Coding State Machine in C++ - c++

Question

I'm attempting to code a state machine based on a gumball machine. I have a interface class of a basic state, while I have specific states that use this interface. I have four states, no_quarter, has_quarter, sold, and sold_out states. I also have a gumball machine class that handles these states and depending on which state my machine is in, it will go that class and do the needed action. Here is my code that is having the problem, I'll post my functions also.
Gumball_Machine.h
class Gumball_Machine
{
private:
int gumball_count;
State *current_state;
No_Quarter_State *nqs;
Has_Quarter_State *hqs;
Sold_State *ss;
Sold_Out_State *sos;
public:
Gumball_Machine(int inventory)
{
gumball_count = inventory;
nqs = new No_Quarter_State(this);
hqs = new Has_Quarter_State(this);
ss = new Sold_State(this);
sos = new Sold_Out_State(this);
if (gumball_count == 0)
set_state(sos);
else
set_state(nqs);
}
void insert_quarter()
{
current_state->insert_quarter();
}
void eject_quarter()
{
current_state->eject_quarter();
}
void turn_crank()
{
current_state->turn_crank();
}
void dispense()
{
current_state->dispense();
}
void set_state(State *new_state)
{
current_state = new_state;
}
State *get_no_quarter_state()
{
return nqs;
}
State *get_has_quarter_state()
{
return hqs;
}
State *get_sold_state()
{
return ss;
}
State *get_sold_out_state()
{
return sos;
}
No_Quarter_State.h
#ifndef NO_QUARTER_STATE_H_INCLUDED
#define NO_QUARTER_STATE_H_INCLUDED
#include "State.h"
class No_Quarter_State: public State
{
public:
No_Quarter_State(Gumball_Machine *gbm);
void insert_quarter();
void eject_quarter();
void turn_crank();
void dispense();
};
#endif // NO_QUARTER_STATE_H_INCLUDED
No_Quarter_State.cpp
#include "No_Quarter_State.h"
#include "Gumball_Machine.h"
No_Quarter_State::No_Quarter_State(Gumball_Machine *machine)
{
machine = machine;
}
void No_Quarter_State::insert_quarter()
{
cout << "You inserted a quarter.\n";
machine->set_state(machine->get_has_quarter_state());
}
void No_Quarter_State::eject_quarter()
{
cout << "You must insert a quarter before you can eject one.\n";
}
void No_Quarter_State::turn_crank()
{
cout << "You must insert a quarter before you can crank the handle.\n";
}
void No_Quarter_State::dispense()
{
cout << "You need to pay first before you can get a gumball.\n";
}
The line I'm having an issue is in the No_Quarter_State.cpp
machine->set_state(machine->get_has_quarter_state());
This is giving me a run-time error. I've seen examples like this but I'm not completely sure if this is legal in C++. I'm attempting to switch the state of my gumball machine object.
The error I get is a generic not responding error: "test.ext has stopped working". I'm using CodeBlocks to code this.

Answer 1

In the constructor, the presumed member variable machine is hidden by the parameter.
No_Quarter_State::No_Quarter_State(Gumball_Machine *machine)
{
machine = machine;
}
You can fix this by using initializer list syntax instead:Thanks Sneftel and NathanOliver
No_Quarter_State::No_Quarter_State(Gumball_Machine *machine)
: machine(machine)
{
}
However, in regular method functions, you would have to use this-> if you named the method parameter the same as the member variable. A typical style used to avoid that issue is to prepend m_ or append _ to member names.