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CppTypeSwitching

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(C++) Type switching

Type switching means that in a class there is an internal variable stored to embody the type of the class. The a switch-statement is used to conclude its behaviour.

'Avoid type switching; prefer polymorphism' [1]

An example is the code below, followed by the equivalent classes using polymorphism.

include <iostream>

include <vector>


enum MyClassType { happy, sad, angry };
class MyClass
{
  private:
  const MyClassType mType;
  public:
  MyClass(const MyClassType& state) : mType(state) { ; }
  void sayHello() const
  {
    switch (mType)
    {
      case happy : std::cout << "Hello!!!" << std::endl; break;
      case sad   : std::cout << "... hello ..." << std::endl; break;
      case angry  : std::cout << "HELLO!!!" << std::endl; break;
      default: assert(!"Unknown value of mType"); std::exit(1);
    }
  }
};
int main()
{
  std::vector<MyBaseClass* > myVector(3);
  myVector[0] = new MyClass(happy);
  myVector[1] = new MyClass(sad);
  myVector[2] = new MyClass(angry);
  const int size = myVector.size();
  for (int i=0; i!=size; ++i) myVector[i]->sayHello();
  //Clean up
  for (int i=0; i!=size; ++i) delete myVector[i];
}

The correct equivalent:

include <iostream>

include <vector>


struct MyBaseClass
{
  virtual void sayHello() const = 0;
};
struct MyHappyClass : public MyBaseClass
{
  void sayHello() const { std::cout << "Hello!!!" << std::endl; }
};
struct MySadClass : public MyBaseClass
{
  void sayHello() const { std::cout << "... hello ..." << std::endl; }
};
struct MyAngryClass : public MyBaseClass
{
  void sayHello() const { std::cout << "HELLO!!!" << std::endl; }
};
int main()
{
  std::vector<MyBaseClass* > myVector(3);
  myVector[0] = new MyHappyClass;
  myVector[1] = new MySadClass;
  myVector[2] = new MyAngryClass;
  const int size = myVector.size();
  for (int i=0; i!=size; ++i) myVector[i]->sayHello();
  //Clean up
  for (int i=0; i!=size; ++i) delete myVector[i];
}

Code links

• #include
• enum
• class
• private
• public
• iostream
• main
• std
• scope operator'::'
• cout
• stream out operator '<<'
• endl
• return

Reference

• 1) Herb Sutter and Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Chapter 90: 'Avoid type switching; prefer polymorphism'.

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