Overload is ambiguous error. How to work around this?

Projects Programming
1

I'm trying to create a base class. I'd like to be able to instantiate the class several different ways but am getting the 'overload is ambiguous error'

Here's a simple class to illustrate. I'd like to be able to instantiate Shapes that either have width/heights or radius

class Shape {
   protected:
      float width, height, radius;
   public:
      Shape( float r=0) {
        radius = r;
      }
      Shape( float a=0, float b=0) {
         width = a;
         height = b;
      }

      // pure virtual function
      virtual float area() = 0;
};

class Circle: public Shape{
   public:
      Circle(float r=0): Shape(r) {}
      float area();
};
class Rectangle: public Shape{
   public:
      Rectangle( float a=0, float b=0):Shape(a, b) { }
      float area();
};

When "Circle" builds off of "Shape", I get the following error:

Circle.h:7: error: call of overloaded 'Shape(float&)' is ambiguous
shape.h:12: note: candidates are: Shape::Shape(float, float)
shape.h:9: note:                 Shape::Shape(float)
shape.h:5: note:                 Shape::Shape(const Shape&)

Of course, if I change one of the class constructors to be "int" and leave the other "float", it works. For example this works but at the expensive of not having the type I want for one of the constructors:

class Shape {
   protected:
      float width, height;
      int radius;
   public:
   Shape( int r=0) {
        radius = r;
      }
      Shape( float a=0, float b=0) {
         width = a;
         height = b;
      }

Question: How do I build the base class overload constructors so that both (or many) can share the same numeric type?

2 
      Shape( float r=0) {
      Shape( float a=0, float b=0) {

Well, they are ambiguous. How about not giving default values?

3

Let's put it this way. If you use this:

shape foo;

Do you expect the first or second constructor to be called? If you don't know, neither does the compiler.

Losing the default values seems a good answer. After all who wants a shape with no width or height (or radius)?

4

It seems to me that your base class is too complex anyway. What is the width and height of a circle? What is the radius of a rectangle?

They should probably be in the constructors of the derived classes.

5

Yes. Thank you.

removing the default values is the correct answer:

class Shape {
   protected:
      float width, height, radius;
   public:
      Shape( float r) {
        radius = r;
      }
      Shape( float a, float b) {
         width = a;
         height = b;
      }

PS. This class was just a gross simplification of the actual code which has nothing to do with 'shapes', but rather robotic motor controllers. I'm attempting to build an abstract base class for the controllers. There will be child classes for each particular type of implemented controller (RoboClaw, Pololu, RoboTEQ, etc.) that will inherit and/or be forced to implement methods of the base class, thus part of the reason for overloaded Constructors.

As an aside, allowing for default values can be implemented in the child classes as I've discovered.