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

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.

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_{NOTE: You should not use leading underscores for any symbols (as in class _tcp)!}