What does const static mean in C and C

Question

const static int foo   42    I saw this in some code here on StackOverflow and I couldn t figure out what it does  Then I saw some confused answers on other forums  My best guess is that it s used in C to hide the constant foo from other modules  Is this correct  If so  why would anyone use it in a C   context where you can just make it private

User · Answer

In C      static const int foo   42    is the preferred way to define  amp  use constants  I e  use this rather than    define foo 42   because it doesn t subvert the type-safety system

User · Answer

It s missing an  int    It should be   const static int foo   42    In C and C    it declares an integer constant with local file scope of value 42     Why 42   If you don t already know  and it s hard to believe you don t   it s a refernce to the Answer to Life  the Universe  and Everything

User · Answer

A lot of people gave the basic answer but nobody pointed out that in C   const defaults to static at namespace level  and some gave wrong information   See the C  98 standard section 3 5 3   First some background   Translation unit  A source file after the pre-processor  recursively  included all its include files   Static linkage  A symbol is only available within its translation unit   External linkage  A symbol is available from other translation units   At namespace level  This includes the global namespace aka global variables    static const int sci   0     sci is explicitly static const int ci   1             ci is implicitly static extern const int eci   2     eci is explicitly extern extern int ei   3            ei is explicitly extern int i   4                    i is implicitly extern static int si   5            si is explicitly static   At function level  static means the value is maintained between function calls  The semantics of function static variables is similar to global variables in that they reside in the program s data-segment  and not the stack or the heap   see this question for more details about static variables  lifetime   At class level  static means the value is shared between all instances of the class and const means it doesn t change

User · Answer

In C      static const int foo   42    is the preferred way to define  amp  use constants  I e  use this rather than    define foo 42   because it doesn t subvert the type-safety system

User · Answer

According to C99 GNU99 specification    static   is storage-class specifier objects of file level scope by default has external linkage objects of file level scope with static specifier has internal linkage  const   is type-qualifier  is a part of type  keyword applied to immediate left instance - i e     MyObj const   myVar  - unqualified pointer to const qualified object type MyObj   const myVar  - const qualified pointer to unqualified object type  Leftmost usage - applied to the object type  not variable   const MyObj   myVar  - unqualified pointer to const qualified object type     THUS   static NSString   const myVar  - constant pointer to immutable string with internal linkage   Absence of the static keyword will make variable name global and might lead to name conflicts within the application

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To all the great answers  I want to add a small detail   If You write plugins  e g  DLLs or  so libraries to be loaded by a CAD system   then static is a life saver that avoids name collisions like this one    The CAD system loads a plugin A  which has a  const int foo   42   in it  The system loads a plugin B  which has  const int foo   23   in it  As a result  plugin B will use the value 42 for foo  because the plugin loader will realize  that there is already a  foo  with external linkage    Even worse  Step 3 may behave differently depending on compiler optimization  plugin load mechanism  etc   I had this issue once with two helper functions  same name  different behaviour  in two plugins  Declaring them static solved the problem

User · Answer

Making it private would still mean it appears in the header  I tend to use  the weakest  way that works  See this classic article by Scott Meyers  http   www ddj com cpp 184401197  it s about functions  but can be applied here as well

User · Answer

According to C99 GNU99 specification    static   is storage-class specifier objects of file level scope by default has external linkage objects of file level scope with static specifier has internal linkage  const   is type-qualifier  is a part of type  keyword applied to immediate left instance - i e     MyObj const   myVar  - unqualified pointer to const qualified object type MyObj   const myVar  - const qualified pointer to unqualified object type  Leftmost usage - applied to the object type  not variable   const MyObj   myVar  - unqualified pointer to const qualified object type     THUS   static NSString   const myVar  - constant pointer to immutable string with internal linkage   Absence of the static keyword will make variable name global and might lead to name conflicts within the application

User · Answer

This ia s global constant visible accessible only in the compilation module   cpp file   BTW using static for this purpose is deprecated  Better use an anonymous namespace and an enum   namespace     enum          foo   42

User · Answer

Making it private would still mean it appears in the header  I tend to use  the weakest  way that works  See this classic article by Scott Meyers  http   www ddj com cpp 184401197  it s about functions  but can be applied here as well

