Array of member functions

This is a discussion on Array of member functions within the C++ Programming forums, part of the General Programming Boards category; Hi, I want to implement a statemachine using an array of member function pointers instead of using switch-case statements: This ...

  1. #1
    Registered User Ward's Avatar
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    Unhappy Array of member functions

    Hi,
    I want to implement a statemachine using an array of member function pointers instead of using switch-case statements:

    This is the original situation:
    Code:
    class MyClass
    {
       private:
        UCHAR m_ucState;
       public:
        BOOL HandleState();
        BOOL HandleState_1();
        BOOL HandleState_2();
        BOOL HandleState_3();
    }
    
    BOOL MyClass::HandleState()
    {
       switch(m_ucState)
       {
          case 1:
             return HandleState_1();
          case 2:
             return HandleState_2();
          case 3:
             return HandleState_3();
       }
    }
    I want to change the HandleState function to something like this:
    Code:
    BOOL MyClass::HandleState()
    {
       return m_myStateHandlers[m_ucState]();
    }
    Can anyone help me how I should declare the 'm_myStateHandlers' array? I should be a membervariable and all HandleState_... functions are not static.

    Thanks in advance,

    Ward
    Greetings.

  2. #2
    Registered User
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    Can anyone help me how I should declare the 'm_myStateHandlers' array
    You declare any array by specifying its size and the type of things you will store in the the array. What does the type 'pointer to member function' look like? See this current thread:

    Pointer to a Member Function
    Last edited by 7stud; 04-04-2006 at 01:58 AM.

  3. #3
    Registered User Ward's Avatar
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    Declaration of m_myStateHandlers

    BOOL (*m_myStateHandlers[3])(void);

    This was not the problem, but I can not fill it up.
    Greetings.

  4. #4
    #define WORLD "sad place" LinuxCoder's Avatar
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    Code:
    BOOL (*m_myStateHandlers[3])(void);
    Shouldn't this be:
    Code:
    static BOOL (MyClass::*m_myStateHandlers[3])(void);
    I guess it must be a pointer to a member function instead of a pointer to a function, also i believe you can use it as static since the methods are not supposed to move and that way you'll only have to initialize them once. About the initialization part i'm still trying to figure it out...

    I can be sooooo wrong though, cheers

  5. #5
    C/C++ homeyg's Avatar
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    Quote Originally Posted by Ward
    BOOL (*m_myStateHandlers[3])(void);

    This was not the problem, but I can not fill it up.
    Code:
    class MyClass
    {
       private:
        UCHAR m_ucState;
        BOOL (*m_myStateHandlers[3])(void);
       public:
        MyClass();
        BOOL HandleState();
        BOOL HandleState_1();
        BOOL HandleState_2();
        BOOL HandleState_3();
    };
    
    MyClass::MyClass()
    {
       int i = 0;
       m_myStateHandlers[i++] = &HandleState_1;
       m_myStateHandlers[i++] = &HandleState_2;
       m_myStateHandlers[i++] = &HandleState_3;
    }
    
    BOOL MyClass::HandleState()
    {
       return (*m_myStateHandlers[m_ucState])();
    }
    Last edited by homeyg; 04-04-2006 at 08:55 PM.

  6. #6
    Just Lurking Dave_Sinkula's Avatar
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    Rather than an index such as m_ucState and a separate array of function pointers, how about using a single pointer to member function to contain the state information? Something along this line:
    Code:
    #include <iostream>
    
    class FSM
    {
       void (FSM::*pmf)(); // pointer to member function for current state
       void init()   { std::cout << "init"   << '\n'; pmf = &FSM::idle;   }
       void idle()   { std::cout << "idle"   << '\n'; pmf = &FSM::active; }
       void active() { std::cout << "active" << '\n'; pmf = &FSM::idle;   }
    public:
       FSM(void (FSM::*pmf_)() = &FSM::init ) : pmf(pmf_) {}
       void run()
       {
          (this->*pmf)();
       }
    };
    
    int main() 
    {
       FSM fsm;
       for ( int i = 0; i < 5; ++i )
       {
          fsm.run();
       }
       return 0;
    }
    
    /* my output
    init
    idle
    active
    idle
    active
    */
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  7. #7
    Registered User Ward's Avatar
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    Response to the latest posts

    Answer to 'homeyg':
    I've tried something similar like you posted last. But I got compiler errors. I will verify my code against yours and test it again.
    Thanks you.

    Answer to 'Dave_Sinkula':
    This is an interesting alternative way of working! At first sight is seems very difficult to perform debugging on it (keeping the overview of the states in sight), but I will surely test this out.
    Thanks.

    Kind regards,

    Ward

  8. #8
    Just Lurking Dave_Sinkula's Avatar
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    Some debuggers might show the current function name, which would tell you the state.
    Attached Images Attached Images  
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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