Use `dynamic_cast` to a pointer type when failure to find the required class is considered a valid alternative

Reason

The dynamic_cast conversion allows to test whether a pointer is pointing at a polymorphic object that has a given class in its hierarchy. Since failure to find the class merely returns a null value, it can be tested during run time. This allows writing code that can choose alternative paths depending on the results.

Contrast with C.147, where failure is an error, and should not be used for conditional execution.

Example

The example below describes the add function of a Shape_owner that takes ownership of constructed Shape objects. The objects are also sorted into views, according to their geometric attributes. In this example, Shape does not inherit from Geometric_attributes. Only its subclasses do.

void add(Shape* const item)
{
  // Ownership is always taken
  owned_shapes.emplace_back(item);

  // Check the Geometric_attributes and add the shape to none/one/some/all of the views

  if (auto even = dynamic_cast<Even_sided*>(item))
  {
    view_of_evens.emplace_back(even);
  }

  if (auto trisym = dynamic_cast<Trilaterally_symmetrical*>(item))
  {
    view_of_trisyms.emplace_back(trisym);
  }
}

Notes

A failure to find the required class will cause dynamic_cast to return a null value, and de-referencing a null-valued pointer will lead to undefined behavior. Therefore the result of the dynamic_cast should always be treated as if it might contain a null value, and tested.

Enforcement

• (Complex) Unless there is a null test on the result of a dynamic_cast of a pointer type, warn upon dereference of the pointer.