What is dynamic type of object

Question

What i think is that dynamic type means dynamically allocated object using new. In the following case, do you say p points to dynamic type or stati

Answer 1

In a statically typed language, such as C++ or Java for example, static may refer to the information known at compilation time while dynamic refers to the information known at runtime.

For example:

struct Base { virtual std::string name() const { return "Base"; } };

struct Derived: Base { std::string name() const { return "Derived"; } };

void print(Base const& b) { std::cout << b.name() << "\n"; }

In the print method, the static type of b is Base const&. Therefore, the compiler will check that all methods called exist in the context of a Base object.

However, when execution comes, the call to name, as the method is virtual, is performed with regard to the dynamic type of the object:

• this may be Base
• this may be Derived
• this may be another derived class from Base that we know not yet

Therefore, in the following example:

int main(int argc, char* argv[]) {
  if (argc == 1) {
    Base base;
    print();
  } else {
    Derived derived;
    print(derived);
  }
};

• The static and dynamic type of base is Base and derived is Derived.
• In the print method, the static type of b is Base (always)
• Depending on the number of arguments, the dynamic of b is either Base or Derived

It is a current mistake to assume that polymorphism is necessarily based on dynamic memory allocation, but the two concepts, while not orthogonal, can be used without one another in some conditions.