tcp_init(uv_default_loop(), tcp);
tcp->data = this; // do not forget it
...
}
and I would move this initialization code to constructor.
You would need such static wrapper for every callback you are going to use with this library. With c++11 you probably can use lambda instead.

Answer 2

The uv_listen() call back connector expects a static or free (outside class) function.
Thus you should declare your function like this
static void listen_uv_listen_uv_connection_cb(uv_stream_t* stream, int status) {
printf("NEW CONNECTION\n");
_tcp* thisStream = static_cast<_tcp*>(stream);
}
Well, the static_cast<> actually requires your _tcp class inherits from uv_stream_t
class _tcp : public uv_stream_t {
// ...
};
To extend on your comment
"Could you please explain to me why does uv_listen expects a static function? Is this the behavior for all function pointer parameters?"
There's a difference made between class member function pointers, that need to be bound to a class instance for calling, and plain function pointers, that work for any function definition.
Why uv_listen() expects a plain function pointer, is hard to tell. May be because it's a native C-API (I actually don't know it), or for sake of flexibility.
NOTE: You should not use leading underscores for any symbols (as in class _tcp)!

Answer 3

void listen_uv_listen_uv_connection_cb(uv_stream_t* stream, int status) {
printf("NEW CONNECTION\n");
}; <<<<<remove ;
There should be no semicolon at the end of function definition.
And you should write constructor/copy ctr/assign operator for this class.