User · Answer

It s a small space optimization   When you say  const int foo   42    You re not defining a constant  but creating a read-only variable  The compiler is smart enough to use 42 whenever it sees foo  but it will also allocate space in the initialized data area for it  This is done because  as defined  foo has external linkage  Another compilation unit can say   extern const int foo   To get access to its value  That s not a good practice since that compilation unit has no idea what the value of foo is  It just knows it s a const int and has to reload the value from memory whenever it is used   Now  by declaring that it is static   static const int foo   42    The compiler can do its usual optimization  but it can also say  hey  nobody outside this compilation unit can see foo and I know it s always 42 so there is no need to allocate any space for it    I should also note that in C    the preferred way to prevent names from escaping the current compilation unit is to use an anonymous namespace   namespace       const int foo   42     same as static definition above

User · Answer

Yes  it hides a variable in a module from other modules  In C    I use it when I don t want need to change a  h file that will trigger an unnecessary rebuild of other files  Also  I put the static first   static const int foo   42    Also  depending on its use  the compiler won t even allocate storage for it and simply  inline  the value where it s used  Without the static  the compiler can t assume it s not being used elsewhere and can t inline

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A lot of people gave the basic answer but nobody pointed out that in C   const defaults to static at namespace level  and some gave wrong information   See the C  98 standard section 3 5 3   First some background   Translation unit  A source file after the pre-processor  recursively  included all its include files   Static linkage  A symbol is only available within its translation unit   External linkage  A symbol is available from other translation units   At namespace level  This includes the global namespace aka global variables    static const int sci   0     sci is explicitly static const int ci   1             ci is implicitly static extern const int eci   2     eci is explicitly extern extern int ei   3            ei is explicitly extern int i   4                    i is implicitly extern static int si   5            si is explicitly static   At function level  static means the value is maintained between function calls  The semantics of function static variables is similar to global variables in that they reside in the program s data-segment  and not the stack or the heap   see this question for more details about static variables  lifetime   At class level  static means the value is shared between all instances of the class and const means it doesn t change

User · Answer

Yes  it hides a variable in a module from other modules  In C    I use it when I don t want need to change a  h file that will trigger an unnecessary rebuild of other files  Also  I put the static first   static const int foo   42    Also  depending on its use  the compiler won t even allocate storage for it and simply  inline  the value where it s used  Without the static  the compiler can t assume it s not being used elsewhere and can t inline

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C  17 inline variables  If you Googled  C   const static   then this is very likely what you really want to use are C  17 inline variables   This awesome C  17 feature allow us to    conveniently use just a single memory address for each constant store it as a constexpr  How to declare constexpr extern  do it in a single line from one header   main cpp   include  lt cassert gt    include  notmain hpp   int main            Both files see the same memory address      assert  amp notmain i    notmain func         assert notmain i    42       notmain hpp   ifndef NOTMAIN HPP  define NOTMAIN HPP  inline constexpr int notmain i   42   const int  notmain func      endif   notmain cpp   include  notmain hpp   const int  notmain func         return  amp notmain i      Compile and run   g   -c -o notmain o -std c  17 -Wall -Wextra -pedantic notmain cpp g   -c -o main o -std c  17 -Wall -Wextra -pedantic main cpp g   -o main -std c  17 -Wall -Wextra -pedantic main o notmain o   main   GitHub upstream   See also  How do inline variables work   C   standard on inline variables  The C   standard guarantees that the addresses will be the same  C  17 N4659 standard draft  10 1 6  The inline specifier       6 An inline function or variable with external linkage shall have the same address in all translation units    cppreference https   en cppreference com w cpp language inline explains that if static is not given  then it has external linkage   GCC inline variable implementation  We can observe how it is implemented with   nm main o notmain o   which contains   main o                   U  GLOBAL OFFSET TABLE                   U  Z12notmain funcv 0000000000000028 r  ZZ4mainE19  PRETTY FUNCTION                    U   assert fail 0000000000000000 T main 0000000000000000 u notmain i  notmain o  0000000000000000 T  Z12notmain funcv 0000000000000000 u notmain i   and man nm says about u       u  The symbol is a unique global symbol   This is a GNU extension to the standard set of ELF symbol bindings   For such a symbol the dynamic linker will make sure that in the entire process                  there is just one symbol with this name and type in use    so we see that there is a dedicated ELF extension for this   Pre-C   17  extern const  Before C   17  and in C  we can achieve a very similar effect with an extern const  which will lead to a single memory location being used   The downsides over inline are    it is not possible to make the variable constexpr with this technique  only inline allows that  How to declare constexpr extern  it is less elegant as you have to declare and define the variable separately in the header and cpp file   main cpp   include  lt cassert gt    include  notmain hpp   int main            Both files see the same memory address      assert  amp notmain i    notmain func         assert notmain i    42       notmain cpp   include  notmain hpp   const int notmain i   42   const int  notmain func         return  amp notmain i      notmain hpp   ifndef NOTMAIN HPP  define NOTMAIN HPP  extern const int notmain i   const int  notmain func      endif   GitHub upstream   Pre-C  17 header only alternatives  These are not as good as the extern solution  but they work and only take up a single memory location   A constexpr function  because constexpr implies inline and inline allows  forces  the definition to appear on every translation unit   constexpr int shared inline constexpr     return 42      and I bet that any decent compiler will inline the call   You can also use a const or constexpr static variable as in    include  lt iostream gt   struct MyClass       static constexpr int i   42      int main         std  cout  lt  lt  MyClass  i  lt  lt  std  endl         undefined reference to  MyClass  i        std  cout  lt  lt   amp MyClass  i  lt  lt  std  endl      but you can t do things like taking its address  or else it becomes odr-used  see also  Defining constexpr static data members  C  In C the situation is the same as C   pre C   17  I ve uploaded an example at  What does  quot static quot  mean in C   The only difference is that in C    const implies static for globals  but it does not in C  C   semantics of  static const  vs  const   Any way to fully inline it   TODO  is there any way to fully inline the variable  without using any memory at all   Much like what the preprocessor does   This would require somehow    forbidding or detecting if the address of the variable is taken add that information to the ELF object files  and let LTO optimize it up   Related    C  11 enum with class members and constexpr link-time optimization   Tested in Ubuntu 18 10  GCC 8 2 0

