does adding a dummy parameter to constructors of a class to solve calling ambiguity, violate any rule ?

Question

take following class and two object definitions:

class Rect{
 public:
  enum centimeter;
  enum meter;
  Rect(double len,double wid,enum centimeter){
   length=(len/100);
   width=(wid/100);
  }
  Rect(int len,int wid,enum meter){
   length=len;
   width=wid;
  }
  //rest of implementation
 private:
  double length;//in meters
  double width;//in meters
};
Rect obj1(10,5,Rect::centimeter());
Rect obj2(10,5,Rect::meter());

two previous constructors have dummy enum parameters to solve calling ambiguity caused in case these dummy parameters didn't exist. Now in spite of possibility of using named constructors here, if I insist on using these dummy parameters, does this violate any coding rule that I should be aware of ?

Answer 1

I think it violates my taste. I would code it like this:

enum Unit {
  Centimeter = 100, 
  Meter      = 1
};

Rect(int len, int wid, Unit unit) {
  length = len / (int) unit;
  width = wid / (int) unit;
}

Rect obj1(10, 5, Rect::Centimeter);
Rect obj2(10, 5, Rect::Meter);

Answer 2

Can't say this breaks a rule, but... It isn't easy to read.

Why can't you declare a

enum metrics {
  centimeter,
  meter
};

and use it as the constructor parameter? Or it can be

class Rect {
public:
  static Rect CreateWithMeters(int width, int height);
  static Rect CreateWithCentimenets(int width, int height);
}

Either is better than current code at my taste.

Answer 3

STL uses that idiom to differentiate iterator types in lieu of concepts.

Answer 4

Rect(int len,int wid,enum centimeter){
  length=(len/100);
  width=(wid/100);
}

In addition to what others wrote, this logic is bad, because Rect(99,99,Rect::centimeter()) is equal to Rect(0,0,Rect::centimeter()).

If you are storing meters internally, do not provide an interface with centimetres, not this way nor any other way.

Answer 5

BOOST_STRONG_TYPEDEF could be the answer here.

BOOST_STRONG_TYPEDEF( double, Meter )
BOOST_STRONG_TYPEDEF( double, Centimeters)
BOOST_STRONG_TYPEDEF( double, Furlongs)

class Rect
{
 public:
  Rect(Meter len, Meter wid) : length(len), width(wid) 
  {};

  Rect(Centimeter len, Centimeter wid) : length(len/100), width(wid/100) 
  {};
}

Rect obj1(Meter(10),Meter(5));
Rect obj1(Centimeter(10),Centimeter(5));