Why should we typedef a struct so often in C

Question

I have seen many programs consisting of structures like the one below  typedef struct        int i      char k    elem   elem user    Why is it needed so often  Any specific reason or applicable area

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Turns out in C99 typedef is required  It is outdated  but a lot of tools  ala HackRank  use c99 as its pure C implementation  And typedef is required there   I m not saying they should change  maybe have two C options  if the requirement changed  those of us studing for interviews on the site would be SOL

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As Greg Hewgill said  the typedef means you no longer have to write struct all over the place  That not only saves keystrokes  it also can make the code cleaner since it provides a smidgen more abstraction   Stuff like  typedef struct     int x  y    Point   Point point new int x  int y      Point a    a x   x    a y   y    return a      becomes cleaner when you don t need to see the  struct  keyword all over the place  it looks more as if there really is a type called  Point  in your language  Which  after the typedef  is the case I guess   Also note that while your example  and mine  omitted naming the struct  itself  actually naming it is also useful for when you want to provide an opaque type  Then you d have code like this in the header  for instance   typedef struct Point Point   Point   point new int x  int y     and then provide the struct definition in the implementation file   struct Point     int x  y      Point   point new int x  int y      Point  p    if  p   malloc sizeof  p      NULL          p- gt x   x      p- gt y   y        return p      In this latter case  you cannot return the Point by value  since its definition is hidden from users of the header file  This is a technique used widely in GTK   for instance   UPDATE Note that there are also highly-regarded C projects where this use of typedef to hide struct is considered a bad idea  the Linux kernel is probably the most well-known such project  See Chapter 5 of The Linux Kernel CodingStyle document for Linus  angry words     My point is that the  should  in the question is perhaps not set in stone  after all

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It turns out that there are pros and cons   A useful source of information is the seminal book  Expert C Programming   Chapter 3   Briefly  in C you have multiple namespaces  tags  types  member names and identifiers  typedef introduces an alias for a type and locates it in the tag namespace  Namely   typedef struct Tag     members     Type    defines two things  One Tag in the tag namespace and one Type in the type namespace  So you can do both Type myType and struct Tag myTagType  Declarations like struct Type myType or Tag myTagType are illegal  In addition  in a declaration like this   typedef Type  Type ptr    we define a pointer to our Type  So if we declare   Type ptr var1  var2  struct Tag  myTagType1  myTagType2    then var1 var2 and myTagType1 are pointers to Type but myTagType2 not   In the above-mentioned book  it mentions that typedefing structs are not very useful as it only saves the programmer from writing the word struct  However  I have an objection  like many other C programmers  Although it sometimes turns to obfuscate some names  that s why it is not advisable in large code bases like the kernel  when you want to implement polymorphism in C it helps a lot look here for details  Example   typedef struct MyWriter t      MyPipe super      MyQueue relative      uint32 t flags       MyWriter    you can do   void my writer func MyPipe  s        MyWriter  self    MyWriter    s      uint32 t myFlags   self- gt flags          So you can access an outer member  flags  by the inner struct  MyPipe  through casting  For me it is less confusing to cast the whole type than doing  struct MyWriter     s  every time you want to perform such functionality  In these cases brief referencing is a big deal especially if you heavily employ the technique in your code   Finally  the last aspect with typedefed types is the inability to extend them  in contrast to macros  If for example  you have     define X char 10  or typedef char Y 10    you can then declare  unsigned X x  but not unsigned Y y    We do not really care for this for structs because it does not apply to storage specifiers  volatile and const

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One other good reason to always typedef enums and structs results from this problem   enum EnumDef     FIRST ITEM    SECOND ITEM     struct StructDef     enum EnuumDef MyEnum    unsigned int MyVar    MyStruct    Notice the typo in EnumDef in the struct  EnuumDef   This compiles without error  or warning  and is  depending on the literal interpretation of the C Standard  correct  The problem is that I just created an new  empty  enumeration definition within my struct  I am not  as intended  using the previous definition EnumDef   With a typdef similar kind of typos would have resulted in a compiler errors for using an unknown type   typedef      FIRST ITEM    SECOND ITEM   EnumDef   typedef struct     EnuumDef MyEnum     compiler error  unknown type       unsigned int MyVar    StructDef  StrructDef MyStruct     compiler error  unknown type       I would advocate ALWAYS typedef ing structs and enumerations    Not only to save some typing  no pun intended      but because it is safer

