Forward declaring an enum in C

Question

I m trying to do something like the following   enum E   void Foo E e    enum E  A  B  C     which the compiler rejects   I ve had a quick look on Google and the consensus seems to be  you can t do it   but I can t understand why   Can anyone explain   Clarification 2  I m doing this as I have private methods in a class that take said enum  and I do not want the enum s values exposed - so  for example  I do not want anyone to know that E is defined as  enum E       FUNCTIONALITY NORMAL  FUNCTIONALITY RESTRICTED  FUNCTIONALITY FOR PROJECT X     as project X is not something I want my users to know about   So  I wanted to forward declare the enum so I could put the private methods in the header file  declare the enum internally in the cpp  and distribute the built library file and header to people   As for the compiler - it s GCC

User · Answer

Just noting that the reason actually is that the size of the enum is not yet known after forward declaration. Well, you use forward declaration of a struct to be able to pass a pointer around or refer to an object from a place that's refered to in the forward declared struct definition itself too.

Forward declaring an enum would not be too useful, because one would wish to be able to pass around the enum by-value. You couldn't even have a pointer to it, because i recently got told some platforms use pointers of different size for char than for int or long. So it all depends on the content of the enum.

The current C++ Standard explicitly disallows doing something like

enum X;

(in 7.1.5.3/1). But the next C++ Standard due to next year allows the following, which convinced me the problem actually has to do with the underlying type:

enum X : int;

It's known as a "opaque" enum declaration. You can even use X by value in the following code. And its enumerators can later be defined in a later redeclaration of the enumeration. See 7.2 in the current working draft.

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There s some dissent since this got bumped  sort of   so here s some relevant bits from the standard   Research shows that the standard doesn t really define forward declaration  nor does it explicitly state that enums can or can t be forward declared  First  from dcl enum  section 7 2   The underlying type of an enumeration is an integral type that can represent all the enumerator values defined in the enumeration  It is implementation-defined which integral type is used as the underlying type for an enumeration except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int  If the enumerator-list is empty  the underlying type is as if the enumeration had a single enumerator with value 0  The value of sizeof   applied to an enumeration type  an object of enumeration type  or an enumerator  is the value of sizeof   applied to the underlying type   So the underlying type of an enum is implementation-defined  with one minor restriction  Next we flip to the section on  quot incomplete types quot   3 9   which is about as close as we come to any standard on forward declarations   A class that has been declared but not defined  or an array of unknown size or of incomplete element type  is an incompletely-defined object type  A class type  such as  quot class X quot   might be incomplete at one point in a translation unit and complete later on  the type  quot class X quot  is the same type at both points  The declared type of an array object might be an array of incomplete class type and therefore incomplete  if the class type is completed later on in the translation unit  the array type becomes complete  the array type at those two points is the same type  The declared type of an array object might be an array of unknown size and therefore be incomplete at one point in a translation unit and complete later on  the array types at those two points   quot array of unknown bound of T quot  and  quot array of N T quot   are different types  The type of a pointer to array of unknown size  or of a type defined by a typedef declaration to be an array of unknown size  cannot be completed   So there  the standard pretty much laid out the types that can be forward declared   Enum wasn t there  so compiler authors generally regard forward declaring as disallowed by the standard due to the variable size of its underlying type  It makes sense  too   Enums are usually referenced in by-value situations  and the compiler would indeed need to know the storage size in those situations   Since the storage size is implementation defined  many compilers may just choose to use 32 bit values for the underlying type of every enum  at which point it becomes possible to forward declare them   An interesting experiment might be to try forward declaring an enum in visual studio  then forcing it to use an underlying type greater than sizeof int  as explained above to see what happens

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Because the enum can be an integral size of varying size  the compiler decides which size a given enum has   the pointer to the enum can also have varying size  since it s an integral type  chars have pointers of a different size on some platforms for instance    So the compiler can t even let you forward-declare the enum and user a pointer to it  because even there  it needs the size of the enum

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For VC  here s the test about forward declaration and specifying underlying type    the following code is compiled ok         typedef int myint      enum T       void foo T   tp                   tp    T 0x12345678            enum T   char               A          But got the warning for  W4  W3 not incur this warning   warning C4480  nonstandard extension used  specifying underlying type for enum  T    VC Microsoft  R  32-bit C C   Optimizing Compiler Version 15 00 30729 01 for 80x86  looks buggy in the above case    when seeing enum T  VC assumes the enum type T uses default 4 bytes int as underlying type  so the generated assembly code is           foo  YAXPAW4T   Z PROC                   foo       File e  work c cpp cpp snippet cpp       Line 13         push    ebp         mov ebp  esp       Line 14         mov eax  DWORD PTR  tp  ebp          mov DWORD PTR  eax   305419896        12345678H       Line 15         pop ebp         ret 0      foo  YAXPAW4T   Z ENDP                   foo   The above assembly code is extracted from  Fatest asm directly  not my personal guess  Do you see the  mov DWORD PTR eax   305419896         12345678H line   the following code snippet proves it        int main int argc  char  argv                union               char ca 4               T t           a          a ca 0    a ca 1    a  ca 2    a ca 3    1          foo   a t            printf    x    x    x    x n    a ca 0   a ca 1   a ca 2   a ca 3             return 0          the result is  0x78  0x56  0x34  0x12   after remove the forward declaration of enum T and move the definition of function foo after the enum T s definition  the result is OK    the above key instruction becomes   mov BYTE PTR  eax   120           00000078H  the final result is  0x78  0x1  0x1  0x1  Note the value is not being overwritten  So using of the forward-declaration of enum in VC is considered harmful   BTW  to not surprise  the syntax for declaration of the underlying type is same as its in C   In pratice I found it s worth to save 3 bytes by specifying the underlying type as char when talk to the embedded system  which is memory limited

