Re: Error while writing State Design Pattern Code

From:

"Daniel T." <daniel_t@earthlink.net>

Newsgroups:

comp.lang.c++

Date:

Sat, 31 May 2008 10:28:22 -0400

Message-ID:

<daniel_t-515354.10282131052008@earthlink.vsrv-sjc.supernews.net>

In article
<2c1ee6ab-5e22-48e5-8a75-f6c6d438a841@w34g2000prm.googlegroups.com>,
Pallav singh <singh.pallav@gmail.com> wrote:

Hi All

i am getting Error while writing following code for state design
Pattern
kindly let me know How to Correct this Error ??

Thanks
Pallav Singh

Results from compiling in: http://www.comeaucomputing.com/tryitout and
my own observations...

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

#include<iostream.h>

(From comeau)
<iostream.h> is not a Standard header, use <iostream> instead.

using namespace std;

class state;

class Machine
{
class state * current;

keyword 'class' unnecessary above.

public :
Machine();

void setcurrentstate(state * s)
   { current = s ; }

void on();
void off();
};

class state
{
   public :
   virtual void on(Machine * m)
     {cout<< " Already On \n"; }

Above, and in general, your use of whitespace is inconsistent,
especially inside quotes.

   virtual void off(Machine * m)
     {cout<<"Already Off \n"; }
};

void Machine::on()
   { current->on(this); }

void Machine::off()
   { current->off(this); }

class ON : public state
{
   public :
   ON() {cout<<"Destructor invoked \n ";}
  ~ON() {cout<<"Constructor invoked \n ";}
   void off(Machine * m);
};

class OFF : public state
{
   public :
   OFF() {cout<<"Destructor invoked \n ";}
  ~OFF() {cout<<"Constructor invoked \n ";}
   void on(Machine * m)
    {cout<<"Going from OFF to ON";
     m->setcurrentstate( new ON() );
     delete this;
    }
};

Machine::Machine()
   {
     current = new OFF();
     cout<<"Machine constructor Called "<<endl;
   }

void ON::off(Machine * m)
   {
     cout<<"Going from ON to OFF";
     m->setcurrentstate( new OFF() );
     delete this;
   }

int main()
{

void (Machine::*ptrs[] )() = { Machine::off, Machine::on }; // ErrorPoint

(From comeau)
error: nonstandard form for taking the address of a member function
   void (Machine::*ptrs[] )() = { Machine::off, Machine::on };
                                  ^

(From comeau)
error: nonstandard form for taking the address of a member function
   void (Machine::*ptrs[] )() = { Machine::off, Machine::on };
                                                ^

  Machine FSM;
  int num;
  while(1)
   { cout <<"Enter 0 / 1 : ";
     cin >> num;
     (FSM.*ptrs[num])();
   }

   return 0;

(From comeau)
warning: statement is unreachable
return 0;
^

}