User · Answer

It s missing an  int    It should be   const static int foo   42    In C and C    it declares an integer constant with local file scope of value 42     Why 42   If you don t already know  and it s hard to believe you don t   it s a refernce to the Answer to Life  the Universe  and Everything

User · Answer

It s a small space optimization   When you say  const int foo   42    You re not defining a constant  but creating a read-only variable  The compiler is smart enough to use 42 whenever it sees foo  but it will also allocate space in the initialized data area for it  This is done because  as defined  foo has external linkage  Another compilation unit can say   extern const int foo   To get access to its value  That s not a good practice since that compilation unit has no idea what the value of foo is  It just knows it s a const int and has to reload the value from memory whenever it is used   Now  by declaring that it is static   static const int foo   42    The compiler can do its usual optimization  but it can also say  hey  nobody outside this compilation unit can see foo and I know it s always 42 so there is no need to allocate any space for it    I should also note that in C    the preferred way to prevent names from escaping the current compilation unit is to use an anonymous namespace   namespace       const int foo   42     same as static definition above

User · Answer

To all the great answers  I want to add a small detail   If You write plugins  e g  DLLs or  so libraries to be loaded by a CAD system   then static is a life saver that avoids name collisions like this one    The CAD system loads a plugin A  which has a  const int foo   42   in it  The system loads a plugin B  which has  const int foo   23   in it  As a result  plugin B will use the value 42 for foo  because the plugin loader will realize  that there is already a  foo  with external linkage    Even worse  Step 3 may behave differently depending on compiler optimization  plugin load mechanism  etc   I had this issue once with two helper functions  same name  different behaviour  in two plugins  Declaring them static solved the problem

User · Answer

It has uses in both C and C     As you guessed  the static part limits its scope to that compilation unit  It also provides for static initialization  const just tells the compiler to not let anybody modify it  This variable is either put in the data or bss segment depending on the architecture  and might be in memory marked read-only   All that is how C treats these variables  or how C   treats namespace variables   In C    a member marked static is shared by all instances of a given class  Whether it s private or not doesn t affect the fact that one variable is shared by multiple instances  Having const on there will warn you if any code would try to modify that   If it was strictly private  then each instance of the class would get its own version  optimizer notwithstanding

