I am wondering what the difference is between typeid
and typeof
in C++. Here's what I know:
typeid
is mentioned in the documentation for type_info which is defined in the C++ header file typeinfo.
typeof
is defined in the GCC extension for C and in the C++ Boost library.
Also, here is test code test that I've created where I've discovered that typeid
does not return what I expected. Why?
main.cpp
#include <iostream>
#include <typeinfo> //for 'typeid' to work
class Person {
public:
// ... Person members ...
virtual ~Person() {}
};
class Employee : public Person {
// ... Employee members ...
};
int main () {
Person person;
Employee employee;
Person *ptr = &employee;
int t = 3;
std::cout << typeid(t).name() << std::endl;
std::cout << typeid(person).name() << std::endl; // Person (statically known at compile-time)
std::cout << typeid(employee).name() << std::endl; // Employee (statically known at compile-time)
std::cout << typeid(ptr).name() << std::endl; // Person * (statically known at compile-time)
std::cout << typeid(*ptr).name() << std::endl; // Employee (looked up dynamically at run-time
// because it is the dereference of a pointer
// to a polymorphic class)
}
output:
bash-3.2$ g++ -Wall main.cpp -o main
bash-3.2$ ./main
i
6Person
8Employee
P6Person
8Employee
C++ language has no such thing as typeof
. You must be looking at some compiler-specific extension. If you are talking about GCC's typeof
, then a similar feature is present in C++11 through the keyword decltype
. Again, C++ has no such typeof
keyword.
typeid
is a C++ language operator which returns type identification information at run time. It basically returns a type_info
object, which is equality-comparable with other type_info
objects.
Note, that the only defined property of the returned type_info
object has is its being equality- and non-equality-comparable, i.e. type_info
objects describing different types shall compare non-equal, while type_info
objects describing the same type have to compare equal. Everything else is implementation-defined. Methods that return various "names" are not guaranteed to return anything human-readable, and even not guaranteed to return anything at all.
Note also, that the above probably implies (although the standard doesn't seem to mention it explicitly) that consecutive applications of typeid
to the same type might return different type_info
objects (which, of course, still have to compare equal).