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Using a typedef avoids having to write struct every time you declare a variable of that type   struct elem    int i   char k     elem user     compile error  struct elem user     this is correct

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Using a typedef avoids having to write struct every time you declare a variable of that type   struct elem    int i   char k     elem user     compile error  struct elem user     this is correct

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It s amazing how many people get this wrong  PLEASE don t typedef structs in C  it needlessly pollutes the global namespace which is typically very polluted already in large C programs   Also  typedef d structs without a tag name are a major cause of needless imposition of ordering relationships among header files   Consider    ifndef FOO H  define FOO H 1   define FOO DEF  0xDEADBABE   struct bar     forward declaration  defined in bar h    struct foo     struct bar  bar       endif   With such a definition  not using typedefs  it is possible for a compiland unit to include foo h to get at the FOO DEF definition   If it doesn t attempt to dereference  the  bar  member of the foo struct then there will be no need to include the  bar h  file   Also  since the namespaces are different between the tag names and the member names  it is possible to write very readable code such as   struct foo  foo   printf  foo- gt bar    p   foo- gt bar     Since the namespaces are separate  there is no conflict in naming variables coincident with their struct tag name   If I have to maintain your code  I will remove your typedef d structs

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the name you  optionally  give the struct is called the tag name and  as has been noted  is not a type in itself  To get to the type requires the struct prefix   GTK  aside  I m not sure the tagname is used anything like as commonly as a typedef to the struct type  so in C   that is recognised and you can omit the struct keyword and use the tagname as the type name too         struct MyStruct             int i                 The following is legal in C        MyStruct obj      obj i   7

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As Greg Hewgill said  the typedef means you no longer have to write struct all over the place  That not only saves keystrokes  it also can make the code cleaner since it provides a smidgen more abstraction   Stuff like  typedef struct     int x  y    Point   Point point new int x  int y      Point a    a x   x    a y   y    return a      becomes cleaner when you don t need to see the  struct  keyword all over the place  it looks more as if there really is a type called  Point  in your language  Which  after the typedef  is the case I guess   Also note that while your example  and mine  omitted naming the struct  itself  actually naming it is also useful for when you want to provide an opaque type  Then you d have code like this in the header  for instance   typedef struct Point Point   Point   point new int x  int y     and then provide the struct definition in the implementation file   struct Point     int x  y      Point   point new int x  int y      Point  p    if  p   malloc sizeof  p      NULL          p- gt x   x      p- gt y   y        return p      In this latter case  you cannot return the Point by value  since its definition is hidden from users of the header file  This is a technique used widely in GTK   for instance   UPDATE Note that there are also highly-regarded C projects where this use of typedef to hide struct is considered a bad idea  the Linux kernel is probably the most well-known such project  See Chapter 5 of The Linux Kernel CodingStyle document for Linus  angry words     My point is that the  should  in the question is perhaps not set in stone  after all

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I don t think forward declarations are even possible with typedef  Use of struct  enum  and union allow for forwarding declarations when dependencies  knows about  is bidirectional   Style  Use of typedef in C   makes quite a bit of sense  It can almost be necessary when dealing with templates that require multiple and or variable parameters  The typedef helps keep the naming straight   Not so in the C programming language  The use of typedef most often serves no purpose but to obfuscate the data structure usage  Since only   struct  6   enum  4   union  5    number of keystrokes are used to declare a data type there is almost no use for the aliasing of the struct  Is that data type a union or a struct  Using the straightforward non-typdefed declaration lets you know right away what type it is    Notice how Linux is written with strict avoidance of this aliasing nonsense typedef brings  The result is a minimalist and clean style

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typedef will not provide a co-dependent set of data structures  This you cannot do with typdef   struct bar  struct foo   struct foo       struct bar  b      struct bar       struct foo  f       Of course you can always add   typedef struct foo foo t  typedef struct bar bar t    What exactly is the point of that

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the name you  optionally  give the struct is called the tag name and  as has been noted  is not a type in itself  To get to the type requires the struct prefix   GTK  aside  I m not sure the tagname is used anything like as commonly as a typedef to the struct type  so in C   that is recognised and you can omit the struct keyword and use the tagname as the type name too         struct MyStruct             int i                 The following is legal in C        MyStruct obj      obj i   7