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My answer is wrong  but I ve left it here because the comments are useful    Forward declaring enums is non-standard  because pointers to different enum types are not guaranteed to be the same size  The compiler may need to see the definition to know what size pointers can be used with this type   In practice  at least on all the popular compilers  pointers to enums are a consistent size  Forward declaration of enums is provided as a language extension by Visual C    for example

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My solution to your problem would be to either   1 - use int instead of enums  Declare your ints in an anonymous namespace in your CPP file  not in the header    namespace      const int FUNCTIONALITY NORMAL   0      const int FUNCTIONALITY RESTRICTED   1      const int FUNCTIONALITY FOR PROJECT X   2       As your methods are private  no one will mess with the data  You could even go further to test if someone sends you an invalid data   namespace      const int FUNCTIONALITY begin   0      const int FUNCTIONALITY NORMAL   0      const int FUNCTIONALITY RESTRICTED   1      const int FUNCTIONALITY FOR PROJECT X   2      const int FUNCTIONALITY end   3       bool isFunctionalityCorrect int i             return  i  gt   FUNCTIONALITY begin   amp  amp   i  lt  FUNCTIONALITY end             2   create a full class with limited const instantiations  like done in Java  Forward declare the class  and then define it in the CPP file  and instanciate only the enum-like values  I did something like that in C    and the result was not as satisfying as desired  as it needed some code to simulate an enum  copy construction  operator    etc     3   As proposed before  use the privately declared enum  Despite the fact an user will see its full definition  it won t be able to use it  nor use the private methods  So you ll usually be able to modify the enum and the content of the existing methods without needing recompiling of code using your class   My guess would be either the solution 3 or 1

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I d do it this way    in the public header   typedef unsigned long E   void Foo E e      in the internal header   enum Econtent   FUNCTIONALITY NORMAL  FUNCTIONALITY RESTRICTED  FUNCTIONALITY FOR PROJECT X    FORCE 32BIT   0xFFFFFFFF      By adding FORCE 32BIT we ensure that Econtent compiles to a long  so it s interchangeable with E

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My answer is wrong  but I ve left it here because the comments are useful    Forward declaring enums is non-standard  because pointers to different enum types are not guaranteed to be the same size  The compiler may need to see the definition to know what size pointers can be used with this type   In practice  at least on all the popular compilers  pointers to enums are a consistent size  Forward declaration of enums is provided as a language extension by Visual C    for example

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I m adding an up-to-date answer here  given recent developments   You can forward-declare an enum in C  11  so long as you declare its storage type at the same time   The syntax looks like this   enum E   short  void foo E e          enum E   short       VALUE 1      VALUE 2               In fact  if the function never refers to the values of the enumeration  you don t need the complete declaration at all at that point   This is supported by G   4 6 and onwards  -std c  0x or -std c  11 in more recent versions    Visual C   2013 supports this  in earlier versions it has some sort of non-standard support that I haven t figured out yet - I found some suggestion that a simple forward declaration is legal  but YMMV

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You define an enumeration to restrict the possible values of elements of the type to a limited set   This restriction is to be enforced at compile time   When forward declaring the fact that you will use a  limited set  later on doesn t add any value  subsequent code needs to know the possible values in order to benefit from it   Although the compiler is concerned about the size of the enumerated type  the intent of the enumeration gets lost when you forward declare it

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Because the enum can be an integral size of varying size  the compiler decides which size a given enum has   the pointer to the enum can also have varying size  since it s an integral type  chars have pointers of a different size on some platforms for instance    So the compiler can t even let you forward-declare the enum and user a pointer to it  because even there  it needs the size of the enum

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The reason the enum can t be forward declared is that without knowing the values  the compiler can t know the storage required for the enum variable   C   Compiler s are allowed to specify the actual storage space based on the size necessary to contain all the values specified   If all that is visible is the forward declaration  the translation unit can t know what storage size will have been chosen - it could be a char or an int  or something else     From Section 7 2 5 of the ISO C   Standard      The underlying type of an enumeration is an integral type that can represent all the enumerator values defined in the enumeration  It is implementation-defined which integral type is used as the underlying type for an enumeration except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int  If the enumerator-list is empty  the underlying type is as if the enumeration had a single enumerator with value 0  The value of sizeof   applied to an enumeration type  an object of enumeration type  or an enumerator  is the value of sizeof   applied to the underlying type    Since the caller to the function must know the sizes of the parameters to correctly setup the call stack  the number of enumerations in an enumeration list must be known before the function prototype   Update  In C  0X a syntax for foreward declaring enum types has been proposed and accepted   You can see the proposal at http   www open-std org jtc1 sc22 wg21 docs papers 2008 n2764 pdf

