C++ typeid Keyword

The typeid keyword in C++ is used to get information about the type of a variable, object, or expression at runtime. It is part of the typeinfo header and is commonly used for runtime type identification (RTTI). The typeid operator returns a reference to a type_info object, which can provide details like the type’s name.

The typeid keyword is used in polymorphic scenarios where you need to determine the actual type of an object at runtime.

Syntax

</>
Copy
typeid(expression).name()
typeid
The operator used to obtain runtime type information.
expression
The variable, object, or expression whose type information is required.
name()
A member function of the type_info object that returns the name of the type as a string.

Examples

Example 1: Getting the Type of a Variable

This example demonstrates how to use typeid to get the type of a variable.

</>
Copy
#include <iostream>
#include <typeinfo> // Required for typeid
using namespace std;

int main() {
    int num = 42;
    float pi = 3.14;
    char letter = 'A';

    cout << "Type of num: " << typeid(num).name() << endl;
    cout << "Type of pi: " << typeid(pi).name() << endl;
    cout << "Type of letter: " << typeid(letter).name() << endl;

    return 0;
}

Output:

Type of num: i
Type of pi: f
Type of letter: c

Explanation:

  1. The typeid operator is used with the variables num, pi, and letter to determine their types.
  2. The name() function of the type_info object returns the names of their respective types: int, float, and char.
  3. The output shows the type names for each variable.

Example 2: Using typeid with Polymorphism

This example demonstrates how typeid can be used in a polymorphic scenario to identify the actual type of an object at runtime.

</>
Copy
#include <iostream>
#include <typeinfo>
using namespace std;

class Base {
public:
    virtual ~Base() {}
};

class Derived : public Base {};

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

    cout << "Type of basePtr: " << typeid(basePtr).name() << endl;
    cout << "Type of *basePtr: " << typeid(*basePtr).name() << endl;

    delete basePtr;
    return 0;
}

Output:

Type of basePtr: P4Base
Type of *basePtr: 7Derived

Explanation:

  1. The class Base has a virtual destructor, enabling runtime type identification.
  2. The pointer basePtr points to an object of the derived class Derived.
  3. typeid(basePtr) returns the type of the pointer, which is Base*.
  4. typeid(*basePtr) dereferences the pointer and returns the type of the actual object it points to, which is Derived.

Key Points to Remember about typeid Keyword

  1. The typeid operator is used to retrieve the type information of variables, objects, or expressions.
  2. It works with polymorphic types when the base class has at least one virtual function.
  3. The typeid operator returns a type_info object, which provides a name() function to get the type’s name.
  4. It is commonly used for debugging, logging, and implementing runtime type identification (RTTI).
  5. To use typeid, include the <typeinfo> header.