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Type Identification

A relatively new feature in C++ is type identification, where it is possible to determine the type of an object at run time. A simple example of this feature is:

#include <typeinfo.h> #include <stdio.h> class A { public: virtual void f(int) {} }; class B : public A { public: virtual void f(int) {} }; int main() { A a; B b; A* ap1 = &a; A* ap2 = &b; if (typeid(*ap1) == typeid(A)) printf("ap1 is A\n"); else printf("ap1 is B\n"); if (typeid(*ap2) == typeid(A)) printf("ap2 is A\n"); else printf("ap2 is B\n"); return 0; }

which produces:

ap1 is A ap2 is B

even though the nominal type of both *ap1 and *ap2 is A. In this example, *ap1 and *ap2 represent polymorphic types, that is, types that can refer to any class type in a hierarchy of derivations. If we omitted the virtual functions in A and B, this program would give different results, considering both *ap1 and *ap2 to be referencing A objects.

typeid() produces an object of type "typeinfo", described in typeinfo.h (or just "typeinfo" in newer systems). This type has operations for testing for equality, and also a member function for returning the name of a type. For example, when this code is executed:

#include <typeinfo.h> #include <stdio.h> int main() { int i; double x[57]; float f1 = 0.0; const float f2 = 0.0; printf("%s\n", typeid(i).name()); printf("%s\n", typeid(x).name()); if (typeid(f1) == typeid(f2)) printf("equal\n"); return 0; }

the result is:

int double [57] equal

Note that the typeid() comparison ignores top-level "const". The form of the name returned by name() is implementation-dependent.

This feature of C++ is quite important, because it represents a partial departure from early binding, that is, fully resolving names at compile time. Sometimes it's necessary to be able to manipulate type names in a running program. A more recent language like Java(tm) has many more features of this type.