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If you really don t want your enum to appear in your header file AND ensure that it is only used by private methods  then one solution can be to go with the pimpl principle   It s a technique that ensure to hide the class internals in the headers by just declaring   class A    public          private      void  pImpl       Then in your implementation file  cpp   you declare a class that will be the representation of the internals   class AImpl   public      AImpl A  pThis   m pThis pThis              all private methods here     private      A  m pThis       You must dynamically create the implementation in the class constructor and delete it in the destructor and when implementing public method  you must use     AImpl  pImpl - gt PrivateMethod      There are pros for using pimpl  one is that it decouple your class header from its implementation  no need to recompile other classes when changing one class implementation  Another is that is speeds up your compilation time because your headers are so simple   But it s a pain to use  so you should really ask yourself if just declaring your enum as private in the header is that much a trouble

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If you really don t want your enum to appear in your header file AND ensure that it is only used by private methods  then one solution can be to go with the pimpl principle   It s a technique that ensure to hide the class internals in the headers by just declaring   class A    public          private      void  pImpl       Then in your implementation file  cpp   you declare a class that will be the representation of the internals   class AImpl   public      AImpl A  pThis   m pThis pThis              all private methods here     private      A  m pThis       You must dynamically create the implementation in the class constructor and delete it in the destructor and when implementing public method  you must use     AImpl  pImpl - gt PrivateMethod      There are pros for using pimpl  one is that it decouple your class header from its implementation  no need to recompile other classes when changing one class implementation  Another is that is speeds up your compilation time because your headers are so simple   But it s a pain to use  so you should really ask yourself if just declaring your enum as private in the header is that much a trouble

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Seems it can not be forward-declared in GCC   Interesting discussion here

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I d do it this way    in the public header   typedef unsigned long E   void Foo E e      in the internal header   enum Econtent   FUNCTIONALITY NORMAL  FUNCTIONALITY RESTRICTED  FUNCTIONALITY FOR PROJECT X    FORCE 32BIT   0xFFFFFFFF      By adding FORCE 32BIT we ensure that Econtent compiles to a long  so it s interchangeable with E

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In my projects  I adopted the Namespace-Bound Enumeration technique to deal with enums from legacy and 3rd-party components  Here is an example   forward h   namespace type       class legacy type      typedef const legacy type amp  type      enum h      May be defined here or pulled in via  include  namespace legacy       enum evil   x   y  z        namespace type       using legacy  evil       class legacy type           public          legacy type evil e                e  e                      operator evil   const                       return e                  private          evil e              foo h    include  forward h   class foo   public      void f type  type t        foo cc    include  foo h    include  lt iostream gt   include  enum h   void foo  f type  type t        switch  t                case legacy  x              std  cout  lt  lt   x   lt  lt  std  endl              break          case legacy  y              std  cout  lt  lt   y   lt  lt  std  endl              break          case legacy  z              std  cout  lt  lt   z   lt  lt  std  endl              break          default              std  cout  lt  lt   default   lt  lt  std  endl            main cc    include  foo h   include  enum h   int main         foo fu      fu f legacy  x        return 0      Note that the foo h header does not have to know anything about legacy  evil  Only the files that use the legacy type legacy  evil  here  main cc  need to include enum h

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You can wrap the enum in a struct  adding in some constructors and type conversions  and forward declare the struct instead    define ENUM CLASS NAME  TYPE  VALUES       struct NAME         enum e   VALUES          explicit NAME TYPE v    val v           NAME e v    val v           operator e   const   return e val           private           TYPE val        This appears to work  http   ideone com TYtP2

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My answer is wrong  but I ve left it here because the comments are useful    Forward declaring enums is non-standard  because pointers to different enum types are not guaranteed to be the same size  The compiler may need to see the definition to know what size pointers can be used with this type   In practice  at least on all the popular compilers  pointers to enums are a consistent size  Forward declaration of enums is provided as a language extension by Visual C    for example

User · Answer

I d do it this way    in the public header   typedef unsigned long E   void Foo E e      in the internal header   enum Econtent   FUNCTIONALITY NORMAL  FUNCTIONALITY RESTRICTED  FUNCTIONALITY FOR PROJECT X    FORCE 32BIT   0xFFFFFFFF      By adding FORCE 32BIT we ensure that Econtent compiles to a long  so it s interchangeable with E

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There is indeed no such thing as a forward declaration of enum  As an enum s definition doesn t contain any code that could depend on other code using the enum  it s usually not a problem to define the enum completely when you re first declaring it   If the only use of your enum is by private member functions  you can implement encapsulation by having the enum itself as a private member of that class  The enum still has to be fully defined at the point of declaration  that is  within the class definition  However  this is not a bigger problem as declaring private member functions there  and is not a worse exposal of implementation internals than that   If you need a deeper degree of concealment for your implementation details  you can break it into an abstract interface  only consisting of pure virtual functions  and a concrete  completely concealed  class implementing  inheriting  the interface  Creation of class instances can be handled by a factory or a static member function of the interface  That way  even the real class name  let alone its private functions  won t be exposed

