C++ expression cannot be used as a function: (_f2)(arg); - c++

Question

A few years ago i had my Open tibia server, it's old source engine, previous compiled at gcc 4.7+, already i try to compile it at newest debian, my gcc version is: 8.3.0 (Debian 8.3.0-6)
error: no match for call to ‘(Task) (Game*&)’
(*_f2)(arg);
error displays: expression cannot be used as a function
(_f2)(arg);
template<class ArgType>
class TCallList : public SchedulerTask{
public:
TCallList(
boost::function<bool(Game*, ArgType)> f1,
Task* f2,
std::list<ArgType>& call_list,
int64_t interval) :
_f1(f1), _f2(f2), _list(call_list), _interval(interval)
{
//
}
virtual void operator()(Game* arg)
{
if(_eventid != 0){
bool ret = _f1(arg, _list.front());
_list.pop_front();
if(ret){
if(_list.empty()){
//callback function
if(_f2){
(_f2)(arg);
delete _f2;
}
}
else{
//fire next task
SchedulerTask* newTask = new TCallList(_f1, _f2, _list, _interval);
newTask->setTicks(_interval);
newTask->setEventId(this->getEventId());
arg->addEvent(newTask);
}
}
}
}
private:
boost::function<bool(Game*, ArgType)> _f1;
Task* _f2;
std::list<ArgType> _list;
int64_t _interval;
};
template<class ArgType>
SchedulerTask* makeTask(int64_t ticks,
boost::function<bool(Game*, ArgType)>* f1,
std::list<ArgType>& call_list,
int64_t interval,
Task* f2)
{
TCallList<ArgType>* t = new TCallList<ArgType>(f1, f2, call_list, interval);
t->setTicks(ticks);
return t;
}
Scheduler.cpp
#ifndef __OTSERV_SCHEDULER_H__
#define __OTSERV_SCHEDULER_H__
#include <boost/function.hpp>
#include <boost/bind.hpp>
#include <functional>
#include "otsystem.h"
class Game;
class SchedulerTask {
public:
SchedulerTask() {
_eventid = 0;
_cycle = 0;
}
virtual ~SchedulerTask()
{
//
}
// definition to make sure lower cycles end up front
// in the priority_queue used in the scheduler
inline bool operator<(const SchedulerTask& other) const
{
return getCycle() > other.getCycle();
}
virtual void operator()(Game* arg) = 0;
virtual void setEventId(uint32_t id)
{
_eventid = id;
}
inline uint32_t getEventId() const
{
return _eventid;
}
virtual void setTicks(const int64_t ticks)
{
_cycle = OTSYS_TIME() + ticks;
}
inline int64_t getCycle() const
{
return _cycle;
}
protected:
uint32_t _eventid;
int64_t _cycle;
};
class TSchedulerTask : public SchedulerTask{
public:
TSchedulerTask(boost::function1<void, Game*> f) :
_f(f)
{
//
}
virtual ~TSchedulerTask()
{
//
}
virtual void operator()(Game* arg)
{
_f(arg);
}
protected:
boost::function1<void, Game*> _f;
};
SchedulerTask* makeTask(boost::function1<void, Game*> f);
SchedulerTask* makeTask(int64_t ticks, boost::function1<void, Game*> f);
class lessSchedTask : public std::binary_function<SchedulerTask*, SchedulerTask*, bool>{
public:
bool operator()(SchedulerTask*& t1, SchedulerTask*& t2)
{
return *t1 < *t2;
}
};
#endif