User · Answer

That line of code can actually appear in several different contexts and alghough it behaves approximately the same  there are small differences   Namespace Scope     foo h static const int i   0     i  will be visible in every translation unit that includes the header   However  unless you actually use the address of the object  for example    amp i    I m pretty sure that the compiler will treat  i  simply as a type safe 0   Where two more more translation units take the   amp i  then the address will be different for each translation unit      foo cc static const int i   0     i  has internal linkage  and so cannot be referred to from outside of this translation unit   However  again unless you use its address it will most likely be treated as a type-safe 0   One thing worth pointing out  is that the following declaration   const int i1   0    is exactly the same as static const int i   0   A variable in a namespace declared with const and not explicitly declared with extern is implicitly static   If you think about this  it was the intention of the C   committee to allow const variables to be declared in header files without always needing the static keyword to avoid breaking the ODR   Class Scope  class A   public    static const int i   0       In the above example  the standard explicitly specifies that  i  does not need to be defined if its address is not required  In other words if you only use  i  as a type-safe 0 then the compiler will not define it   One difference between the class and namespace versions is that the address of  i   if used in two ore more translation units  will be the same for the class member   Where the address is used  you must have a definition for it      a h class A   public    static const int i   0         a cc  include  a h  const int A  i                Definition so that we can take the address

User · Answer

To all the great answers  I want to add a small detail   If You write plugins  e g  DLLs or  so libraries to be loaded by a CAD system   then static is a life saver that avoids name collisions like this one    The CAD system loads a plugin A  which has a  const int foo   42   in it  The system loads a plugin B  which has  const int foo   23   in it  As a result  plugin B will use the value 42 for foo  because the plugin loader will realize  that there is already a  foo  with external linkage    Even worse  Step 3 may behave differently depending on compiler optimization  plugin load mechanism  etc   I had this issue once with two helper functions  same name  different behaviour  in two plugins  Declaring them static solved the problem

User · Answer

This ia s global constant visible accessible only in the compilation module   cpp file   BTW using static for this purpose is deprecated  Better use an anonymous namespace and an enum   namespace     enum          foo   42

User · Answer

A lot of people gave the basic answer but nobody pointed out that in C   const defaults to static at namespace level  and some gave wrong information   See the C  98 standard section 3 5 3   First some background   Translation unit  A source file after the pre-processor  recursively  included all its include files   Static linkage  A symbol is only available within its translation unit   External linkage  A symbol is available from other translation units   At namespace level  This includes the global namespace aka global variables    static const int sci   0     sci is explicitly static const int ci   1             ci is implicitly static extern const int eci   2     eci is explicitly extern extern int ei   3            ei is explicitly extern int i   4                    i is implicitly extern static int si   5            si is explicitly static   At function level  static means the value is maintained between function calls  The semantics of function static variables is similar to global variables in that they reside in the program s data-segment  and not the stack or the heap   see this question for more details about static variables  lifetime   At class level  static means the value is shared between all instances of the class and const means it doesn t change

User · Answer

In C      static const int foo   42    is the preferred way to define  amp  use constants  I e  use this rather than    define foo 42   because it doesn t subvert the type-safety system

User · Answer

It s missing an  int    It should be   const static int foo   42    In C and C    it declares an integer constant with local file scope of value 42     Why 42   If you don t already know  and it s hard to believe you don t   it s a refernce to the Answer to Life  the Universe  and Everything

User · Answer

That line of code can actually appear in several different contexts and alghough it behaves approximately the same  there are small differences   Namespace Scope     foo h static const int i   0     i  will be visible in every translation unit that includes the header   However  unless you actually use the address of the object  for example    amp i    I m pretty sure that the compiler will treat  i  simply as a type safe 0   Where two more more translation units take the   amp i  then the address will be different for each translation unit      foo cc static const int i   0     i  has internal linkage  and so cannot be referred to from outside of this translation unit   However  again unless you use its address it will most likely be treated as a type-safe 0   One thing worth pointing out  is that the following declaration   const int i1   0    is exactly the same as static const int i   0   A variable in a namespace declared with const and not explicitly declared with extern is implicitly static   If you think about this  it was the intention of the C   committee to allow const variables to be declared in header files without always needing the static keyword to avoid breaking the ODR   Class Scope  class A   public    static const int i   0       In the above example  the standard explicitly specifies that  i  does not need to be defined if its address is not required  In other words if you only use  i  as a type-safe 0 then the compiler will not define it   One difference between the class and namespace versions is that the address of  i   if used in two ore more translation units  will be the same for the class member   Where the address is used  you must have a definition for it      a h class A   public    static const int i   0         a cc  include  a h  const int A  i                Definition so that we can take the address

