typeid versus typeof in C

Question

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  lt iostream gt     include  lt typeinfo gt     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    amp employee        int t   3         std  cout  lt  lt  typeid t  name    lt  lt  std  endl        std  cout  lt  lt  typeid person  name    lt  lt  std  endl       Person  statically known at compile-time        std  cout  lt  lt  typeid employee  name    lt  lt  std  endl     Employee  statically known at compile-time        std  cout  lt  lt  typeid ptr  name    lt  lt  std  endl          Person    statically known at compile-time        std  cout  lt  lt  typeid  ptr  name    lt  lt  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

User · Answer

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

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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

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You can use Boost demangle to accomplish a nice looking name    include  lt boost units detail utility hpp gt    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

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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

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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

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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  lt  lt  typeid t  name    lt  lt  std  endl      i std  cout  lt  lt  typeid person  name    lt  lt  std  endl       6Person std  cout  lt  lt  typeid employee  name    lt  lt  std  endl     8Employee std  cout  lt  lt  typeid ptr  name    lt  lt  std  endl          P6Person std  cout  lt  lt  typeid  ptr  name    lt  lt  std  endl        8Employee

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

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