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You define an enumeration to restrict the possible values of elements of the type to a limited set   This restriction is to be enforced at compile time   When forward declaring the fact that you will use a  limited set  later on doesn t add any value  subsequent code needs to know the possible values in order to benefit from it   Although the compiler is concerned about the size of the enumerated type  the intent of the enumeration gets lost when you forward declare it

User · Answer

You define an enumeration to restrict the possible values of elements of the type to a limited set   This restriction is to be enforced at compile time   When forward declaring the fact that you will use a  limited set  later on doesn t add any value  subsequent code needs to know the possible values in order to benefit from it   Although the compiler is concerned about the size of the enumerated type  the intent of the enumeration gets lost when you forward declare it

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My solution to your problem would be to either   1 - use int instead of enums  Declare your ints in an anonymous namespace in your CPP file  not in the header    namespace      const int FUNCTIONALITY NORMAL   0      const int FUNCTIONALITY RESTRICTED   1      const int FUNCTIONALITY FOR PROJECT X   2       As your methods are private  no one will mess with the data  You could even go further to test if someone sends you an invalid data   namespace      const int FUNCTIONALITY begin   0      const int FUNCTIONALITY NORMAL   0      const int FUNCTIONALITY RESTRICTED   1      const int FUNCTIONALITY FOR PROJECT X   2      const int FUNCTIONALITY end   3       bool isFunctionalityCorrect int i             return  i  gt   FUNCTIONALITY begin   amp  amp   i  lt  FUNCTIONALITY end             2   create a full class with limited const instantiations  like done in Java  Forward declare the class  and then define it in the CPP file  and instanciate only the enum-like values  I did something like that in C    and the result was not as satisfying as desired  as it needed some code to simulate an enum  copy construction  operator    etc     3   As proposed before  use the privately declared enum  Despite the fact an user will see its full definition  it won t be able to use it  nor use the private methods  So you ll usually be able to modify the enum and the content of the existing methods without needing recompiling of code using your class   My guess would be either the solution 3 or 1

User · Answer

Seems it can not be forward-declared in GCC   Interesting discussion here

User · Answer

Just noting that the reason actually is that the size of the enum is not yet known after forward declaration. Well, you use forward declaration of a struct to be able to pass a pointer around or refer to an object from a place that's refered to in the forward declared struct definition itself too.

Forward declaring an enum would not be too useful, because one would wish to be able to pass around the enum by-value. You couldn't even have a pointer to it, because i recently got told some platforms use pointers of different size for char than for int or long. So it all depends on the content of the enum.

The current C++ Standard explicitly disallows doing something like

enum X;

(in 7.1.5.3/1). But the next C++ Standard due to next year allows the following, which convinced me the problem actually has to do with the underlying type:

enum X : int;

It's known as a "opaque" enum declaration. You can even use X by value in the following code. And its enumerators can later be defined in a later redeclaration of the enumeration. See 7.2 in the current working draft.

User · Answer

You can wrap the enum in a struct  adding in some constructors and type conversions  and forward declare the struct instead    define ENUM CLASS NAME  TYPE  VALUES       struct NAME         enum e   VALUES          explicit NAME TYPE v    val v           NAME e v    val v           operator e   const   return e val           private           TYPE val        This appears to work  http   ideone com TYtP2

User · Answer

Because the enum can be an integral size of varying size  the compiler decides which size a given enum has   the pointer to the enum can also have varying size  since it s an integral type  chars have pointers of a different size on some platforms for instance    So the compiler can t even let you forward-declare the enum and user a pointer to it  because even there  it needs the size of the enum

User · Answer

In my projects  I adopted the Namespace-Bound Enumeration technique to deal with enums from legacy and 3rd-party components  Here is an example   forward h   namespace type       class legacy type      typedef const legacy type amp  type      enum h      May be defined here or pulled in via  include  namespace legacy       enum evil   x   y  z        namespace type       using legacy  evil       class legacy type           public          legacy type evil e                e  e                      operator evil   const                       return e                  private          evil e              foo h    include  forward h   class foo   public      void f type  type t        foo cc    include  foo h    include  lt iostream gt   include  enum h   void foo  f type  type t        switch  t                case legacy  x              std  cout  lt  lt   x   lt  lt  std  endl              break          case legacy  y              std  cout  lt  lt   y   lt  lt  std  endl              break          case legacy  z              std  cout  lt  lt   z   lt  lt  std  endl              break          default              std  cout  lt  lt   default   lt  lt  std  endl            main cc    include  foo h   include  enum h   int main         foo fu      fu f legacy  x        return 0      Note that the foo h header does not have to know anything about legacy  evil  Only the files that use the legacy type legacy  evil  here  main cc  need to include enum h

