[c++] 'typeid' versus 'typeof' in C++

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

The primary difference between the two is the following

• typeof is a compile time construct and returns the type as defined at compile time
• typeid is a runtime construct and hence gives information about the runtime type of the value.

typeof Reference: http://www.delorie.com/gnu/docs/gcc/gcc_36.html

typeid Reference: https://en.wikipedia.org/wiki/Typeid

Answering the additional question:

my following test code for typeid does not output the correct type name. what's wrong?

There isn't anything wrong. What you see is the string representation of the type name. The standard C++ doesn't force compilers to emit the exact name of the class, it is just up to the implementer(compiler vendor) to decide what is suitable. In short, the names are up to the compiler.

These are two different tools. typeof returns the type of an expression, but it is not standard. In C++0x there is something called decltype which does the same job AFAIK.

decltype(0xdeedbeef) number = 0; // number is of type int!
decltype(someArray[0]) element = someArray[0];

Whereas typeid is used with polymorphic types. For example, lets say that cat derives animal:

animal* a = new cat; // animal has to have at least one virtual function
...
if( typeid(*a) == typeid(cat) )
{
    // the object is of type cat! but the pointer is base pointer.
}

You can use Boost demangle to accomplish a nice looking name:

#include <boost/units/detail/utility.hpp>

and something like

To_main_msg_evt ev("Failed to initialize cards in " + boost::units::detail::demangle(typeid(*_IO_card.get()).name()) + ".\n", true, this);

typeid can operate at runtime, and return an object describing the run time type of the object, which must be a pointer to an object of a class with virtual methods in order for RTTI (run-time type information) to be stored in the class. It can also give the compile time type of an expression or a type name, if not given a pointer to a class with run-time type information.

typeof is a GNU extension, and gives you the type of any expression at compile time. This can be useful, for instance, in declaring temporary variables in macros that may be used on multiple types. In C++, you would usually use templates instead.

typeid provides the type of the data at runtime, when asked for. Typedef is a compile time construct that defines a new type as stated after that. There is no typeof in C++ Output appears as (shown as inscribed comments):

std::cout << typeid(t).name() << std::endl;  // i
std::cout << typeid(person).name() << std::endl;   // 6Person
std::cout << typeid(employee).name() << std::endl; // 8Employee
std::cout << typeid(ptr).name() << std::endl;      // P6Person
std::cout << typeid(*ptr).name() << std::endl;     //8Employee