User · Answer

It has uses in both C and C     As you guessed  the static part limits its scope to that compilation unit  It also provides for static initialization  const just tells the compiler to not let anybody modify it  This variable is either put in the data or bss segment depending on the architecture  and might be in memory marked read-only   All that is how C treats these variables  or how C   treats namespace variables   In C    a member marked static is shared by all instances of a given class  Whether it s private or not doesn t affect the fact that one variable is shared by multiple instances  Having const on there will warn you if any code would try to modify that   If it was strictly private  then each instance of the class would get its own version  optimizer notwithstanding

User · Answer

It has uses in both C and C     As you guessed  the static part limits its scope to that compilation unit  It also provides for static initialization  const just tells the compiler to not let anybody modify it  This variable is either put in the data or bss segment depending on the architecture  and might be in memory marked read-only   All that is how C treats these variables  or how C   treats namespace variables   In C    a member marked static is shared by all instances of a given class  Whether it s private or not doesn t affect the fact that one variable is shared by multiple instances  Having const on there will warn you if any code would try to modify that   If it was strictly private  then each instance of the class would get its own version  optimizer notwithstanding

User · Answer

It s missing an  int    It should be   const static int foo   42    In C and C    it declares an integer constant with local file scope of value 42     Why 42   If you don t already know  and it s hard to believe you don t   it s a refernce to the Answer to Life  the Universe  and Everything

User · Answer

Yes  it hides a variable in a module from other modules  In C    I use it when I don t want need to change a  h file that will trigger an unnecessary rebuild of other files  Also  I put the static first   static const int foo   42    Also  depending on its use  the compiler won t even allocate storage for it and simply  inline  the value where it s used  Without the static  the compiler can t assume it s not being used elsewhere and can t inline

User · Answer

That line of code can actually appear in several different contexts and alghough it behaves approximately the same  there are small differences   Namespace Scope     foo h static const int i   0     i  will be visible in every translation unit that includes the header   However  unless you actually use the address of the object  for example    amp i    I m pretty sure that the compiler will treat  i  simply as a type safe 0   Where two more more translation units take the   amp i  then the address will be different for each translation unit      foo cc static const int i   0     i  has internal linkage  and so cannot be referred to from outside of this translation unit   However  again unless you use its address it will most likely be treated as a type-safe 0   One thing worth pointing out  is that the following declaration   const int i1   0    is exactly the same as static const int i   0   A variable in a namespace declared with const and not explicitly declared with extern is implicitly static   If you think about this  it was the intention of the C   committee to allow const variables to be declared in header files without always needing the static keyword to avoid breaking the ODR   Class Scope  class A   public    static const int i   0       In the above example  the standard explicitly specifies that  i  does not need to be defined if its address is not required  In other words if you only use  i  as a type-safe 0 then the compiler will not define it   One difference between the class and namespace versions is that the address of  i   if used in two ore more translation units  will be the same for the class member   Where the address is used  you must have a definition for it      a h class A   public    static const int i   0         a cc  include  a h  const int A  i                Definition so that we can take the address

User · Answer

In C      static const int foo   42    is the preferred way to define  amp  use constants  I e  use this rather than    define foo 42   because it doesn t subvert the type-safety system

User · Answer

It s a small space optimization   When you say  const int foo   42    You re not defining a constant  but creating a read-only variable  The compiler is smart enough to use 42 whenever it sees foo  but it will also allocate space in the initialized data area for it  This is done because  as defined  foo has external linkage  Another compilation unit can say   extern const int foo   To get access to its value  That s not a good practice since that compilation unit has no idea what the value of foo is  It just knows it s a const int and has to reload the value from memory whenever it is used   Now  by declaring that it is static   static const int foo   42    The compiler can do its usual optimization  but it can also say  hey  nobody outside this compilation unit can see foo and I know it s always 42 so there is no need to allocate any space for it    I should also note that in C    the preferred way to prevent names from escaping the current compilation unit is to use an anonymous namespace   namespace       const int foo   42     same as static definition above

User · Answer

Yes  it hides a variable in a module from other modules  In C    I use it when I don t want need to change a  h file that will trigger an unnecessary rebuild of other files  Also  I put the static first   static const int foo   42    Also  depending on its use  the compiler won t even allocate storage for it and simply  inline  the value where it s used  Without the static  the compiler can t assume it s not being used elsewhere and can t inline

User · Answer

This ia s global constant visible accessible only in the compilation module   cpp file   BTW using static for this purpose is deprecated  Better use an anonymous namespace and an enum   namespace     enum          foo   42