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I d do it this way    in the public header   typedef unsigned long E   void Foo E e      in the internal header   enum Econtent   FUNCTIONALITY NORMAL  FUNCTIONALITY RESTRICTED  FUNCTIONALITY FOR PROJECT X    FORCE 32BIT   0xFFFFFFFF      By adding FORCE 32BIT we ensure that Econtent compiles to a long  so it s interchangeable with E

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There s some dissent since this got bumped  sort of   so here s some relevant bits from the standard   Research shows that the standard doesn t really define forward declaration  nor does it explicitly state that enums can or can t be forward declared  First  from dcl enum  section 7 2   The underlying type of an enumeration is an integral type that can represent all the enumerator values defined in the enumeration  It is implementation-defined which integral type is used as the underlying type for an enumeration except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int  If the enumerator-list is empty  the underlying type is as if the enumeration had a single enumerator with value 0  The value of sizeof   applied to an enumeration type  an object of enumeration type  or an enumerator  is the value of sizeof   applied to the underlying type   So the underlying type of an enum is implementation-defined  with one minor restriction  Next we flip to the section on  quot incomplete types quot   3 9   which is about as close as we come to any standard on forward declarations   A class that has been declared but not defined  or an array of unknown size or of incomplete element type  is an incompletely-defined object type  A class type  such as  quot class X quot   might be incomplete at one point in a translation unit and complete later on  the type  quot class X quot  is the same type at both points  The declared type of an array object might be an array of incomplete class type and therefore incomplete  if the class type is completed later on in the translation unit  the array type becomes complete  the array type at those two points is the same type  The declared type of an array object might be an array of unknown size and therefore be incomplete at one point in a translation unit and complete later on  the array types at those two points   quot array of unknown bound of T quot  and  quot array of N T quot   are different types  The type of a pointer to array of unknown size  or of a type defined by a typedef declaration to be an array of unknown size  cannot be completed   So there  the standard pretty much laid out the types that can be forward declared   Enum wasn t there  so compiler authors generally regard forward declaring as disallowed by the standard due to the variable size of its underlying type  It makes sense  too   Enums are usually referenced in by-value situations  and the compiler would indeed need to know the storage size in those situations   Since the storage size is implementation defined  many compilers may just choose to use 32 bit values for the underlying type of every enum  at which point it becomes possible to forward declare them   An interesting experiment might be to try forward declaring an enum in visual studio  then forcing it to use an underlying type greater than sizeof int  as explained above to see what happens

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Forward declaring things in C   is very useful because it dramatically speeds up compilation time   You can forward declare several things in C   including  struct  class  function  etc     But can you forward declare an enum in C     No you can t     But why not allow it   If it were allowed you could define your enum type in your header file  and your enum values in your source file   Sounds like it should be allowed right   Wrong   In C   there is no default type for enum like there is in C   int    In C   your enum type will be determined by the compiler to be any type that will fit the range of values you have for your enum    What does that mean    It means that your enum s underlying type cannot be fully determined until you have all of the values of the enum defined   Which mans you cannot separate the declaration and definition of your enum   And therefore you cannot forward declare an enum in C     The ISO C   standard S7 2 5      The underlying type of an enumeration is an integral type that can represent all the enumerator values defined in the enumeration  It is implementation-defined which integral type is used as the underlying type for an enumeration except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int  If the enumerator-list is empty  the underlying type is as if the enumeration had a single enumerator with value 0  The value of sizeof   applied to an enumeration type  an object of enumeration type  or an enumerator  is the value of sizeof   applied to the underlying type    You can determine the size of an enumerated type in C   by using the sizeof operator   The size of the enumerated type is the size of its underlying type    In this way you can guess which type your compiler is using for your enum      What if you specify the type of your enum explicitly like this   enum Color   char   Red 0  Green 1  Blue 2   assert sizeof Color    1     Can you then forward declare your enum    No   But why not   Specifying the type of an enum is not actually part of the current C   standard   It is a VC   extension    It will be part of C  0x though   Source

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Forward declaring things in C   is very useful because it dramatically speeds up compilation time   You can forward declare several things in C   including  struct  class  function  etc     But can you forward declare an enum in C     No you can t     But why not allow it   If it were allowed you could define your enum type in your header file  and your enum values in your source file   Sounds like it should be allowed right   Wrong   In C   there is no default type for enum like there is in C   int    In C   your enum type will be determined by the compiler to be any type that will fit the range of values you have for your enum    What does that mean    It means that your enum s underlying type cannot be fully determined until you have all of the values of the enum defined   Which mans you cannot separate the declaration and definition of your enum   And therefore you cannot forward declare an enum in C     The ISO C   standard S7 2 5      The underlying type of an enumeration is an integral type that can represent all the enumerator values defined in the enumeration  It is implementation-defined which integral type is used as the underlying type for an enumeration except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int  If the enumerator-list is empty  the underlying type is as if the enumeration had a single enumerator with value 0  The value of sizeof   applied to an enumeration type  an object of enumeration type  or an enumerator  is the value of sizeof   applied to the underlying type    You can determine the size of an enumerated type in C   by using the sizeof operator   The size of the enumerated type is the size of its underlying type    In this way you can guess which type your compiler is using for your enum      What if you specify the type of your enum explicitly like this   enum Color   char   Red 0  Green 1  Blue 2   assert sizeof Color    1     Can you then forward declare your enum    No   But why not   Specifying the type of an enum is not actually part of the current C   standard   It is a VC   extension    It will be part of C  0x though   Source