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From an old article by Dan Saks  http   www ddj com cpp 184403396 pgno 3         The C language rules for naming   structs are a little eccentric  but   they re pretty harmless  However  when   extended to classes in C    those same   rules open little cracks for bugs to   crawl through       In C  the name s appearing in  struct s                            is a tag  A tag name is not a type   name  Given the definition above    declarations such as  s x        error in C    s  p       error in C          are errors in C  You must write them   as  struct s x         OK    struct s  p        OK          The names of unions and enumerations   are also tags rather than types       In C  tags are distinct from all other   names  for functions  types    variables  and enumeration constants     C compilers maintain tags in a symbol   table that s conceptually if not   physically separate from the table   that holds all other names  Thus  it   is possible for a C program to have   both a tag and an another name with   the same spelling in the same scope    For example   struct s s        is a valid declaration which declares   variable s of type struct s  It may   not be good practice  but C compilers   must accept it  I have never seen a   rationale for why C was designed this   way  I have always thought it was a   mistake  but there it is       Many programmers  including yours   truly  prefer to think of struct names   as type names  so they define an alias   for the tag using a typedef  For   example  defining  struct s                      typedef struct s S        lets you use S in place of struct s    as in  S x  S  p        A program cannot use S as the name of   both a type and a variable  or   function or enumeration constant    S S        error       This is good       The tag name in a struct  union  or   enum definition is optional  Many   programmers fold the struct definition   into the typedef and dispense with the   tag altogether  as in   typedef struct                     S       The linked article also has a discussion about how the C   behavior of not requireing a typedef can cause subtle name hiding problems   To prevent these problems  it s a good idea to typedef your classes and structs in C    too  even though at first glance it appears to be unnecessary   In C    with the typedef the name hiding become an error that the compiler tells you about rather than a hidden source of potential problems

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the name you  optionally  give the struct is called the tag name and  as has been noted  is not a type in itself  To get to the type requires the struct prefix   GTK  aside  I m not sure the tagname is used anything like as commonly as a typedef to the struct type  so in C   that is recognised and you can omit the struct keyword and use the tagname as the type name too         struct MyStruct             int i                 The following is legal in C        MyStruct obj      obj i   7

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Let s start with the basics and work our way up   Here is an example of Structure definition   struct point         int x  y         Here the name point is optional   A Structure can be declared during its definition or after   Declaring during definition  struct point         int x  y      first point  second point    Declaring after definition  struct point         int x  y       struct point first point  second point    Now  carefully note the last case above  you need to write struct point to declare Structures of that type if you decide to create that type at a later point in your code   Enter typedef  If you intend to create new Structure   Structure is a custom data-type  at a later time in your program using the same blueprint  using typedef during its definition might be a good idea since you can save some typing moving forward   typedef struct point         int x  y      Points   Points first point  second point    A word of caution while naming your custom type  Nothing prevents you from using  t suffix at the end of your custom type name but POSIX standard reserves the use of suffix  t to denote standard library type names

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At all  in C language  struct union enum are macro instruction processed by the C language preprocessor  do not mistake with the preprocessor that treat   include  and other   so    struct a      int i      struct b      struct a     int i     int j       struct b is expended as something like this    struct b       struct a               int i             int i      int j      and so  at compile time it evolve on stack as something like  b  int ai int i int j  that also why it s dificult to have selfreferent structs  C preprocessor round in a d  claration loop that can t terminate   typedef are type specifier  that means only C compiler process it and it can do like he want for optimise assembler code implementation  It also dont expend member of type par stupidly like pr  processor do with structs but use more complex reference construction algorithm  so construction like    typedef struct a A    anticipated declaration for member declaration  typedef struct a   Implemented declaration       A  b     member declaration  A    is permited and fully functional  This implementation give also access to compilator type conversion and remove some bugging effects when execution thread leave the application field of initialisation functions    This mean that in C typedefs are more near as C   class than lonely structs

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In  C  programming language the keyword  typedef  is used to declare a new name for some object struct  array  function  enum type   For example  I will use a  struct-s   In  C  we often declare a  struct  outside of the  main  function  For example   struct complex  int real part  img part  COMPLEX   main      struct KOMPLEKS number     number type is now a struct type  number real part   3   number img part   -1   printf  Number   d  d i  n  number real part  number img part        Each time I decide to use a struct type I will need this keyword  struct  something   name   typedef  will simply rename that type and I can use that new name in my program every time I want  So our code will be   typedef struct complex int real part  img part   COMPLEX    now COMPLEX is the new name for this structure and if I want to use it without    a keyword like in the first example  struct complex number    main     COMPLEX number     number is now the same type as in the first example number real part   1  number img part   5  printf   d  d  n   number real part  number img part        If you have some local object struct  array  valuable  that will be used in your entire program you can simply give it a name using a  typedef

