8.6. Determining an Object’s Type at Runtime
Problem
At runtime, you need to interrogate dynamically the type of particular class.
Solution
Use runtime type identification (commonly referred to as RTTI) to query the address of the object for the type of object it points to. Example 8-6 shows how.
Example 8-6. Using runtime type identification
#include <iostream>
#include <typeinfo>
using namespace std;
class Base {};
class Derived : public Base {};
int main() {
Base b, bb;
Derived d;
// Use typeid to test type equality
if (typeid(b) == typeid(d)) { // No
cout << "b and d are of the same type.\n";
}
if (typeid(b) == typeid(bb)) { // Yes
cout << "b and bb are of the same type.\n";
}
if (typeid(d) == typeid(Derived)) { // Yes
cout << "d is of type Derived.\n";
}
}Discussion
Example 8-6 shows you how to use the
operator typeid to determine and compare the type of an
object. typeid takes an expression or a type and
returns a reference to an object of type_info or a
subclass of it (which is implementation defined). You can use what is returned to test for
equality or retrieve a string representation of the type’s name. For example, you can
compare the types of two objects like this:
if (typeid(b) == typeid(d)) {This will return true if the type_info objects
returned by both of these are equal. This is because typeid returns a reference to a static object, so if you call it on two objects that are the same type, you will get two references to the same thing, which is why the equality test ...