typedef struct vs struct definitions

Question

I m a beginner in C programming  but I was wondering what s the difference between using typedef when defining a structure versus not using typedef  It seems to me like there s really no difference  they accomplish the same goal   struct myStruct      int one      int two       vs   typedef struct      int one      int two   myStruct

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In C  the type specifier keywords of structures  unions and enumerations are mandatory  ie you always have to prefix the type s name  its tag  with struct  union or enum when referring to the type   You can get rid of the keywords by using a typedef  which is a form of information hiding as the actual type of an object will no longer be visible when declaring it   It is therefore recommended  see eg the Linux kernel coding style guide  Chapter 5  to only do this when  you actually want to hide this information and not just to save a few keystrokes   An example of when you should use a typedef would be an opaque type which is only ever used with corresponding accessor functions macros

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The typedef  as it is with other constructs  is used to give a data type a new name   In this case it is mostly done in order to make the code cleaner   struct myStruct blah    vs   myStruct blah

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In C  not C     you have to declare struct variables like   struct myStruct myVariable    In order to be able to use myStruct myVariable  instead  you can typedef the struct   typedef struct myStruct someStruct  someStruct myVariable    You can combine struct definition and typedefs it in a single statement which declares an anonymous struct and typedefs it   typedef struct         myStruct

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The common idiom is using both    typedef struct S        int x     S    They are different definitions  To make the discussion clearer I will split the sentence   struct S        int x       typedef struct S S    In the first line you are defining the identifier S within the struct name space  not in the C   sense   You can use it and define variables or function arguments of the newly defined type by defining the type of the argument as struct S   void f  struct S argument       struct is required here   The second line adds a type alias S in the global name space and thus allows you to just write   void f  S argument       struct keyword no longer needed   Note that since both identifier name spaces are different  defining S both in the structs and global spaces is not an error  as it is not redefining the same identifier  but rather creating a different identifier in a different place   To make the difference clearer   typedef struct S        int x     T   void S          correct    void T         error  symbol T already defined as an alias to  struct S    You can define a function with the same name of the struct as the identifiers are kept in different spaces  but you cannot define a function with the same name as a typedef as those identifiers collide   In C    it is slightly different as the rules to locate a symbol have changed subtly  C   still keeps the two different identifier spaces  but unlike in C  when you only define the symbol within the class identifier space  you are not required to provide the struct class keyword       C   struct S        int x         S defined as a class  void f  S a       correct  struct is optional   What changes are the search rules  not where the identifiers are defined  The compiler will search the global identifier table and after S has not been found it will search for S within the class identifiers   The code presented before behaves in the same way   typedef struct S        int x     T   void S         correct        void T         error  symbol T already defined as an alias to  struct S    After the definition of the S function in the second line  the struct S cannot be resolved automatically by the compiler  and to create an object or define an argument of that type you must fall back to including the struct keyword      previous code here    int main         S         struct S s

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With the latter example you omit the struct keyword when using the structure  So everywhere in your code  you can write    myStruct a    instead of  struct myStruct a    This save some typing  and might be more readable  but this is a matter of taste