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At all  in C language  struct union enum are macro instruction processed by the C language preprocessor  do not mistake with the preprocessor that treat   include  and other   so    struct a      int i      struct b      struct a     int i     int j       struct b is expended as something like this    struct b       struct a               int i             int i      int j      and so  at compile time it evolve on stack as something like  b  int ai int i int j  that also why it s dificult to have selfreferent structs  C preprocessor round in a d  claration loop that can t terminate   typedef are type specifier  that means only C compiler process it and it can do like he want for optimise assembler code implementation  It also dont expend member of type par stupidly like pr  processor do with structs but use more complex reference construction algorithm  so construction like    typedef struct a A    anticipated declaration for member declaration  typedef struct a   Implemented declaration       A  b     member declaration  A    is permited and fully functional  This implementation give also access to compilator type conversion and remove some bugging effects when execution thread leave the application field of initialisation functions    This mean that in C typedefs are more near as C   class than lonely structs

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As Greg Hewgill said  the typedef means you no longer have to write struct all over the place  That not only saves keystrokes  it also can make the code cleaner since it provides a smidgen more abstraction   Stuff like  typedef struct     int x  y    Point   Point point new int x  int y      Point a    a x   x    a y   y    return a      becomes cleaner when you don t need to see the  struct  keyword all over the place  it looks more as if there really is a type called  Point  in your language  Which  after the typedef  is the case I guess   Also note that while your example  and mine  omitted naming the struct  itself  actually naming it is also useful for when you want to provide an opaque type  Then you d have code like this in the header  for instance   typedef struct Point Point   Point   point new int x  int y     and then provide the struct definition in the implementation file   struct Point     int x  y      Point   point new int x  int y      Point  p    if  p   malloc sizeof  p      NULL          p- gt x   x      p- gt y   y        return p      In this latter case  you cannot return the Point by value  since its definition is hidden from users of the header file  This is a technique used widely in GTK   for instance   UPDATE Note that there are also highly-regarded C projects where this use of typedef to hide struct is considered a bad idea  the Linux kernel is probably the most well-known such project  See Chapter 5 of The Linux Kernel CodingStyle document for Linus  angry words     My point is that the  should  in the question is perhaps not set in stone  after all

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Turns out in C99 typedef is required  It is outdated  but a lot of tools  ala HackRank  use c99 as its pure C implementation  And typedef is required there   I m not saying they should change  maybe have two C options  if the requirement changed  those of us studing for interviews on the site would be SOL

User · Answer

Using a typedef avoids having to write struct every time you declare a variable of that type   struct elem    int i   char k     elem user     compile error  struct elem user     this is correct

User · Answer

typedef will not provide a co-dependent set of data structures  This you cannot do with typdef   struct bar  struct foo   struct foo       struct bar  b      struct bar       struct foo  f       Of course you can always add   typedef struct foo foo t  typedef struct bar bar t    What exactly is the point of that

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Using a typedef avoids having to write struct every time you declare a variable of that type   struct elem    int i   char k     elem user     compile error  struct elem user     this is correct

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It s amazing how many people get this wrong  PLEASE don t typedef structs in C  it needlessly pollutes the global namespace which is typically very polluted already in large C programs   Also  typedef d structs without a tag name are a major cause of needless imposition of ordering relationships among header files   Consider    ifndef FOO H  define FOO H 1   define FOO DEF  0xDEADBABE   struct bar     forward declaration  defined in bar h    struct foo     struct bar  bar       endif   With such a definition  not using typedefs  it is possible for a compiland unit to include foo h to get at the FOO DEF definition   If it doesn t attempt to dereference  the  bar  member of the foo struct then there will be no need to include the  bar h  file   Also  since the namespaces are different between the tag names and the member names  it is possible to write very readable code such as   struct foo  foo   printf  foo- gt bar    p   foo- gt bar     Since the namespaces are separate  there is no conflict in naming variables coincident with their struct tag name   If I have to maintain your code  I will remove your typedef d structs