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The reason the enum can t be forward declared is that without knowing the values  the compiler can t know the storage required for the enum variable   C   Compiler s are allowed to specify the actual storage space based on the size necessary to contain all the values specified   If all that is visible is the forward declaration  the translation unit can t know what storage size will have been chosen - it could be a char or an int  or something else     From Section 7 2 5 of the ISO C   Standard      The underlying type of an enumeration is an integral type that can represent all the enumerator values defined in the enumeration  It is implementation-defined which integral type is used as the underlying type for an enumeration except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int  If the enumerator-list is empty  the underlying type is as if the enumeration had a single enumerator with value 0  The value of sizeof   applied to an enumeration type  an object of enumeration type  or an enumerator  is the value of sizeof   applied to the underlying type    Since the caller to the function must know the sizes of the parameters to correctly setup the call stack  the number of enumerations in an enumeration list must be known before the function prototype   Update  In C  0X a syntax for foreward declaring enum types has been proposed and accepted   You can see the proposal at http   www open-std org jtc1 sc22 wg21 docs papers 2008 n2764 pdf

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My answer is wrong  but I ve left it here because the comments are useful    Forward declaring enums is non-standard  because pointers to different enum types are not guaranteed to be the same size  The compiler may need to see the definition to know what size pointers can be used with this type   In practice  at least on all the popular compilers  pointers to enums are a consistent size  Forward declaration of enums is provided as a language extension by Visual C    for example

User · Answer

Seems it can not be forward-declared in GCC   Interesting discussion here

User · Answer

Because the enum can be an integral size of varying size  the compiler decides which size a given enum has   the pointer to the enum can also have varying size  since it s an integral type  chars have pointers of a different size on some platforms for instance    So the compiler can t even let you forward-declare the enum and user a pointer to it  because even there  it needs the size of the enum

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I m adding an up-to-date answer here  given recent developments   You can forward-declare an enum in C  11  so long as you declare its storage type at the same time   The syntax looks like this   enum E   short  void foo E e          enum E   short       VALUE 1      VALUE 2               In fact  if the function never refers to the values of the enumeration  you don t need the complete declaration at all at that point   This is supported by G   4 6 and onwards  -std c  0x or -std c  11 in more recent versions    Visual C   2013 supports this  in earlier versions it has some sort of non-standard support that I haven t figured out yet - I found some suggestion that a simple forward declaration is legal  but YMMV

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There is indeed no such thing as a forward declaration of enum  As an enum s definition doesn t contain any code that could depend on other code using the enum  it s usually not a problem to define the enum completely when you re first declaring it   If the only use of your enum is by private member functions  you can implement encapsulation by having the enum itself as a private member of that class  The enum still has to be fully defined at the point of declaration  that is  within the class definition  However  this is not a bigger problem as declaring private member functions there  and is not a worse exposal of implementation internals than that   If you need a deeper degree of concealment for your implementation details  you can break it into an abstract interface  only consisting of pure virtual functions  and a concrete  completely concealed  class implementing  inheriting  the interface  Creation of class instances can be handled by a factory or a static member function of the interface  That way  even the real class name  let alone its private functions  won t be exposed

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To anyone facing this for iOS Mac Xcode  If you are facing this while integrating C C   headers in XCode with Objective-C  just change the extension of your file from  mm to  m

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Forward declaration of enums is possible since C  11  Previously  the reason enum types couldn t be forward declared is because the size of the enumeration depends on its contents  As long as the size of the enumeration is specified by the application  it can be forward declared   enum Enum1                      Illegal in C   and C  0x  no size is explicitly specified  enum Enum2   unsigned int       Legal in C  0x  enum class Enum3                Legal in C  0x  because enum class declarations have a default type of  int   enum class Enum4  unsigned int    Legal C  0x  enum Enum2   unsigned short     Illegal in C  0x  because Enum2 was previously declared with a different type

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Seems it can not be forward-declared in GCC   Interesting discussion here

