Why do you use typedef when declaring an enum in C

Question

I haven t written any C   in years and now I m trying to get back into it  I then ran across this and thought about giving up   typedef enum TokenType       blah1     0x00000000      blah2     0X01000000      blah3     0X02000000   TokenType    What is this  Why is the typedef keyword used here  Why does the name TokenType appear twice in this declaration  How are the semantics different from this   enum TokenType       blah1   0x00000000      blah2 0x01000000      blah3 0x02000000

User · Answer

Holdover from C

User · Answer

You do not need to do it  In C  not C    you were required to use enum Enumname to refer to a data element of the enumerated type  To simplify it you were allowed to typedef it to a single name data type   typedef enum MyEnum              MyEnum    allowed functions taking a parameter of the enum to be defined as  void f  MyEnum x     instead of the longer  void f  enum MyEnum x     Note that the name of the typename does not need to be equal to the name of the enum  The same happens with structs   In C    on the other hand  it is not required  as enums  classes and structs can be accessed directly as types by their names      C   enum MyEnum                void f  MyEnum x       Correct C    Error in C

User · Answer

You do not need to do it  In C  not C    you were required to use enum Enumname to refer to a data element of the enumerated type  To simplify it you were allowed to typedef it to a single name data type   typedef enum MyEnum              MyEnum    allowed functions taking a parameter of the enum to be defined as  void f  MyEnum x     instead of the longer  void f  enum MyEnum x     Note that the name of the typename does not need to be equal to the name of the enum  The same happens with structs   In C    on the other hand  it is not required  as enums  classes and structs can be accessed directly as types by their names      C   enum MyEnum                void f  MyEnum x       Correct C    Error in C

User · Answer

The actual answer to the  why  question  which is surprisingly ignored by the existing answers top this old question  is that this enum declaration is probably located in a header file that is intended to be cross-compilable as both C and C   code  i e  included into both C and C   implementation fiules   The art of writing such header files relies on the author s ability to select language features that have the proper compatible meaning in both languages

User · Answer

Holdover from C

User · Answer

It s a C heritage  in C  if you do    enum TokenType       blah1     0x00000000      blah2     0X01000000      blah3     0X02000000      you ll have to use it doing something like    enum TokenType foo    But if you do this    typedef enum e TokenType       blah1     0x00000000      blah2     0X01000000      blah3     0X02000000   TokenType    You ll be able to declare    TokenType foo    But in C    you can use only the former definition and use it as if it were in a C typedef

User · Answer

This is kind of old  but anyway  I hope you ll appreciate the link that I am about to type as I appreciated it when I came across it earlier this year   Here it is  I should quote the explanation that is always in my mind when I have to grasp some nasty typedefs      In variable declarations  the introduced names are instances of the corresponding types          However  when the typedef keyword precedes the declaration  the introduced names are aliases of the corresponding types   As many people previously said  there is no need to use typedefs declaring enums in C    But that s the explanation of the typedef s syntax  I hope it helps  Probably not OP  since it s been almost 10 years  but anyone that is struggling to understand these kind of things

User · Answer

In C  declaring your enum the first way allows you to use it like so   TokenType my type    If you use the second style  you ll be forced to declare your variable like this   enum TokenType my type    As mentioned by others  this doesn t make a difference in C    My guess is that either the person who wrote this is a C programmer at heart  or you re compiling C code as C    Either way  it won t affect the behaviour of your code

User · Answer

In C  declaring your enum the first way allows you to use it like so   TokenType my type    If you use the second style  you ll be forced to declare your variable like this   enum TokenType my type    As mentioned by others  this doesn t make a difference in C    My guess is that either the person who wrote this is a C programmer at heart  or you re compiling C code as C    Either way  it won t affect the behaviour of your code

User · Answer

Some people say C doesn t have namespaces but that is not technically correct  It has three    Tags  enum  union  and struct  Labels  everything else    typedef enum     XYZ  declares an anonymous enumeration and imports it into the global namespace with the name XYZ   typedef enum ABC     XYZ  declares an enum named ABC in the tag namespace  then imports it into the global namespace as XYZ   Some people don t want to bother with the separate namespaces so they typedef everything  Others never typedef because they want the namespacing

User · Answer

In C  it is good style because you can change the type to something besides an enum   typedef enum e TokenType       blah1     0x00000000      blah2     0X01000000      blah3     0X02000000   TokenType   foo enum e TokenType token       this can only be passed as an enum     foo TokenType token      TokenType can be defined to something else later                          without changing this declaration      In C   you can define the enum so that it will compile as C   or C

User · Answer

This is kind of old  but anyway  I hope you ll appreciate the link that I am about to type as I appreciated it when I came across it earlier this year   Here it is  I should quote the explanation that is always in my mind when I have to grasp some nasty typedefs      In variable declarations  the introduced names are instances of the corresponding types          However  when the typedef keyword precedes the declaration  the introduced names are aliases of the corresponding types   As many people previously said  there is no need to use typedefs declaring enums in C    But that s the explanation of the typedef s syntax  I hope it helps  Probably not OP  since it s been almost 10 years  but anyone that is struggling to understand these kind of things

