Difference between struct and typedef struct in C

Question

In C    is there any difference between   struct Foo            and   typedef struct         Foo

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You can t use forward declaration with the typedef struct   The struct itself is an anonymous type  so you don t have an actual name to forward declare   typedef struct      int one      int two   myStruct    A forward declaration like this wont work   struct myStruct    forward declaration fails  void blah myStruct  pStruct      error C2371   myStruct    redefinition  different basic types

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In this DDJ article  Dan Saks explains one small area where bugs can creep through if you do not typedef your structs  and classes        If you want  you can imagine that C     generates a typedef for every tag   name  such as  typedef class string string        Unfortunately  this is not entirely   accurate  I wish it were that simple    but it s not  C   can t generate such   typedefs for structs  unions  or enums   without introducing incompatibilities   with C       For example  suppose a C program   declares both a function and a struct   named status   int status    struct status        Again  this may be bad practice  but   it is C  In this program  status  by   itself  refers to the function  struct   status refers to the type       If C   did automatically generate   typedefs for tags  then when you   compiled this program as C    the   compiler would generate   typedef struct status status        Unfortunately  this type name would   conflict with the function name  and   the program would not compile  That s   why C   can t simply generate a   typedef for each tag       In C    tags act just like typedef   names  except that a program can   declare an object  function  or   enumerator with the same name and the   same scope as a tag  In that case  the   object  function  or enumerator name   hides the tag name  The program can   refer to the tag name only by using   the keyword class  struct  union  or   enum  as appropriate  in front of the   tag name  A type name consisting of   one of these keywords followed by a   tag is an elaborated-type-specifier    For instance  struct status and enum   month are elaborated-type-specifiers        Thus  a C program that contains both   int status    struct status        behaves the same when compiled as C      The name status alone refers to the   function  The program can refer to the   type only by using the   elaborated-type-specifier struct   status       So how does this allow bugs to creep   into programs  Consider the program in   Listing 1  This program defines a   class foo with a default constructor    and a conversion operator that   converts a foo object to char const      The expression  p   foo          in main should construct a foo object   and apply the conversion operator  The   subsequent output statement  cout  lt  lt  p  lt  lt    n         should display class foo  but it   doesn t  It displays function foo       This surprising result occurs because   the program includes header lib h   shown in Listing 2  This header   defines a function also named foo  The   function name foo hides the class name   foo  so the reference to foo in main   refers to the function  not the class    main can refer to the class only by   using an elaborated-type-specifier  as   in  p   class foo          The way to avoid such confusion   throughout the program is to add the   following typedef for the class name   foo   typedef class foo foo        immediately before or after the class   definition  This typedef causes a   conflict between the type name foo and   the function name foo  from the   library  that will trigger a   compile-time error       I know of no one who actually writes   these typedefs as a matter of course    It requires a lot of discipline  Since   the incidence of errors such as the   one in Listing 1 is probably pretty   small  you many never run afoul of   this problem  But if an error in your   software might cause bodily injury    then you should write the typedefs no   matter how unlikely the error        I can t imagine why anyone would ever   want to hide a class name with a   function or object name in the same   scope as the class  The hiding rules   in C were a mistake  and they should   not have been extended to classes in   C    Indeed  you can correct the   mistake  but it requires extra   programming discipline and effort that   should not be necessary

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An important difference between a  typedef struct  and a  struct  in C   is that inline member initialisation in  typedef structs  will not work      the  x  in this struct will NOT be initialised to zero typedef struct   int x   0    Foo      the  x  in this struct WILL be initialised to zero struct Foo   int x   0

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In C    there is only a subtle difference   It s a holdover from C  in which it makes a difference   The C language standard  C89   3 1 2 3  C99   6 2 3  and C11   6 2 3  mandates separate namespaces for different categories of identifiers  including tag identifiers  for struct union enum  and ordinary identifiers  for typedef and other identifiers     If you just said   struct Foo          Foo x    you would get a compiler error  because Foo is only defined in the tag namespace    You d have to declare it as   struct Foo x    Any time you want to refer to a Foo  you d always have to call it a struct Foo   This gets annoying fast  so you can add a typedef   struct Foo          typedef struct Foo Foo    Now struct Foo  in the tag namespace  and just plain Foo  in the ordinary identifier namespace  both refer to the same thing  and you can freely declare objects of type Foo without the struct keyword     The construct   typedef struct Foo         Foo    is just an abbreviation for the declaration and typedef     Finally   typedef struct         Foo    declares an anonymous structure and creates a typedef for it   Thus  with this construct  it doesn t have a name in the tag namespace  only a name in the typedef namespace   This means it also cannot be forward-declared   If you want to make a forward declaration  you have to give it a name in the tag namespace     In C    all struct union enum class declarations act like they are implicitly typedef ed  as long as the name is not hidden by another declaration with the same name   See Michael Burr s answer for the full details

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Struct is to create a data type  The typedef is to set a nickname for a data type

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There is a difference  but subtle  Look at it this way  struct Foo introduces a new type  The second one creates an alias called Foo  and not a new type  for an unnamed struct type      7 1 3 The typedef specifier      1            A name declared with the typedef specifier becomes a typedef-name  Within the scope of its declaration  a   typedef-name is syntactically equivalent to a keyword and names the type associated with the identifier in   the way described in Clause 8  A typedef-name is thus a synonym for another type  A typedef-name does not introduce a new type the way a class declaration  9 1  or enum declaration does       8 If the typedef declaration defines an unnamed class  or enum   the first typedef-name declared by the declaration   to be that class type  or enum type  is used to denote the class type  or enum type  for linkage   purposes only  3 5     Example    typedef struct      ps  S     S is the class name for linkage purposes   So  a typedef always is used as an placeholder synonym for another type

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There is no difference in C    but I believe in C it would allow you to declare instances of the struct Foo without explicitly doing   struct Foo bar

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One more important difference  typedefs cannot be forward declared  So for the typedef option you must  include the file containing the typedef  meaning everything that  includes your  h also includes that file whether it directly needs it or not  and so on  It can definitely impact your build times on larger projects   Without the typedef  in some cases you can just add a forward declaration of struct Foo  at the top of your  h file  and only  include the struct definition in your  cpp file

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