C++ "Incomplete type is not allowed" trying to create array of functions inside a class

Question

How do I get the below code example, that compiles and works just fine to work inside a class?

---

Below code works just fine

#include <iostream>
using namespace std;

typedef int (*IntFunctionWithOneParameter) (int a);

int function(int a){ return a; }
int functionTimesTwo(int a){ return a*2; }
int functionDivideByTwo(int a){ return a/2; }

void main()
{
    IntFunctionWithOneParameter functions[] = 
    {
        function, 
        functionTimesTwo, 
        functionDivideByTwo
    };

    for(int i = 0; i < 3; ++i)
    {
        cout << functions[i](8) << endl;
    }
}

---

So the above code works fine, but I want to move it inside a class in a separate file, similar to the below NON-WORKING IDEA, where I get "incomplete type is not allowed" error at "functions[] =";

class myClass {
private:

  typedef int (*IntFunctionWithOneParameter) (int a);

  int function(int a){ return a; }
  int functionTimesTwo(int a){ return a*2; }
  int functionDivideByTwo(int a){ return a/2; }

  IntFunctionWithOneParameter functions[] = 
    {
        function, 
        functionTimesTwo, 
        functionDivideByTwo
    };
};

So my question is how can I get it to work inside my class, where it is the ONLY place the functions are needed, meaning I do need to access the functions in main() or other places!

---

EDIT

Here is why I need an "array of functions". To save time spent on "if's" or more exactly "switches" as I am making a software (vst) synthesizer, and the less time spent in the processing, the more notes (polyphonic) the user can play at any given time. And multiply the 44100 times per second the function is run, with 8 tone generators, which each can have up to 16 unison voices, so actually the function needed, may be called up to 5,644,800 times per second, per note played! The exact function needed inside this main loop is known BEFORE entering loop, and ONLY changes when the user adjust a knob, so I do want to avoid ifs and switches. Now had it only been one function that occasionally changes, i could just duplicate main loop with variations for each function possible, HOWEVER the main audio processing loop, has several areas, each with a variety of ever growing functions possible, each which ONLY changes when user changes various knobs. So although I could, I am not going to make 5 * 20 * 23 (and growing) different versions of a main loop, to avoid if's and switches.

Answer 1

There's a bunch of things wrong with the code that you posted:

• No semicolon after class definition.
• Class instead of class
• No fixed size set for the functions member, which is not allowed. You need to explicitly set the size of the array.
• Member function pointers are not the same as "regular" function pointers. Member function pointers have an implicit this as first argument, since they need an object to be invoked on. So myFunction is not of type myArrayOfFunctions. If you make myFunction and myFunction2 static, then they can be stored as regular function pointers. Is this an option?
• The name myArrayOfFunctions is very confusing, since it's not an array at all.

All but the last of these will cause your code not to compile.

Answer 2

This example may be what you've needed.

Note: I've changed typedef statement to using and changed function's signatures to take in plain int for testing convinience sake.

class myClass {
public:
    using myArrayOfFunctions = float(myClass::*)(int a, int b, float c);

    float myFunction1 (int a, int b, float c)
    {
        return a * b * c;
    }

    float myFunction2 (int a, int b, float c)
    {
        return a + b + c;
    }

    myArrayOfFunctions functions[2];

    myClass()
    {
        functions[0] = &myClass::myFunction1;
        functions[1] = &myClass::myFunction2;
    };

    void Invoke()
    {
        (this->*functions[0])(1, 2, 3);
        (this->*functions[1])(3, 2, 1);
    }
};

int main()
{
    myClass a;
    a.Invoke();
    (a.*(a.functions[0]))(4, 5, 6);
    return 0;
}

As you see, I'm getting the pointer to the class function but to call it I need to call it with an actual object (this in invoke() function and a object in main()).

Answer 3

You can write this:

class myClass
{
public:
    typedef float (*myArrayOfStaticFunctions) (int& a, int& b, float& c);
    typedef float (myClass::*myArrayOfFunctions) (int& a, int& b, float& c);

    static float myFunction1 (int& a, int& b, float& c){cout<<"myFunction1"<<endl; return 0;}
    static float myFunction2 (int& a, int& b, float& c){ cout<<"myFunction2"<<endl; return 0;}
    float myFunction3 (int& a, int& b, float& c){ cout<<"myFunction3"<<endl; return 0;}
    float myFunction4 (int& a, int& b, float& c){ cout<<"myFunction4"<<endl; return 0;}

    myArrayOfStaticFunctions    StaticArrayfunctions[2];
    myArrayOfFunctions          Arrayfunctions[2];

    myClass (){
        StaticArrayfunctions [0] =myFunction1;
        StaticArrayfunctions [1] =myFunction2;
        Arrayfunctions [0] = &myClass::myFunction3;
        Arrayfunctions [1] = &myClass::myFunction4;
    }
};

int _tmain(int argc, _TCHAR* argv[])
{
    myClass m;
    int a =0, b=0; float c;

    m.StaticArrayfunctions[0] (a,b,c);
    m.StaticArrayfunctions[1] (a,b,c);

    myClass::myArrayOfFunctions func3 = m.Arrayfunctions[0];
    myClass::myArrayOfFunctions func4 = m.Arrayfunctions[1];

    (m.*func3)(a,b,c);
    (m.*func4)(a,b,c);

    return 0;
}