Undefined reference to static class member

Question

Can anyone explain why following code won t compile  At least on g   4 2 4   And more interesting  why it will compile when I cast MEMBER to int    include  lt vector gt   class Foo     public        static const int MEMBER   1        int main          vector lt int gt  v        v push back  Foo  MEMBER             undefined reference to  Foo  MEMBER      v push back   int  Foo  MEMBER       OK       return 0

User · Answer

With C  11  the above would be possible for basic types as  class Foo   public      static constexpr int MEMBER   1         The constexpr part creates a static expression as opposed to a static variable - and that behaves just like an extremely simple inline method definition  The approach proved a bit wobbly with C-string constexprs inside template classes  though

User · Answer

The C   standard requires a definition for your static const member if the definition is somehow needed     The definition is required  for example if it s address is used   push back takes its parameter by const reference  and so strictly the compiler needs the address of your member and you need to define it in the namespace   When you explicitly cast the constant  you re creating a temporary and it s this temporary which is bound to the reference  under special rules in the standard    This is a really interesting case  and I actually think it s worth raising an issue so that the std be changed to have the same behaviour for your constant member   Although  in a weird kind of way this could be seen as a legitimate use of the unary     operator   Basically the result of the unary   is an rvalue and so the rules for binding of rvalues to const references apply and we don t use the address of our static const member   v push back   Foo  MEMBER

User · Answer

The C   standard requires a definition for your static const member if the definition is somehow needed     The definition is required  for example if it s address is used   push back takes its parameter by const reference  and so strictly the compiler needs the address of your member and you need to define it in the namespace   When you explicitly cast the constant  you re creating a temporary and it s this temporary which is bound to the reference  under special rules in the standard    This is a really interesting case  and I actually think it s worth raising an issue so that the std be changed to have the same behaviour for your constant member   Although  in a weird kind of way this could be seen as a legitimate use of the unary     operator   Basically the result of the unary   is an rvalue and so the rules for binding of rvalues to const references apply and we don t use the address of our static const member   v push back   Foo  MEMBER

User · Answer

No idea why the cast works  but Foo  MEMBER isn t allocated until the first time Foo is loaded  and since you re never loading it  it s never allocated   If you had a reference to a Foo somewhere  it would probably work

User · Answer

The problem comes because of an interesting clash of new C   features and what you re trying to do   First  let s take a look at the push back signature   void push back const T amp     It s expecting a reference to an object of type T   Under the old system of initialization  such a member exists   For example  the following code compiles just fine    include  lt vector gt   class Foo   public      static const int MEMBER      const int Foo  MEMBER   1    int main        std  vector lt int gt  v      v push back  Foo  MEMBER             undefined reference to  Foo  MEMBER      v push back   int  Foo  MEMBER       OK       return 0      This is because there is an actual object somewhere that has that value stored in it   If  however  you switch to the new method of specifying static const members  like you have above  Foo  MEMBER is no longer an object   It is a constant  somewhat akin to    define MEMBER 1   But without the headaches of a preprocessor macro  and with type safety    That means that the vector  which is expecting a reference  can t get one

User · Answer

You need to actually define the static member somewhere  after the class definition    Try this   class Foo                 const int Foo  MEMBER   int main                   That should get rid of the undefined reference

User · Answer

No idea why the cast works  but Foo  MEMBER isn t allocated until the first time Foo is loaded  and since you re never loading it  it s never allocated   If you had a reference to a Foo somewhere  it would probably work

User · Answer

No idea why the cast works  but Foo  MEMBER isn t allocated until the first time Foo is loaded  and since you re never loading it  it s never allocated   If you had a reference to a Foo somewhere  it would probably work

User · Answer

Aaa h  class Aaa    protected       static Aaa  defaultAaa        Aaa cpp     You must define an actual variable in your program for the static members of the classes  static Aaa  Aaa  defaultAaa

User · Answer

The C   standard requires a definition for your static const member if the definition is somehow needed     The definition is required  for example if it s address is used   push back takes its parameter by const reference  and so strictly the compiler needs the address of your member and you need to define it in the namespace   When you explicitly cast the constant  you re creating a temporary and it s this temporary which is bound to the reference  under special rules in the standard    This is a really interesting case  and I actually think it s worth raising an issue so that the std be changed to have the same behaviour for your constant member   Although  in a weird kind of way this could be seen as a legitimate use of the unary     operator   Basically the result of the unary   is an rvalue and so the rules for binding of rvalues to const references apply and we don t use the address of our static const member   v push back   Foo  MEMBER

User · Answer

No idea why the cast works  but Foo  MEMBER isn t allocated until the first time Foo is loaded  and since you re never loading it  it s never allocated   If you had a reference to a Foo somewhere  it would probably work

