Function pointers in classes

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  18 years ago

  by Kithera

  Grant Wagner

  Crofton Maryland USA, United States

  Joined 18 years ago

  I'm currently working on some real time graphic generations using Direct X and Visual C++ 6.0. I'm famular with general function pointers as shown in this example...
  

  
  int add(int rhs, int lhs);
  int sub(int rhs, int lhs);
  

  
  int (*math)(int rhs, int lhs);
  

  
  void main (void)
  {
     int a = 5, b = 7;
     string temp;
     Thing MyThing;
  

  
     cout << "Current a:" << a << " b:" << b << "." << endl;
  

  
     cout << "Adding." << endl;
     math = add;
     a = math(a, b);
     cout << "Current a:" << a << " b:" << b << "." << endl;
  

  
     cout << "Subtracting." << endl;
     math = sub;
     a = math(a, b);
     cout << "Current a:" << a << " b:" << b << "." << endl;
  

  
     cin >> temp;
     return;
  }
  

  
  int add(int rhs, int lhs) { return(rhs + lhs);}
  int sub(int rhs, int lhs) { return(rhs - lhs);}
  

  
  However, when I create a class, the syntax no longer works, and I am unable to get it to compile. Such as....
  

  
  class Thing
  {
  public:
     int add(int rhs, int lhs);
     int sub(int rhs, int lhs);
  

  
     int (Thing::*math)(int rhs, int lhs);
  };
  

  
  void main (void)
  {
     int a = 5, b = 7;
     string temp;
     Thing MyThing;
  

  
     cout << "Current a:" << a << " b:" << b << "." << endl;
  

  
     cout << "Adding." << endl;
     MyThing.math = MyThing.add;
     a = MyThing.math(a, b);
     cout << "Current a:" << a << " b:" << b << "." << endl;
  

  
     cout << "Subtracting." << endl;
     MyThing.math = MyThing.sub;
     a = MyThing.math(a, b);
     cout << "Current a:" << a << " b:" << b << "." << endl;
  

  
     cin >> temp;
     return;
  }
  

  
  int Thing::add(int rhs, int lhs) { return(rhs + lhs);}
  int Thing::sub(int rhs, int lhs) { return(rhs - lhs);}
  

  
  

  
  This code will not compile with an error code saying that MyThing.math never evaluates to a function. What is it that I'm missing? I have noticed that nearlly every C++ book out there seems to just gloss this part over. Any help would be greatly appreciated.
  

  
  Grant
  

  
  

  
  

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• • Reply

  18 years ago

  by qwijibow

  Chris Stones

  England Nottingham, United Kingdom

  Joined 19 years ago

  hahaha, i had the same problem, ur gonna kik urself when i tel you...
  

  
  god knows why, But u know how u dont need a ";" after the last "}" in a function,
  

  
  well you do in a class !!!!
  

  
  so as you know a function looks like...
  

  
  int main()
  {
    //a load of code
    return 0;
  }
  

  
  and a class would look like...
  

  
  class CSomeClass
  {
     private:
     //sum private variables and functions
     public:
    //sum public functions
  };
  

  
  

  
  see,, so slap that little ; in there, and it should work.
  

  
  

  
  

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• • Reply

  18 years ago

  by qwijibow

  Chris Stones

  England Nottingham, United Kingdom

  Joined 19 years ago

  sorry, was looking at the wrong part
  ahh well im an idiot ! sorry.

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