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 ...
Get C++ Cookbook now with the O’Reilly learning platform.
O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.