Defining static const integer members in class definition

Question

My understanding is that C   allows static const members to be defined inside a class so long as it s an integer type   Why  then  does the following code give me a linker error    include  lt algorithm gt   include  lt iostream gt   class test   public      static const int N   10      int main         std  cout  lt  lt  test  N  lt  lt    n       std  min 9  test  N       The error I get is   test cpp   text 0x130   undefined reference to  test  N  collect2  ld returned 1 exit status   Interestingly  if I comment out the call to std  min  the code compiles and links just fine  even though test  N is also referenced on the previous line    Any idea as to what s going on   My compiler is gcc 4 4 on Linux

User · Accepted Answer

My understanding is that C   allows static const members to be defined inside a class so long as it s an integer type   You are sort of correct   You are allowed to initialize static const integrals in the class declaration but that is not a definition   http   publib boulder ibm com infocenter comphelp v8v101 index jsp topic  com ibm xlcpp8a doc language ref cplr038 htm  Interestingly  if I comment out the call to std  min  the code compiles and links just fine  even though test  N is also referenced on the previous line    Any idea as to what s going on   std  min takes its parameters by const reference   If it took them by value you d not have this problem but since you need a reference you also need a definition   Here s chapter verse   9 4 2 4 - If a static data member is of const integral or const enumeration type  its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression  5 19    In that case  the member can appear in integral constant expressions   The member shall still be defined in a namespace scope if it is used in the program and the namespace scope definition shall not contain an initializer   See Chu s answer for a possible workaround

User · Answer

Not just int s   But you can t define the value in the class declaration   If you have   class classname       public         static int const N      in the  h file then you must have   int const classname  N   10    in the  cpp file

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C   allows static const members to be defined inside a class   Nope  3 1   2 says      A declaration is a definition unless it declares a function without specifying the function s body  8 4   it contains the extern specifier  7 1 1  or a linkage-specification  7 5  and neither an initializer nor a functionbody  it declares a static data member in a class definition  9 4   it is a class name declaration  9 1   it is an opaque-enum-declaration  7 2   or it is a typedef declaration  7 1 3   a using-declaration  7 3 3   or a using-directive  7 3 4

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As of C  11 you can use   static constexpr int N   10   This theoretically still requires you to define the constant in a  cpp file  but as long as you don t take the address of N it is very unlikely that any compiler implementation will produce an error

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Here s another way to work around the problem   std  min 9  int test  N       I think Crazy Eddie s answer correctly describes why the problem exists

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Bjarne Stroustrup s example in his C   FAQ suggests you are correct  and only need a definition if you take the address     class AE              public      static const int c6   7      static const int c7   31      const int AE  c7       definition  int f         const int  p1    amp AE  c6        error  c6 not an lvalue     const int  p2    amp AE  c7        ok                He says  You can take the address of a static member if  and only if  it has an out-of-class definition    Which suggests it would work otherwise   Maybe your min function invokes addresses somehow behind the scenes

User · Answer

Another way to do this  for integer types anyway  is to define constants as enums in the class   class test   public      enum   N   10

[c++] Defining static const integer members in class definition

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