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In  C  programming language the keyword  typedef  is used to declare a new name for some object struct  array  function  enum type   For example  I will use a  struct-s   In  C  we often declare a  struct  outside of the  main  function  For example   struct complex  int real part  img part  COMPLEX   main      struct KOMPLEKS number     number type is now a struct type  number real part   3   number img part   -1   printf  Number   d  d i  n  number real part  number img part        Each time I decide to use a struct type I will need this keyword  struct  something   name   typedef  will simply rename that type and I can use that new name in my program every time I want  So our code will be   typedef struct complex int real part  img part   COMPLEX    now COMPLEX is the new name for this structure and if I want to use it without    a keyword like in the first example  struct complex number    main     COMPLEX number     number is now the same type as in the first example number real part   1  number img part   5  printf   d  d  n   number real part  number img part        If you have some local object struct  array  valuable  that will be used in your entire program you can simply give it a name using a  typedef

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I don t think forward declarations are even possible with typedef  Use of struct  enum  and union allow for forwarding declarations when dependencies  knows about  is bidirectional   Style  Use of typedef in C   makes quite a bit of sense  It can almost be necessary when dealing with templates that require multiple and or variable parameters  The typedef helps keep the naming straight   Not so in the C programming language  The use of typedef most often serves no purpose but to obfuscate the data structure usage  Since only   struct  6   enum  4   union  5    number of keystrokes are used to declare a data type there is almost no use for the aliasing of the struct  Is that data type a union or a struct  Using the straightforward non-typdefed declaration lets you know right away what type it is    Notice how Linux is written with strict avoidance of this aliasing nonsense typedef brings  The result is a minimalist and clean style

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Linux kernel coding style Chapter 5 gives great pros and cons  mostly cons  of using typedef   Please don t use things like  quot vps t quot   It s a mistake to use typedef for structures and pointers  When you see a vps t a   in the source  what does it mean  In contrast  if it says struct virtual container  a   you can actually tell what  quot a quot  is  Lots of people think that typedefs  quot help readability quot   Not so  They are useful only for   a  totally opaque objects  where the typedef is actively used to hide what the object is   Example   quot pte t quot  etc  opaque objects that you can only access using the proper accessor functions  NOTE  Opaqueness and  quot accessor functions quot  are not good in themselves  The reason we have them for things like pte t etc  is that there really is absolutely zero portably accessible information there   b  Clear integer types  where the abstraction helps avoid confusion whether it is  quot int quot  or  quot long quot   u8 u16 u32 are perfectly fine typedefs  although they fit into category  d  better than here  NOTE  Again - there needs to be a reason for this  If something is  quot unsigned long quot   then there s no reason to do typedef unsigned long myflags t   but if there is a clear reason for why it under certain circumstances might be an  quot unsigned int quot  and under other configurations might be  quot unsigned long quot   then by all means go ahead and use a typedef   c  when you use sparse to literally create a new type for type-checking   d  New types which are identical to standard C99 types  in certain exceptional circumstances  Although it would only take a short amount of time for the eyes and brain to become accustomed to the standard types like  uint32 t   some people object to their use anyway  Therefore  the Linux-specific  u8 u16 u32 u64  types and their signed equivalents which are identical to standard types are permitted -- although they are not mandatory in new code of your own  When editing existing code which already uses one or the other set of types  you should conform to the existing choices in that code   e  Types safe for use in userspace  In certain structures which are visible to userspace  we cannot require C99 types and cannot use the  u32  form above  Thus  we use   u32 and similar types in all structures which are shared with userspace  Maybe there are other cases too  but the rule should basically be to NEVER EVER use a typedef unless you can clearly match one of those rules  In general  a pointer  or a struct that has elements that can reasonably be directly accessed should never be a typedef

