How to switch with pointer to member functions?

Question

Well, all I want to do is a "switch" with a function pointer, but with methods pointers. The switch is that if I call the method Run(), it will either redirect to A::RunOn() or A::RunOff() according to Run ptr is pointing to these member functions.

I know it can be done. I did it in plain c but I have searched and googled to do the same thing in c++ but no luck.

class A
{
    typedef (void)(A::*RunPtr)(int);
    RunPtr RunMethod;

public:
    RunMethod Run;

    A()
    {
        Run = RunOff;
    }

    void SetOn(bool value)
    {
        if (value)
            Run = RunOn;
        else
            Run = RunOff;
    }

    void RunOn(int)
    {
        // RunOn stuff here
    }

    void RunOff(int)
    {
        // RunOff stuff here
    }
};

So I can call Run() and there will be a switch between the function calls, which I think is more efficient than just doing:

if (on)
    RunOn();
else
    RunOff();

Don't know how to do it!

Answer 1

Your member function pointer typedef is wrong (Despite the other issues in the shown code). You need

typedef void(A::*RunPtr)(int);

Or you can provide the alias for the member function pointer of class A with the help of using keyword as follows:

using RunPtr = void(A::*)(int);
RunPtr RunMethod;

Now in the SetOn you can do member pointer assignment as follows

void SetOn(bool value)
{
    RunMethod = value ? &A::RunOn : &A::RunOff;
}

---

Now, in order to call the stored member function pointer, you may/ can provide a Run member function as follows:

void  Run(int arg)
{
    std::invoke(RunMethod, this, arg);
    // do something...
}

The call to member function is a bit tricky. However, this can be done using more generic std::invoke from <functional> header (Since c++17).

Here is the complete example:

#include <iostream>
#include <functional> //  std::invoke

class A
{
    using RunPtr = void(A::*)(int);
    // or with typedef
    // typedef void(A::*RunPtr)(int);
    RunPtr RunMethod;

public:
    void SetOn(bool value)
    {
        RunMethod = value ?  &A::RunOn : &A::RunOff;
    }

    void  Run(int arg)
    {
        std::invoke(RunMethod, this, arg);
        // do something...
    }

    void RunOn(int arg) { std::cout << "RunOn: " << arg << "\n"; }

    void RunOff(int arg) { std::cout << "RunOff: " << arg << "\n";  }
};

int main()
{
    A obj;
    obj.SetOn(true);
    obj.Run(1);       // prints: RunOn: 1  
    obj.SetOn(false);
    obj.Run(0);       // prints: RunOff: 0
}

(See a Demo)