C++ Downcasting to Derived Class based off Variable

Question

Suppose I have a base class "Shape", and derived classes "Triangle", "Square", and "Circle". A member of "Shape" is an int "shapeType".

If shapeType==1, then it's a triangle. If shapeType==2, then it's a square. If shapeType==3, then it's a circle.

I'm interested in knowing given I have just a "Shape" object that was once a derived-object, if there is a way to "dynamically" down-cast to the proper derived class by using the shapeType value.

I know I can do a hard-code switch statement, roughly like:

Triangle* t;
Square* s;
Circle* c;

switch (shape->shapeType) {
case 1:
   t = (Triangle*)shape;
case 2: 
   ...
}

However, the above requires me to make a pointer of EVERY derived class possibility. I am wondering if there is a way to do this without hard-coding every class, but instead somehow determine the class type map where the key is the shapeType and the value is the class type.

Answer 1

If they've virtual functions, then use dynamic_cast:

t = dynamic_cast<Triangle*>(shape);
if ( t )
{
     //use t
}

But take a note: you should try defining the classes and virtual functions in such a way that you would hardly need to use dynamic_cast. Prefer well-defined interface, and polymorphism, in general.

Here is one example,

class Shape
{
   public:
     virtual ~Shape() {} //destructor must be virtual - important!
     virtual double Area() const = 0;
};

class Triangle : public Shape
{
   public:
     Triangle(double a, double b, double c);
     virtual double Area() const 
     {
         //calculate area and return it!
     }
};

Shape *s = new Triangle(10, 20, 30);
double aread = s->Area(); //calls Triangle::Area()

No need to use shapeType variable.