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For VC  here s the test about forward declaration and specifying underlying type    the following code is compiled ok         typedef int myint      enum T       void foo T   tp                   tp    T 0x12345678            enum T   char               A          But got the warning for  W4  W3 not incur this warning   warning C4480  nonstandard extension used  specifying underlying type for enum  T    VC Microsoft  R  32-bit C C   Optimizing Compiler Version 15 00 30729 01 for 80x86  looks buggy in the above case    when seeing enum T  VC assumes the enum type T uses default 4 bytes int as underlying type  so the generated assembly code is           foo  YAXPAW4T   Z PROC                   foo       File e  work c cpp cpp snippet cpp       Line 13         push    ebp         mov ebp  esp       Line 14         mov eax  DWORD PTR  tp  ebp          mov DWORD PTR  eax   305419896        12345678H       Line 15         pop ebp         ret 0      foo  YAXPAW4T   Z ENDP                   foo   The above assembly code is extracted from  Fatest asm directly  not my personal guess  Do you see the  mov DWORD PTR eax   305419896         12345678H line   the following code snippet proves it        int main int argc  char  argv                union               char ca 4               T t           a          a ca 0    a ca 1    a  ca 2    a ca 3    1          foo   a t            printf    x    x    x    x n    a ca 0   a ca 1   a ca 2   a ca 3             return 0          the result is  0x78  0x56  0x34  0x12   after remove the forward declaration of enum T and move the definition of function foo after the enum T s definition  the result is OK    the above key instruction becomes   mov BYTE PTR  eax   120           00000078H  the final result is  0x78  0x1  0x1  0x1  Note the value is not being overwritten  So using of the forward-declaration of enum in VC is considered harmful   BTW  to not surprise  the syntax for declaration of the underlying type is same as its in C   In pratice I found it s worth to save 3 bytes by specifying the underlying type as char when talk to the embedded system  which is memory limited

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If you really don t want your enum to appear in your header file AND ensure that it is only used by private methods  then one solution can be to go with the pimpl principle   It s a technique that ensure to hide the class internals in the headers by just declaring   class A    public          private      void  pImpl       Then in your implementation file  cpp   you declare a class that will be the representation of the internals   class AImpl   public      AImpl A  pThis   m pThis pThis              all private methods here     private      A  m pThis       You must dynamically create the implementation in the class constructor and delete it in the destructor and when implementing public method  you must use     AImpl  pImpl - gt PrivateMethod      There are pros for using pimpl  one is that it decouple your class header from its implementation  no need to recompile other classes when changing one class implementation  Another is that is speeds up your compilation time because your headers are so simple   But it s a pain to use  so you should really ask yourself if just declaring your enum as private in the header is that much a trouble

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There is indeed no such thing as a forward declaration of enum  As an enum s definition doesn t contain any code that could depend on other code using the enum  it s usually not a problem to define the enum completely when you re first declaring it   If the only use of your enum is by private member functions  you can implement encapsulation by having the enum itself as a private member of that class  The enum still has to be fully defined at the point of declaration  that is  within the class definition  However  this is not a bigger problem as declaring private member functions there  and is not a worse exposal of implementation internals than that   If you need a deeper degree of concealment for your implementation details  you can break it into an abstract interface  only consisting of pure virtual functions  and a concrete  completely concealed  class implementing  inheriting  the interface  Creation of class instances can be handled by a factory or a static member function of the interface  That way  even the real class name  let alone its private functions  won t be exposed

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Forward declaration of enums is possible since C  11  Previously  the reason enum types couldn t be forward declared is because the size of the enumeration depends on its contents  As long as the size of the enumeration is specified by the application  it can be forward declared   enum Enum1                      Illegal in C   and C  0x  no size is explicitly specified  enum Enum2   unsigned int       Legal in C  0x  enum class Enum3                Legal in C  0x  because enum class declarations have a default type of  int   enum class Enum4  unsigned int    Legal C  0x  enum Enum2   unsigned short     Illegal in C  0x  because Enum2 was previously declared with a different type

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You define an enumeration to restrict the possible values of elements of the type to a limited set   This restriction is to be enforced at compile time   When forward declaring the fact that you will use a  limited set  later on doesn t add any value  subsequent code needs to know the possible values in order to benefit from it   Although the compiler is concerned about the size of the enumerated type  the intent of the enumeration gets lost when you forward declare it

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The reason the enum can t be forward declared is that without knowing the values  the compiler can t know the storage required for the enum variable   C   Compiler s are allowed to specify the actual storage space based on the size necessary to contain all the values specified   If all that is visible is the forward declaration  the translation unit can t know what storage size will have been chosen - it could be a char or an int  or something else     From Section 7 2 5 of the ISO C   Standard      The underlying type of an enumeration is an integral type that can represent all the enumerator values defined in the enumeration  It is implementation-defined which integral type is used as the underlying type for an enumeration except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int  If the enumerator-list is empty  the underlying type is as if the enumeration had a single enumerator with value 0  The value of sizeof   applied to an enumeration type  an object of enumeration type  or an enumerator  is the value of sizeof   applied to the underlying type    Since the caller to the function must know the sizes of the parameters to correctly setup the call stack  the number of enumerations in an enumeration list must be known before the function prototype   Update  In C  0X a syntax for foreward declaring enum types has been proposed and accepted   You can see the proposal at http   www open-std org jtc1 sc22 wg21 docs papers 2008 n2764 pdf

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There is indeed no such thing as a forward declaration of enum  As an enum s definition doesn t contain any code that could depend on other code using the enum  it s usually not a problem to define the enum completely when you re first declaring it   If the only use of your enum is by private member functions  you can implement encapsulation by having the enum itself as a private member of that class  The enum still has to be fully defined at the point of declaration  that is  within the class definition  However  this is not a bigger problem as declaring private member functions there  and is not a worse exposal of implementation internals than that   If you need a deeper degree of concealment for your implementation details  you can break it into an abstract interface  only consisting of pure virtual functions  and a concrete  completely concealed  class implementing  inheriting  the interface  Creation of class instances can be handled by a factory or a static member function of the interface  That way  even the real class name  let alone its private functions  won t be exposed

