Pointer to member type incompatible with object type → What is the cause?

问题

Recently I ran into a compiler (GNU g++ 4.9.2) error like this:

ProceduralTimerTaskAdapter.cpp:25:13: error: pointer to member type ‘void (Poco::Util::Timer::)(Poco::Util::TimerTask&)’ incompatible with object type ‘Poco::Util::ProceduralTimerTaskAdapter’

Here is the relevant code (which is almost self-contained, save for the necessary Poco libs):

ProceduralTimerTaskAdapter.h:

#include <Poco/Util/Timer.h>
#include <Poco/Util/TimerTask.h>
#include <Poco/Util/TimerTaskAdapter.h>

#ifndef PROCEDURALTIMERTASKADAPTER_H
#define PROCEDURALTIMERTASKADAPTER_H

using namespace std;
using namespace Poco::Util;

typedef void (*Callback) (TimerTask&);

namespace Poco {
  namespace Util {
    class ProceduralTimerTaskAdapter : public TimerTaskAdapter <Timer> {
    public:
      ProceduralTimerTaskAdapter (Callback procedure); // Constructor

      void run (); // Method defining the main thread
    protected:
      ~ProceduralTimerTaskAdapter (); // Destructor (not for general use)
    private:
      ProceduralTimerTaskAdapter (); // Default constructor (not for general use)

      Callback procedure; // The callback procedure called by the timer.
    };
  }
}

#endif

ProceduralTimerTaskAdapter.cpp:

// This is the implementation of the ProceduralTimerTaskAdapter class.

#include <iostream>
#include <Poco/Util/Timer.h>
#include <Poco/Util/TimerTask.h>
#include <Poco/Util/TimerTaskAdapter.h>
#include "ProceduralTimerTaskAdapter.h"

using namespace std;
using namespace Poco::Util;

ProceduralTimerTaskAdapter::ProceduralTimerTaskAdapter (Callback procedure) : TimerTaskAdapter<Timer>::TimerTaskAdapter (*(new Timer ()), procedure)
{
  this -> procedure = procedure;
}

ProceduralTimerTaskAdapter::~ProceduralTimerTaskAdapter ()
{
}

void ProceduralTimerTaskAdapter::run ()
{
  TimerTask &task = *this;

  (this ->* procedure) (task);
}

What I wanna do is, in fact, build an extension of the well-known TimerTaskAdapter to handle callback functions, which are not tied to a specific class (because they are situated in main.cpp, for instance). I override the virtual method run () with a very simple self-made one, which calls the callback. After having handled several different errors, I ended up with this apparent class mismatch I can't solve myself. I even don't understand why the compiler states a class name, whose name is Poco::Util::Timer:: (Why does it end with ::?). As ProceduralTimerTaskAdapter defines a member named procedure, why does the compiler expect another class?

Thank you.

回答1:

Derive from Poco::Util::TimerTask (like in Poco::Util::TimerTaskAdapter class) and override run method in which you will call procedures.

class ProcedureAdapter : public Poco::Util::TimerTask {
public:

    typedef void (*Callback)(TimerTask&);

    ProcedureAdapter (Callback c) : callback(c) {;}

    void run () {
        callback(*this); // call some procedure which takes TimerTask
    }

    Callback callback;
};

void fun (Poco::Util::TimerTask&) {
    cout << "fun was invoked" << endl;
}

void fun2 (Poco::Util::TimerTask&) {
    cout << "fun2 was invoked" << endl; 
}

int main()
{
    Poco::Util::Timer t;
    t.schedule (new ProcedureAdapter{&fun},1,1);
    t.schedule (new ProcedureAdapter{&fun2},1,1);

回答2:

The syntax ->* expects a left-hand operator of type pointer to class object (such as this) and a right-hand operator of type pointer to member function of that class. But in

TimerTask &task = *this;          // line 24
(this ->* procedure) (task);      // line 25

procedure is not a pointer to a member function of ProceduralTimerTaskAdapter. So your code is ill-formed. procedure is simply a pointer to a free (non-member) function taking a TimerTask& and returning void. If ProceduralTimerTaskAdapter is derived from TimerTask then the following code should compile

TimerTask &task = *this;
(this -> procedure) (task);

or shorter

procedure(*this);

using the fact that pointers to functions can syntactically be used like the function.

Edit. It appears (from your comments to another answer) that your code was ill-formed in yet another way, namely that ProceduralTimerTaskAdapter was not derived from TimerTask. Then, of course already line 24 (not just 25) should produce an error. It seems, therefore, that you didn't show us the precise same code as the one that created the error message, or not all the errors it causes.

来源：https://stackoverflow.com/questions/53141938/pointer-to-member-type-incompatible-with-object-type-%e2%86%92-what-is-the-cause

标签

c++

function-pointers

poco-libraries