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It turns out that there are pros and cons   A useful source of information is the seminal book  Expert C Programming   Chapter 3   Briefly  in C you have multiple namespaces  tags  types  member names and identifiers  typedef introduces an alias for a type and locates it in the tag namespace  Namely   typedef struct Tag     members     Type    defines two things  One Tag in the tag namespace and one Type in the type namespace  So you can do both Type myType and struct Tag myTagType  Declarations like struct Type myType or Tag myTagType are illegal  In addition  in a declaration like this   typedef Type  Type ptr    we define a pointer to our Type  So if we declare   Type ptr var1  var2  struct Tag  myTagType1  myTagType2    then var1 var2 and myTagType1 are pointers to Type but myTagType2 not   In the above-mentioned book  it mentions that typedefing structs are not very useful as it only saves the programmer from writing the word struct  However  I have an objection  like many other C programmers  Although it sometimes turns to obfuscate some names  that s why it is not advisable in large code bases like the kernel  when you want to implement polymorphism in C it helps a lot look here for details  Example   typedef struct MyWriter t      MyPipe super      MyQueue relative      uint32 t flags       MyWriter    you can do   void my writer func MyPipe  s        MyWriter  self    MyWriter    s      uint32 t myFlags   self- gt flags          So you can access an outer member  flags  by the inner struct  MyPipe  through casting  For me it is less confusing to cast the whole type than doing  struct MyWriter     s  every time you want to perform such functionality  In these cases brief referencing is a big deal especially if you heavily employ the technique in your code   Finally  the last aspect with typedefed types is the inability to extend them  in contrast to macros  If for example  you have     define X char 10  or typedef char Y 10    you can then declare  unsigned X x  but not unsigned Y y    We do not really care for this for structs because it does not apply to storage specifiers  volatile and const

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As Greg Hewgill said  the typedef means you no longer have to write struct all over the place  That not only saves keystrokes  it also can make the code cleaner since it provides a smidgen more abstraction   Stuff like  typedef struct     int x  y    Point   Point point new int x  int y      Point a    a x   x    a y   y    return a      becomes cleaner when you don t need to see the  struct  keyword all over the place  it looks more as if there really is a type called  Point  in your language  Which  after the typedef  is the case I guess   Also note that while your example  and mine  omitted naming the struct  itself  actually naming it is also useful for when you want to provide an opaque type  Then you d have code like this in the header  for instance   typedef struct Point Point   Point   point new int x  int y     and then provide the struct definition in the implementation file   struct Point     int x  y      Point   point new int x  int y      Point  p    if  p   malloc sizeof  p      NULL          p- gt x   x      p- gt y   y        return p      In this latter case  you cannot return the Point by value  since its definition is hidden from users of the header file  This is a technique used widely in GTK   for instance   UPDATE Note that there are also highly-regarded C projects where this use of typedef to hide struct is considered a bad idea  the Linux kernel is probably the most well-known such project  See Chapter 5 of The Linux Kernel CodingStyle document for Linus  angry words     My point is that the  should  in the question is perhaps not set in stone  after all

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A  a typdef aids in the meaning and documentation of a program by allowing creation of more meaningful synonyms for data types  In addition  they help parameterize a program against portability problems  K amp R  pg147  C prog lang    B  a structure defines a type  Structs allows convenient grouping of a collection of vars for convenience of handling  K amp R  pg127  C prog lang   as a single unit  C  typedef ing a struct is explained in A above   D  To me  structs are custom types or containers or collections or namespaces or complex types  whereas a typdef is just a means to create more nicknames

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A  a typdef aids in the meaning and documentation of a program by allowing creation of more meaningful synonyms for data types  In addition  they help parameterize a program against portability problems  K amp R  pg147  C prog lang    B  a structure defines a type  Structs allows convenient grouping of a collection of vars for convenience of handling  K amp R  pg127  C prog lang   as a single unit  C  typedef ing a struct is explained in A above   D  To me  structs are custom types or containers or collections or namespaces or complex types  whereas a typdef is just a means to create more nicknames

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the name you  optionally  give the struct is called the tag name and  as has been noted  is not a type in itself  To get to the type requires the struct prefix   GTK  aside  I m not sure the tagname is used anything like as commonly as a typedef to the struct type  so in C   that is recognised and you can omit the struct keyword and use the tagname as the type name too         struct MyStruct             int i                 The following is legal in C        MyStruct obj      obj i   7