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My solution to your problem would be to either   1 - use int instead of enums  Declare your ints in an anonymous namespace in your CPP file  not in the header    namespace      const int FUNCTIONALITY NORMAL   0      const int FUNCTIONALITY RESTRICTED   1      const int FUNCTIONALITY FOR PROJECT X   2       As your methods are private  no one will mess with the data  You could even go further to test if someone sends you an invalid data   namespace      const int FUNCTIONALITY begin   0      const int FUNCTIONALITY NORMAL   0      const int FUNCTIONALITY RESTRICTED   1      const int FUNCTIONALITY FOR PROJECT X   2      const int FUNCTIONALITY end   3       bool isFunctionalityCorrect int i             return  i  gt   FUNCTIONALITY begin   amp  amp   i  lt  FUNCTIONALITY end             2   create a full class with limited const instantiations  like done in Java  Forward declare the class  and then define it in the CPP file  and instanciate only the enum-like values  I did something like that in C    and the result was not as satisfying as desired  as it needed some code to simulate an enum  copy construction  operator    etc     3   As proposed before  use the privately declared enum  Despite the fact an user will see its full definition  it won t be able to use it  nor use the private methods  So you ll usually be able to modify the enum and the content of the existing methods without needing recompiling of code using your class   My guess would be either the solution 3 or 1

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My solution to your problem would be to either   1 - use int instead of enums  Declare your ints in an anonymous namespace in your CPP file  not in the header    namespace      const int FUNCTIONALITY NORMAL   0      const int FUNCTIONALITY RESTRICTED   1      const int FUNCTIONALITY FOR PROJECT X   2       As your methods are private  no one will mess with the data  You could even go further to test if someone sends you an invalid data   namespace      const int FUNCTIONALITY begin   0      const int FUNCTIONALITY NORMAL   0      const int FUNCTIONALITY RESTRICTED   1      const int FUNCTIONALITY FOR PROJECT X   2      const int FUNCTIONALITY end   3       bool isFunctionalityCorrect int i             return  i  gt   FUNCTIONALITY begin   amp  amp   i  lt  FUNCTIONALITY end             2   create a full class with limited const instantiations  like done in Java  Forward declare the class  and then define it in the CPP file  and instanciate only the enum-like values  I did something like that in C    and the result was not as satisfying as desired  as it needed some code to simulate an enum  copy construction  operator    etc     3   As proposed before  use the privately declared enum  Despite the fact an user will see its full definition  it won t be able to use it  nor use the private methods  So you ll usually be able to modify the enum and the content of the existing methods without needing recompiling of code using your class   My guess would be either the solution 3 or 1

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If you really don t want your enum to appear in your header file AND ensure that it is only used by private methods  then one solution can be to go with the pimpl principle   It s a technique that ensure to hide the class internals in the headers by just declaring   class A    public          private      void  pImpl       Then in your implementation file  cpp   you declare a class that will be the representation of the internals   class AImpl   public      AImpl A  pThis   m pThis pThis              all private methods here     private      A  m pThis       You must dynamically create the implementation in the class constructor and delete it in the destructor and when implementing public method  you must use     AImpl  pImpl - gt PrivateMethod      There are pros for using pimpl  one is that it decouple your class header from its implementation  no need to recompile other classes when changing one class implementation  Another is that is speeds up your compilation time because your headers are so simple   But it s a pain to use  so you should really ask yourself if just declaring your enum as private in the header is that much a trouble

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The reason the enum can t be forward declared is that without knowing the values  the compiler can t know the storage required for the enum variable   C   Compiler s are allowed to specify the actual storage space based on the size necessary to contain all the values specified   If all that is visible is the forward declaration  the translation unit can t know what storage size will have been chosen - it could be a char or an int  or something else     From Section 7 2 5 of the ISO C   Standard      The underlying type of an enumeration is an integral type that can represent all the enumerator values defined in the enumeration  It is implementation-defined which integral type is used as the underlying type for an enumeration except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int  If the enumerator-list is empty  the underlying type is as if the enumeration had a single enumerator with value 0  The value of sizeof   applied to an enumeration type  an object of enumeration type  or an enumerator  is the value of sizeof   applied to the underlying type    Since the caller to the function must know the sizes of the parameters to correctly setup the call stack  the number of enumerations in an enumeration list must be known before the function prototype   Update  In C  0X a syntax for foreward declaring enum types has been proposed and accepted   You can see the proposal at http   www open-std org jtc1 sc22 wg21 docs papers 2008 n2764 pdf

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To anyone facing this for iOS Mac Xcode  If you are facing this while integrating C C   headers in XCode with Objective-C  just change the extension of your file from  mm to  m

[c++] Forward declaring an enum in C++

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