C++ typeid Explained: Purpose and Usage

The typeid in C++ is an operator used to retrieve the type information of an expression. It returns a std::type_info object that contains the type information of the expression, including the name of the type.

The typeid is commonly used with dynamic_cast and std::type_info to identify the actual type of an object at runtime, allowing for polymorphism.

Here is an example of how to use typeid:

#include <iostream>
#include <typeinfo>

class Base {
   virtual void foo() {}
};

class Derived : public Base {};

int main() {
   Base* basePtr = new Derived();

   // 使用typeid获取basePtr所指对象的类型信息
   const std::type_info& type = typeid(*basePtr);

   // 打印类型的名称
   std::cout << "Object type: " << type.name() << std::endl;

   // 使用typeid进行类型判断
   if (type == typeid(Base)) {
      std::cout << "Object is of type Base" << std::endl;
   }
   else if (type == typeid(Derived)) {
      std::cout << "Object is of type Derived" << std::endl;
   }

   delete basePtr;
   return 0;
}

The output result:

Object type: class Derived
Object is of type Derived

In the example above, typeid(*basePtr) returns a type_info object with the name “class Derived”, indicating that the actual type of the object pointed to by basePtr is Derived.