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Linux kernel coding style Chapter 5 gives great pros and cons  mostly cons  of using typedef   Please don t use things like  quot vps t quot   It s a mistake to use typedef for structures and pointers  When you see a vps t a   in the source  what does it mean  In contrast  if it says struct virtual container  a   you can actually tell what  quot a quot  is  Lots of people think that typedefs  quot help readability quot   Not so  They are useful only for   a  totally opaque objects  where the typedef is actively used to hide what the object is   Example   quot pte t quot  etc  opaque objects that you can only access using the proper accessor functions  NOTE  Opaqueness and  quot accessor functions quot  are not good in themselves  The reason we have them for things like pte t etc  is that there really is absolutely zero portably accessible information there   b  Clear integer types  where the abstraction helps avoid confusion whether it is  quot int quot  or  quot long quot   u8 u16 u32 are perfectly fine typedefs  although they fit into category  d  better than here  NOTE  Again - there needs to be a reason for this  If something is  quot unsigned long quot   then there s no reason to do typedef unsigned long myflags t   but if there is a clear reason for why it under certain circumstances might be an  quot unsigned int quot  and under other configurations might be  quot unsigned long quot   then by all means go ahead and use a typedef   c  when you use sparse to literally create a new type for type-checking   d  New types which are identical to standard C99 types  in certain exceptional circumstances  Although it would only take a short amount of time for the eyes and brain to become accustomed to the standard types like  uint32 t   some people object to their use anyway  Therefore  the Linux-specific  u8 u16 u32 u64  types and their signed equivalents which are identical to standard types are permitted -- although they are not mandatory in new code of your own  When editing existing code which already uses one or the other set of types  you should conform to the existing choices in that code   e  Types safe for use in userspace  In certain structures which are visible to userspace  we cannot require C99 types and cannot use the  u32  form above  Thus  we use   u32 and similar types in all structures which are shared with userspace  Maybe there are other cases too  but the rule should basically be to NEVER EVER use a typedef unless you can clearly match one of those rules  In general  a pointer  or a struct that has elements that can reasonably be directly accessed should never be a typedef

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One other good reason to always typedef enums and structs results from this problem   enum EnumDef     FIRST ITEM    SECOND ITEM     struct StructDef     enum EnuumDef MyEnum    unsigned int MyVar    MyStruct    Notice the typo in EnumDef in the struct  EnuumDef   This compiles without error  or warning  and is  depending on the literal interpretation of the C Standard  correct  The problem is that I just created an new  empty  enumeration definition within my struct  I am not  as intended  using the previous definition EnumDef   With a typdef similar kind of typos would have resulted in a compiler errors for using an unknown type   typedef      FIRST ITEM    SECOND ITEM   EnumDef   typedef struct     EnuumDef MyEnum     compiler error  unknown type       unsigned int MyVar    StructDef  StrructDef MyStruct     compiler error  unknown type       I would advocate ALWAYS typedef ing structs and enumerations    Not only to save some typing  no pun intended      but because it is safer

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From an old article by Dan Saks  http   www ddj com cpp 184403396 pgno 3         The C language rules for naming   structs are a little eccentric  but   they re pretty harmless  However  when   extended to classes in C    those same   rules open little cracks for bugs to   crawl through       In C  the name s appearing in  struct s                            is a tag  A tag name is not a type   name  Given the definition above    declarations such as  s x        error in C    s  p       error in C          are errors in C  You must write them   as  struct s x         OK    struct s  p        OK          The names of unions and enumerations   are also tags rather than types       In C  tags are distinct from all other   names  for functions  types    variables  and enumeration constants     C compilers maintain tags in a symbol   table that s conceptually if not   physically separate from the table   that holds all other names  Thus  it   is possible for a C program to have   both a tag and an another name with   the same spelling in the same scope    For example   struct s s        is a valid declaration which declares   variable s of type struct s  It may   not be good practice  but C compilers   must accept it  I have never seen a   rationale for why C was designed this   way  I have always thought it was a   mistake  but there it is       Many programmers  including yours   truly  prefer to think of struct names   as type names  so they define an alias   for the tag using a typedef  For   example  defining  struct s                      typedef struct s S        lets you use S in place of struct s    as in  S x  S  p        A program cannot use S as the name of   both a type and a variable  or   function or enumeration constant    S S        error       This is good       The tag name in a struct  union  or   enum definition is optional  Many   programmers fold the struct definition   into the typedef and dispense with the   tag altogether  as in   typedef struct                     S       The linked article also has a discussion about how the C   behavior of not requireing a typedef can cause subtle name hiding problems   To prevent these problems  it s a good idea to typedef your classes and structs in C    too  even though at first glance it appears to be unnecessary   In C    with the typedef the name hiding become an error that the compiler tells you about rather than a hidden source of potential problems