User · Answer

It has uses in both C and C     As you guessed  the static part limits its scope to that compilation unit  It also provides for static initialization  const just tells the compiler to not let anybody modify it  This variable is either put in the data or bss segment depending on the architecture  and might be in memory marked read-only   All that is how C treats these variables  or how C   treats namespace variables   In C    a member marked static is shared by all instances of a given class  Whether it s private or not doesn t affect the fact that one variable is shared by multiple instances  Having const on there will warn you if any code would try to modify that   If it was strictly private  then each instance of the class would get its own version  optimizer notwithstanding

User · Answer

Making it private would still mean it appears in the header  I tend to use  the weakest  way that works  See this classic article by Scott Meyers  http   www ddj com cpp 184401197  it s about functions  but can be applied here as well

User · Answer

It s a small space optimization   When you say  const int foo   42    You re not defining a constant  but creating a read-only variable  The compiler is smart enough to use 42 whenever it sees foo  but it will also allocate space in the initialized data area for it  This is done because  as defined  foo has external linkage  Another compilation unit can say   extern const int foo   To get access to its value  That s not a good practice since that compilation unit has no idea what the value of foo is  It just knows it s a const int and has to reload the value from memory whenever it is used   Now  by declaring that it is static   static const int foo   42    The compiler can do its usual optimization  but it can also say  hey  nobody outside this compilation unit can see foo and I know it s always 42 so there is no need to allocate any space for it    I should also note that in C    the preferred way to prevent names from escaping the current compilation unit is to use an anonymous namespace   namespace       const int foo   42     same as static definition above

User · Answer

This ia s global constant visible accessible only in the compilation module   cpp file   BTW using static for this purpose is deprecated  Better use an anonymous namespace and an enum   namespace     enum          foo   42

User · Answer

Making it private would still mean it appears in the header  I tend to use  the weakest  way that works  See this classic article by Scott Meyers  http   www ddj com cpp 184401197  it s about functions  but can be applied here as well

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C  17 inline variables  If you Googled  C   const static   then this is very likely what you really want to use are C  17 inline variables   This awesome C  17 feature allow us to    conveniently use just a single memory address for each constant store it as a constexpr  How to declare constexpr extern  do it in a single line from one header   main cpp   include  lt cassert gt    include  notmain hpp   int main            Both files see the same memory address      assert  amp notmain i    notmain func         assert notmain i    42       notmain hpp   ifndef NOTMAIN HPP  define NOTMAIN HPP  inline constexpr int notmain i   42   const int  notmain func      endif   notmain cpp   include  notmain hpp   const int  notmain func         return  amp notmain i      Compile and run   g   -c -o notmain o -std c  17 -Wall -Wextra -pedantic notmain cpp g   -c -o main o -std c  17 -Wall -Wextra -pedantic main cpp g   -o main -std c  17 -Wall -Wextra -pedantic main o notmain o   main   GitHub upstream   See also  How do inline variables work   C   standard on inline variables  The C   standard guarantees that the addresses will be the same  C  17 N4659 standard draft  10 1 6  The inline specifier       6 An inline function or variable with external linkage shall have the same address in all translation units    cppreference https   en cppreference com w cpp language inline explains that if static is not given  then it has external linkage   GCC inline variable implementation  We can observe how it is implemented with   nm main o notmain o   which contains   main o                   U  GLOBAL OFFSET TABLE                   U  Z12notmain funcv 0000000000000028 r  ZZ4mainE19  PRETTY FUNCTION                    U   assert fail 0000000000000000 T main 0000000000000000 u notmain i  notmain o  0000000000000000 T  Z12notmain funcv 0000000000000000 u notmain i   and man nm says about u       u  The symbol is a unique global symbol   This is a GNU extension to the standard set of ELF symbol bindings   For such a symbol the dynamic linker will make sure that in the entire process                  there is just one symbol with this name and type in use    so we see that there is a dedicated ELF extension for this   Pre-C   17  extern const  Before C   17  and in C  we can achieve a very similar effect with an extern const  which will lead to a single memory location being used   The downsides over inline are    it is not possible to make the variable constexpr with this technique  only inline allows that  How to declare constexpr extern  it is less elegant as you have to declare and define the variable separately in the header and cpp file   main cpp   include  lt cassert gt    include  notmain hpp   int main            Both files see the same memory address      assert  amp notmain i    notmain func         assert notmain i    42       notmain cpp   include  notmain hpp   const int notmain i   42   const int  notmain func         return  amp notmain i      notmain hpp   ifndef NOTMAIN HPP  define NOTMAIN HPP  extern const int notmain i   const int  notmain func      endif   GitHub upstream   Pre-C  17 header only alternatives  These are not as good as the extern solution  but they work and only take up a single memory location   A constexpr function  because constexpr implies inline and inline allows  forces  the definition to appear on every translation unit   constexpr int shared inline constexpr     return 42      and I bet that any decent compiler will inline the call   You can also use a const or constexpr static variable as in    include  lt iostream gt   struct MyClass       static constexpr int i   42      int main         std  cout  lt  lt  MyClass  i  lt  lt  std  endl         undefined reference to  MyClass  i        std  cout  lt  lt   amp MyClass  i  lt  lt  std  endl      but you can t do things like taking its address  or else it becomes odr-used  see also  Defining constexpr static data members  C  In C the situation is the same as C   pre C   17  I ve uploaded an example at  What does  quot static quot  mean in C   The only difference is that in C    const implies static for globals  but it does not in C  C   semantics of  static const  vs  const   Any way to fully inline it   TODO  is there any way to fully inline the variable  without using any memory at all   Much like what the preprocessor does   This would require somehow    forbidding or detecting if the address of the variable is taken add that information to the ELF object files  and let LTO optimize it up   Related    C  11 enum with class members and constexpr link-time optimization   Tested in Ubuntu 18 10  GCC 8 2 0

