Design problem

This is a discussion on Design problem within the C++ Programming forums, part of the General Programming Boards category; Code: Term::Term(const Term& t) { std::vector<Factor*> x = t.factors; if (x.empty()) return; Factor* f; for (std::vector<Factor*>::iterator i = x.begin();i != ...

  1. #1
    Just because ygfperson's Avatar
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    Design problem

    Code:
    Term::Term(const Term& t) {
      std::vector<Factor*> x = t.factors;
      if (x.empty()) return;
      Factor* f;
      for (std::vector<Factor*>::iterator i = x.begin();i != x.end();++i) {
        if ((*i)->who_am_i() == Factor::FLOATING) {
          f = new Float(*((Float*)(*i)));
          factors.push_back(f);
        }
        if ((*i)->who_am_i() == Factor::FACTOR_EXPRESSION) {
          f = new Factor_Expression(*((Factor_Expression*)*i));
          factors.push_back(f);
        }
        if ((*i)->who_am_i() == Factor::FUNCTION) {
          f = new Function(*((Function*)*i));
          factors.push_back(f);
        }
        if ((*i)->who_am_i() == Factor::VARIABLE) {
          f = new Variable(*((Variable*)*i));
          factors.push_back(f);
        }
      }
    }
    Four classes (Variable, Function, Factor_Expression, and Float) derive from the class Factor.

    The goal of this constructor is to create new objects as you can see and put them into a vector array. I'm sure there's a simpler way to do this, something more OOP friendly, but I can't think of it.

  2. #2
    Skunkmeister Stoned_Coder's Avatar
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    it would be a little short sighted to comment without knowing more about the whole structure of your app.
    Can you give a synopsis of your classes, their responsibility and how they are related to other classes.
    Do you know and understand object factories?
    Free the weed!! Class B to class C is not good enough!!
    And the FAQ is here :- http://faq.cprogramming.com/cgi-bin/smartfaq.cgi

  3. #3
    S Sang-drax's Avatar
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    You can use dynamic_cast<>() and typeid() instead of the who_am_i() function.

    You should use dynamic_cast<>() instead of the C-style casts.
    Last edited by Sang-drax : Tomorrow at 02:21 AM. Reason: Time travelling

  4. #4
    Just because ygfperson's Avatar
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    Do you know and understand object factories?
    No... I haven't heard the term before.

    The purpose of this function is to create a new object for each object pointer in the vector array 'factors'.

    I think there should be a way to simplify this. Right now, the function checks each pointer to determine its object type, then creates a new appropriate object, and puts it into the vector array 'factors'.

    So my question is, is there a way to let the object itself decide the new type of object to be produced?

    I'm thinking of something like this:
    Code:
    Factor* Variable::new_object() {
      return new Factor(*this);
    }
    And doing it for each class. But I'm not sure if that's the right way to do it.

    //edit: thanks, sang, i'll look into it

  5. #5
    S Sang-drax's Avatar
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    Perhaps this is a little better:
    Code:
    Term::Term(const Term& t) {
      std::vector<Factor*> x = t.factors;
      if (x.empty()) return;
    
      Float* float;
      Expression* expression;
      Function* function;
      Variable* variable;
      
      for (int i = 0;i < x.size(); ++i)
      {
        if (float = dynamic_cast<Float*>(x[i])) 
          factors.push_back(new Float( *float ));
    
        else if (expression = dynamic_cast<Expression*>(x[i])) 
          factors.push_back(new Expression( *expression ));
    
        else if (function = dynamic_cast<Function*>(x[i])) 
          factors.push_back(new Function( *function ));
          
        else if (variable = dynamic_cast<Variable*>(x[i])) 
          factors.push_back(new Variable( *variable ));
    
      }
    }
    EDIT: The new_object() solution could work, make it virtual and you can get rid of all the if-statements!
    Last edited by Sang-drax : Tomorrow at 02:21 AM. Reason: Time travelling

  6. #6
    Skunkmeister Stoned_Coder's Avatar
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    read this carefully. It explains a lot. If you get lost then start again and reread it. You will have some new ideas after reading thru this sample chapter of Andrei Alexandrescus Modern cpp design.
    Free the weed!! Class B to class C is not good enough!!
    And the FAQ is here :- http://faq.cprogramming.com/cgi-bin/smartfaq.cgi

  7. #7
    S Sang-drax's Avatar
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    Looks interesting; I've printed the pages and I'll read them later.
    Last edited by Sang-drax : Tomorrow at 02:21 AM. Reason: Time travelling

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