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I see some clarification is in order on this   C and C   do not define types differently   C   was originally nothing more than an additional set of includes on top of C   The problem that virtually all C C   developers have today  is a  universities are no longer teaching the fundamentals  and b  people don t understand the difference between a definition and a declaration   The only reason such declarations and definitions exist is so that the linker can calculate address offsets to the fields in the structure   This is why most people get away with code that is actually written incorrectly-- because the compiler is able to determine addressing   The problem arises when someone tries to do something advance  like a queue  or a linked list  or piggying-backing an O S structure   A declaration begins with  struct   a definition begins with  typedef    Further  a struct has a forward declaration label  and a defined label   Most people don t know this and use the forward declaration label as a define label   Wrong   struct myStruct         int field 1                 They ve just used the forward declaration to label the structure-- so now the compiler is aware of it-- but it isn t an actual defined type   The compiler can calculate the addressing-- but this isn t how it was intended to be used  for reasons I will show momentarily   People who use this form of declaration  must always put  struct  in practicly every reference to it-- because it isn t an offical new type   Instead  any structure that does not reference itself  should be declared and defined this way only   typedef struct         field 1             myStruct    Now it s an actual type  and when used you can use at as  myStruct  without having to prepend it with the word  struct    If you want a pointer variable to that structure  then include a secondary label   typedef struct         field 1             myStruct  myStructP    Now you have a pointer variable to that structure  custom to it   FORWARD DECLARATION--  Now  here s the fancy stuff  how the forward declaration works   If you want to create a type that refers to itself  like a linked list or queue element  you have to use a forward declaration   The compiler doesn t consider the structure defined until it gets to the semicolon at the very end  so it s just declared before that point   typedef struct myStructElement         myStructElement   nextSE     field 1             myStruct    Now  the compiler knows that although it doesn t know what the whole type is yet  it can still reference it using the forward reference   Please declare and typedef your structures correctly   There s actually a reason

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Another difference not pointed out is that giving the struct a name  i e  struct myStruct  also enables you to provide forward declarations of the struct   So in some other file  you could write   struct myStruct  void doit struct myStruct  ptr     without having to have access to the definition   What I recommend is you combine your two examples   typedef struct myStruct      int one      int two    myStruct    This gives you the convenience of the more concise typedef name but still allows you to use the full struct name if you need

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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 won t work   struct myStruct    forward declaration fails  void blah myStruct  pStruct      error C2371   myStruct    redefinition  different basic types

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The difference comes in when you use the struct   The first way you have to do   struct myStruct aName    The second way allows you to remove the keyword struct   myStruct aName

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The following code creates an anonymous struct with the alias myStruct   typedef struct      int one      int two    myStruct    You can t refer it without the alias because you don t specify an identifier for the structure

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If you use struct without typedef  you ll always have to write  struct mystruct myvar    It s illegal to write  mystruct myvar    If you use the typedef you don t need the struct prefix anymore

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struct and typedef are two very different things   The struct keyword is used to define  or to refer to  a structure type  For example  this   struct foo       int n       creates a new type called struct foo  The name foo is a tag  it s meaningful only when it s immediately preceded by the struct keyword  because tags and other identifiers are in distinct name spaces   This is similar to  but much more restricted than  the C   concept of namespaces    A typedef  in spite of the name  does not define a new type  it merely creates a new name for an existing type  For example  given   typedef int my int    my int is a new name for int  my int and int are exactly the same type  Similarly  given the struct definition above  you can write   typedef struct foo foo    The type already has a name  struct foo   The typedef declaration gives the same type a new name  foo   The syntax allows you to combine a struct and typedef into a single declaration   typedef struct bar       int n    bar    This is a common idiom  Now you can refer to this structure type either as struct bar or just as bar   Note that the typedef name doesn t become visible until the end of the declaration  If the structure contains a pointer to itself  you have use the struct version to refer to it   typedef struct node       int data      struct node  next     can t use just  node  next  here      node    Some programmers will use distinct identifiers for the struct tag and for the typedef name  In my opinion  there s no good reason for that  using the same name is perfectly legal and makes it clearer that they re the same type   If you must use different identifiers  at least use a consistent convention   typedef struct node s                   node     Personally  I prefer to omit the typedef and refer to the type as struct bar  The typedef save a little typing  but it hides the fact that it s a structure type   If you want the type to be opaque  this can be a good thing  If client code is going to be referring to the member n by name  then it s not opaque  it s visibly a structure  and in my opinion it makes sense to refer to it as a structure  But plenty of smart programmers disagree with me on this point  Be prepared to read and understand code written either way     C   has different rules  Given a declaration of struct blah  you can refer to the type as just blah  even without a typedef  Using a typedef might make your C code a little more C  -like -- if you think that s a good thing

[c] typedef struct vs struct definitions

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