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Let s start with the basics and work our way up   Here is an example of Structure definition   struct point         int x  y         Here the name point is optional   A Structure can be declared during its definition or after   Declaring during definition  struct point         int x  y      first point  second point    Declaring after definition  struct point         int x  y       struct point first point  second point    Now  carefully note the last case above  you need to write struct point to declare Structures of that type if you decide to create that type at a later point in your code   Enter typedef  If you intend to create new Structure   Structure is a custom data-type  at a later time in your program using the same blueprint  using typedef during its definition might be a good idea since you can save some typing moving forward   typedef struct point         int x  y      Points   Points first point  second point    A word of caution while naming your custom type  Nothing prevents you from using  t suffix at the end of your custom type name but POSIX standard reserves the use of suffix  t to denote standard library type names

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From an old article by Dan Saks  http   www ddj com cpp 184403396 pgno 3         The C language rules for naming   structs are a little eccentric  but   they re pretty harmless  However  when   extended to classes in C    those same   rules open little cracks for bugs to   crawl through       In C  the name s appearing in  struct s                            is a tag  A tag name is not a type   name  Given the definition above    declarations such as  s x        error in C    s  p       error in C          are errors in C  You must write them   as  struct s x         OK    struct s  p        OK          The names of unions and enumerations   are also tags rather than types       In C  tags are distinct from all other   names  for functions  types    variables  and enumeration constants     C compilers maintain tags in a symbol   table that s conceptually if not   physically separate from the table   that holds all other names  Thus  it   is possible for a C program to have   both a tag and an another name with   the same spelling in the same scope    For example   struct s s        is a valid declaration which declares   variable s of type struct s  It may   not be good practice  but C compilers   must accept it  I have never seen a   rationale for why C was designed this   way  I have always thought it was a   mistake  but there it is       Many programmers  including yours   truly  prefer to think of struct names   as type names  so they define an alias   for the tag using a typedef  For   example  defining  struct s                      typedef struct s S        lets you use S in place of struct s    as in  S x  S  p        A program cannot use S as the name of   both a type and a variable  or   function or enumeration constant    S S        error       This is good       The tag name in a struct  union  or   enum definition is optional  Many   programmers fold the struct definition   into the typedef and dispense with the   tag altogether  as in   typedef struct                     S       The linked article also has a discussion about how the C   behavior of not requireing a typedef can cause subtle name hiding problems   To prevent these problems  it s a good idea to typedef your classes and structs in C    too  even though at first glance it appears to be unnecessary   In C    with the typedef the name hiding become an error that the compiler tells you about rather than a hidden source of potential problems

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From an old article by Dan Saks  http   www ddj com cpp 184403396 pgno 3         The C language rules for naming   structs are a little eccentric  but   they re pretty harmless  However  when   extended to classes in C    those same   rules open little cracks for bugs to   crawl through       In C  the name s appearing in  struct s                            is a tag  A tag name is not a type   name  Given the definition above    declarations such as  s x        error in C    s  p       error in C          are errors in C  You must write them   as  struct s x         OK    struct s  p        OK          The names of unions and enumerations   are also tags rather than types       In C  tags are distinct from all other   names  for functions  types    variables  and enumeration constants     C compilers maintain tags in a symbol   table that s conceptually if not   physically separate from the table   that holds all other names  Thus  it   is possible for a C program to have   both a tag and an another name with   the same spelling in the same scope    For example   struct s s        is a valid declaration which declares   variable s of type struct s  It may   not be good practice  but C compilers   must accept it  I have never seen a   rationale for why C was designed this   way  I have always thought it was a   mistake  but there it is       Many programmers  including yours   truly  prefer to think of struct names   as type names  so they define an alias   for the tag using a typedef  For   example  defining  struct s                      typedef struct s S        lets you use S in place of struct s    as in  S x  S  p        A program cannot use S as the name of   both a type and a variable  or   function or enumeration constant    S S        error       This is good       The tag name in a struct  union  or   enum definition is optional  Many   programmers fold the struct definition   into the typedef and dispense with the   tag altogether  as in   typedef struct                     S       The linked article also has a discussion about how the C   behavior of not requireing a typedef can cause subtle name hiding problems   To prevent these problems  it s a good idea to typedef your classes and structs in C    too  even though at first glance it appears to be unnecessary   In C    with the typedef the name hiding become an error that the compiler tells you about rather than a hidden source of potential problems

[c] Why should we typedef a struct so often in C?

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