User · Answer

You need to actually define the static member somewhere  after the class definition    Try this   class Foo                 const int Foo  MEMBER   int main                   That should get rid of the undefined reference

User · Answer

The C   standard requires a definition for your static const member if the definition is somehow needed     The definition is required  for example if it s address is used   push back takes its parameter by const reference  and so strictly the compiler needs the address of your member and you need to define it in the namespace   When you explicitly cast the constant  you re creating a temporary and it s this temporary which is bound to the reference  under special rules in the standard    This is a really interesting case  and I actually think it s worth raising an issue so that the std be changed to have the same behaviour for your constant member   Although  in a weird kind of way this could be seen as a legitimate use of the unary     operator   Basically the result of the unary   is an rvalue and so the rules for binding of rvalues to const references apply and we don t use the address of our static const member   v push back   Foo  MEMBER

User · Answer

Aaa h  class Aaa    protected       static Aaa  defaultAaa        Aaa cpp     You must define an actual variable in your program for the static members of the classes  static Aaa  Aaa  defaultAaa

User · Answer

With C  11  the above would be possible for basic types as  class Foo   public      static constexpr int MEMBER   1         The constexpr part creates a static expression as opposed to a static variable - and that behaves just like an extremely simple inline method definition  The approach proved a bit wobbly with C-string constexprs inside template classes  though

User · Answer

The problem comes because of an interesting clash of new C   features and what you re trying to do   First  let s take a look at the push back signature   void push back const T amp     It s expecting a reference to an object of type T   Under the old system of initialization  such a member exists   For example  the following code compiles just fine    include  lt vector gt   class Foo   public      static const int MEMBER      const int Foo  MEMBER   1    int main        std  vector lt int gt  v      v push back  Foo  MEMBER             undefined reference to  Foo  MEMBER      v push back   int  Foo  MEMBER       OK       return 0      This is because there is an actual object somewhere that has that value stored in it   If  however  you switch to the new method of specifying static const members  like you have above  Foo  MEMBER is no longer an object   It is a constant  somewhat akin to    define MEMBER 1   But without the headaches of a preprocessor macro  and with type safety    That means that the vector  which is expecting a reference  can t get one

User · Answer

Regarding the second question  push ref takes reference as a parameter  and you cannot have a reference to static const memeber of a class struct  Once you call static cast  a temporary variable is created  And a reference to this object can be passed  everything works just fine   Or at least my colleague who resolved this said so

User · Answer

You need to actually define the static member somewhere  after the class definition    Try this   class Foo                 const int Foo  MEMBER   int main                   That should get rid of the undefined reference

User · Answer

Regarding the second question  push ref takes reference as a parameter  and you cannot have a reference to static const memeber of a class struct  Once you call static cast  a temporary variable is created  And a reference to this object can be passed  everything works just fine   Or at least my colleague who resolved this said so

User · Answer

You need to actually define the static member somewhere  after the class definition    Try this   class Foo                 const int Foo  MEMBER   int main                   That should get rid of the undefined reference

User · Answer

The problem comes because of an interesting clash of new C   features and what you re trying to do   First  let s take a look at the push back signature   void push back const T amp     It s expecting a reference to an object of type T   Under the old system of initialization  such a member exists   For example  the following code compiles just fine    include  lt vector gt   class Foo   public      static const int MEMBER      const int Foo  MEMBER   1    int main        std  vector lt int gt  v      v push back  Foo  MEMBER             undefined reference to  Foo  MEMBER      v push back   int  Foo  MEMBER       OK       return 0      This is because there is an actual object somewhere that has that value stored in it   If  however  you switch to the new method of specifying static const members  like you have above  Foo  MEMBER is no longer an object   It is a constant  somewhat akin to    define MEMBER 1   But without the headaches of a preprocessor macro  and with type safety    That means that the vector  which is expecting a reference  can t get one

User · Answer

The problem comes because of an interesting clash of new C   features and what you re trying to do   First  let s take a look at the push back signature   void push back const T amp     It s expecting a reference to an object of type T   Under the old system of initialization  such a member exists   For example  the following code compiles just fine    include  lt vector gt   class Foo   public      static const int MEMBER      const int Foo  MEMBER   1    int main        std  vector lt int gt  v      v push back  Foo  MEMBER             undefined reference to  Foo  MEMBER      v push back   int  Foo  MEMBER       OK       return 0      This is because there is an actual object somewhere that has that value stored in it   If  however  you switch to the new method of specifying static const members  like you have above  Foo  MEMBER is no longer an object   It is a constant  somewhat akin to    define MEMBER 1   But without the headaches of a preprocessor macro  and with type safety    That means that the vector  which is expecting a reference  can t get one

[c++] Undefined reference to static class member

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