Question

To store the functions of a class in an array the following link http://sourcemaking.com/design_patterns/state/cpp/1 contains the code like below (Machine is the class name).

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

The example in that link does not compile with the g++ compiler throwing error as below

$ g++ state.cpp 
state.cpp: In function ‘int main()’:
state.cpp:89:18: error: invalid use of non-static member function ‘void Machine::off()’
state.cpp:89:32: error: invalid use of non-static member function ‘void Machine::on()’
state.cpp:97:15: error: expected unqualified-id before ‘*’ token

I am using g++ version 4.5.2

$ g++ --version
g++ (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Can an array be defined like this, i am not able to find an array defination like this any where else. If the example is correct why does not it compile.

Answer 1

If you define a typedef for your member functions you will simplify your code greatly.

class Machine
{
public:
    void on(){}
    void off(){}
};


int main()
{

    typedef void (Machine::*MachineFunctionPtr)();

    MachineFunctionPtr temp[] = { &Machine::off , &Machine::on };


   //To invoke a function use this syntax
    Machine mymachine;
   ((mymachine).*(temp[1]))();

The being said your error is due to missing “&” before the function name.

If you don’t want to use typedef the correct way is something like

void(Machine:: *ptrs[])() = 
{
    &Machine::off, &Machine::on
};
Machine fsm;
int num;
while (1)
{
    cout << "Enter 0/1: ";
    cin >> num;
    ((fsm).*(ptrs[num]))();
}