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A lot of people gave the basic answer but nobody pointed out that in C   const defaults to static at namespace level  and some gave wrong information   See the C  98 standard section 3 5 3   First some background   Translation unit  A source file after the pre-processor  recursively  included all its include files   Static linkage  A symbol is only available within its translation unit   External linkage  A symbol is available from other translation units   At namespace level  This includes the global namespace aka global variables    static const int sci   0     sci is explicitly static const int ci   1             ci is implicitly static extern const int eci   2     eci is explicitly extern extern int ei   3            ei is explicitly extern int i   4                    i is implicitly extern static int si   5            si is explicitly static   At function level  static means the value is maintained between function calls  The semantics of function static variables is similar to global variables in that they reside in the program s data-segment  and not the stack or the heap   see this question for more details about static variables  lifetime   At class level  static means the value is shared between all instances of the class and const means it doesn t change

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That line of code can actually appear in several different contexts and alghough it behaves approximately the same  there are small differences   Namespace Scope     foo h static const int i   0     i  will be visible in every translation unit that includes the header   However  unless you actually use the address of the object  for example    amp i    I m pretty sure that the compiler will treat  i  simply as a type safe 0   Where two more more translation units take the   amp i  then the address will be different for each translation unit      foo cc static const int i   0     i  has internal linkage  and so cannot be referred to from outside of this translation unit   However  again unless you use its address it will most likely be treated as a type-safe 0   One thing worth pointing out  is that the following declaration   const int i1   0    is exactly the same as static const int i   0   A variable in a namespace declared with const and not explicitly declared with extern is implicitly static   If you think about this  it was the intention of the C   committee to allow const variables to be declared in header files without always needing the static keyword to avoid breaking the ODR   Class Scope  class A   public    static const int i   0       In the above example  the standard explicitly specifies that  i  does not need to be defined if its address is not required  In other words if you only use  i  as a type-safe 0 then the compiler will not define it   One difference between the class and namespace versions is that the address of  i   if used in two ore more translation units  will be the same for the class member   Where the address is used  you must have a definition for it      a h class A   public    static const int i   0         a cc  include  a h  const int A  i                Definition so that we can take the address

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To all the great answers  I want to add a small detail   If You write plugins  e g  DLLs or  so libraries to be loaded by a CAD system   then static is a life saver that avoids name collisions like this one    The CAD system loads a plugin A  which has a  const int foo   42   in it  The system loads a plugin B  which has  const int foo   23   in it  As a result  plugin B will use the value 42 for foo  because the plugin loader will realize  that there is already a  foo  with external linkage    Even worse  Step 3 may behave differently depending on compiler optimization  plugin load mechanism  etc   I had this issue once with two helper functions  same name  different behaviour  in two plugins  Declaring them static solved the problem

[c++] What does 'const static' mean in C and C++?

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