User · Answer

Holdover from C

User · Answer

The actual answer to the  why  question  which is surprisingly ignored by the existing answers top this old question  is that this enum declaration is probably located in a header file that is intended to be cross-compilable as both C and C   code  i e  included into both C and C   implementation fiules   The art of writing such header files relies on the author s ability to select language features that have the proper compatible meaning in both languages

User · Answer

Holdover from C

User · Answer

You do not need to do it  In C  not C    you were required to use enum Enumname to refer to a data element of the enumerated type  To simplify it you were allowed to typedef it to a single name data type   typedef enum MyEnum              MyEnum    allowed functions taking a parameter of the enum to be defined as  void f  MyEnum x     instead of the longer  void f  enum MyEnum x     Note that the name of the typename does not need to be equal to the name of the enum  The same happens with structs   In C    on the other hand  it is not required  as enums  classes and structs can be accessed directly as types by their names      C   enum MyEnum                void f  MyEnum x       Correct C    Error in C

User · Answer

It s a C heritage  in C  if you do    enum TokenType       blah1     0x00000000      blah2     0X01000000      blah3     0X02000000      you ll have to use it doing something like    enum TokenType foo    But if you do this    typedef enum e TokenType       blah1     0x00000000      blah2     0X01000000      blah3     0X02000000   TokenType    You ll be able to declare    TokenType foo    But in C    you can use only the former definition and use it as if it were in a C typedef

User · Answer

Some people say C doesn t have namespaces but that is not technically correct  It has three    Tags  enum  union  and struct  Labels  everything else    typedef enum     XYZ  declares an anonymous enumeration and imports it into the global namespace with the name XYZ   typedef enum ABC     XYZ  declares an enum named ABC in the tag namespace  then imports it into the global namespace as XYZ   Some people don t want to bother with the separate namespaces so they typedef everything  Others never typedef because they want the namespacing

User · Answer

In C  declaring your enum the first way allows you to use it like so   TokenType my type    If you use the second style  you ll be forced to declare your variable like this   enum TokenType my type    As mentioned by others  this doesn t make a difference in C    My guess is that either the person who wrote this is a C programmer at heart  or you re compiling C code as C    Either way  it won t affect the behaviour of your code

User · Answer

In some C codestyle guide the typedef version is said to be preferred for  clarity  and  simplicity   I disagree  because the typedef obfuscates the real nature of the declared object  In fact  I don t use typedefs because when declaring a C variable I want to be clear about what the object actually is  This choice helps myself to remember faster what an old piece of code actually does  and will help others when maintaining the code in the future

User · Answer

In C  it is good style because you can change the type to something besides an enum   typedef enum e TokenType       blah1     0x00000000      blah2     0X01000000      blah3     0X02000000   TokenType   foo enum e TokenType token       this can only be passed as an enum     foo TokenType token      TokenType can be defined to something else later                          without changing this declaration      In C   you can define the enum so that it will compile as C   or C

User · Answer

You do not need to do it  In C  not C    you were required to use enum Enumname to refer to a data element of the enumerated type  To simplify it you were allowed to typedef it to a single name data type   typedef enum MyEnum              MyEnum    allowed functions taking a parameter of the enum to be defined as  void f  MyEnum x     instead of the longer  void f  enum MyEnum x     Note that the name of the typename does not need to be equal to the name of the enum  The same happens with structs   In C    on the other hand  it is not required  as enums  classes and structs can be accessed directly as types by their names      C   enum MyEnum                void f  MyEnum x       Correct C    Error in C

User · Answer

It s a C heritage  in C  if you do    enum TokenType       blah1     0x00000000      blah2     0X01000000      blah3     0X02000000      you ll have to use it doing something like    enum TokenType foo    But if you do this    typedef enum e TokenType       blah1     0x00000000      blah2     0X01000000      blah3     0X02000000   TokenType    You ll be able to declare    TokenType foo    But in C    you can use only the former definition and use it as if it were in a C typedef

User · Answer

In some C codestyle guide the typedef version is said to be preferred for  clarity  and  simplicity   I disagree  because the typedef obfuscates the real nature of the declared object  In fact  I don t use typedefs because when declaring a C variable I want to be clear about what the object actually is  This choice helps myself to remember faster what an old piece of code actually does  and will help others when maintaining the code in the future

User · Answer

In C  declaring your enum the first way allows you to use it like so   TokenType my type    If you use the second style  you ll be forced to declare your variable like this   enum TokenType my type    As mentioned by others  this doesn t make a difference in C    My guess is that either the person who wrote this is a C programmer at heart  or you re compiling C code as C    Either way  it won t affect the behaviour of your code

User · Answer

It s a C heritage  in C  if you do    enum TokenType       blah1     0x00000000      blah2     0X01000000      blah3     0X02000000      you ll have to use it doing something like    enum TokenType foo    But if you do this    typedef enum e TokenType       blah1     0x00000000      blah2     0X01000000      blah3     0X02000000   TokenType    You ll be able to declare    TokenType foo    But in C    you can use only the former definition and use it as if it were in a C typedef

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