Error when iterating through an array of pointers to functions

Question

ISSUE

When i am iterating through an array of pointers to functions i am always getting an error in this example case of code. But if i wont iterate through them everything work fine as it ca be seen by the code below too.

Code

class OutterClass {
private:
   
  class InnerClass{
    public:
    
    InnerClass(){}
    
    float Func1 (const float &x ) { return (x > 0) ? x : 0            ;}
    float Func2 (const float &x ) { return 1 / (1 + exp(-x))          ;}
    float Func3 (const float &x ) { return (x > 0) ? x : (exp(x) - 1) ;}

    void test_ISSUE(){  
      //this doesn't work
      /*
      for(int i=0; i<3; i++)
        Serial.println(((this)->*(Function_ptrs)[i])(2));
      */

      // this works
      Serial.println(((this)->*(Function_ptrs)[0])(2));
      Serial.println(((this)->*(Function_ptrs)[1])(2));
      Serial.println(((this)->*(Function_ptrs)[2])(2));
      
    }  
    
  };
  typedef float (InnerClass::*method_function) (const float &);
  static const method_function (Function_ptrs)[3] = {
    &InnerClass::Func1,
    &InnerClass::Func2,
    &InnerClass::Func3
  };
  
public:
  
  InnerClass *Reference;
  OutterClass(){ Reference = new InnerClass();}  
};


void setup() { 
  Serial.begin(9600); 
  
  OutterClass *Obj = new OutterClass();
  Obj->Reference->test_ISSUE();
}

void loop () {}

Recreating the Error

just uncomment for(int i=0; i<3; i++) ...

Error

...: In function `test_ISSUE':

.../sketch_jan23a.ino:17: undefined reference to `OutterClass::Function_ptrs'
.../sketch_jan23a.ino:17: undefined reference to `OutterClass::Function_ptrs'

collect2.exe: error: ld returned 1 exit status

exit status 1
Error compiling for board Arduino Uno.

Any idea?

UPDATE SOLUTION

just used inline static const method_function (Function_ptrs)[3] as stated here and now it works..