Does static constexpr variable inside a function make sense

Question

If I have a variable inside a function  say  a large array   does it make sense to declare it both static and constexpr  constexpr guarantees that the array is created at compile time  so would the static be useless   void f         static constexpr int x                   a few thousand elements               do something with the array     Is the static actually doing anything there in terms of generated code or semantics

User · Accepted Answer

The short answer is that not only is static useful  it is pretty well always going to be desired   First  note that static and constexpr are completely independent of each other  static defines the object s lifetime during execution  constexpr specifies that the object should be available during compilation  Compilation and execution are disjoint and discontiguous  both in time and space  So once the program is compiled  constexpr is no longer relevant   Every variable declared constexpr is implicitly const but const and static are almost orthogonal  except for the interaction with static const integers    The C   object model   sect 1 9  requires that all objects other than bit-fields occupy at least one byte of memory and have addresses  furthermore all such objects observable in a program at a given moment must have distinct addresses  paragraph 6   This does not quite require the compiler to create a new array on the stack for every invocation of a function with a local non-static const array  because the compiler could take refuge in the as-if principle provided it can prove that no other such object can be observed   That s not going to be easy to prove  unfortunately  unless the function is trivial  for example  it does not call any other function whose body is not visible within the translation unit  because arrays  more or less by definition  are addresses  So in most cases  the non-static const expr  array will have to be recreated on the stack at every invocation  which defeats the point of being able to compute it at compile time   On the other hand  a local static const object is shared by all observers  and furthermore may be initialized even if the function it is defined in is never called  So none of the above applies  and a compiler is free not only to generate only a single instance of it  it is free to generate a single instance of it in read-only storage   So you should definitely use static constexpr in your example   However  there is one case where you wouldn t want to use static constexpr  Unless a constexpr declared object is either ODR-used or declared static  the compiler is free to not include it at all  That s pretty useful  because it allows the use of compile-time temporary constexpr arrays without polluting the compiled program with unnecessary bytes  In that case  you would clearly not want to use static  since static is likely to force the object to exist at runtime

User · Answer

In addition to given answer  it s worth noting that compiler is not required to initialize constexpr variable at compile time  knowing that the difference between constexpr and static constexpr is that to use static constexpr you ensure the variable is initialized only once   Following code demonstrates how constexpr variable is initialized multiple times  with same value though   while static constexpr is surely initialized only once   In addition the code compares the advantage of constexpr against const in combination with static    include  lt iostream gt   include  lt string gt   include  lt cassert gt   include  lt sstream gt   const short const short   0  constexpr short constexpr short   0      print only last 3 address value numbers const short addr offset   3      This function will print name  value and address for given parameter void print properties std  string ref name  const short  param  short offset           determine initial size of strings     std  string title    value    address of        const size t ref size   ref name size        const size t title size   title size        assert title size  gt  ref size           create title  resize      title append ref name       title append   is         title append title size - ref size                extract last  offset  values from address     std  stringstream addr      addr  lt  lt  param      const std  string addr str   addr str        const size t addr size   addr str size        assert addr size - offset  gt  0           print title   ref value   address at offset     std  cout  lt  lt  title  lt  lt   param  lt  lt       lt  lt  addr str substr addr size - offset   lt  lt  std  endl        here we test initialization of const variable  runtime  void const value const short counter        static short temp   const short      const short const var     temp      print properties  const    amp const var  addr offset        if  counter          const value counter - 1         here we test initialization of static variable  runtime  void static value const short counter        static short temp   const short      static short static var     temp      print properties  static    amp static var  addr offset        if  counter          static value counter - 1         here we test initialization of static const variable  runtime  void static const value const short counter        static short temp   const short      static const short static var     temp      print properties  static const    amp static var  addr offset        if  counter          static const value counter - 1         here we test initialization of constexpr variable  compile time  void constexpr value const short counter        constexpr short constexpr var   constexpr short      print properties  constexpr    amp constexpr var  addr offset        if  counter          constexpr value counter - 1         here we test initialization of static constexpr variable  compile time  void static constexpr value const short counter        static constexpr short static constexpr var   constexpr short      print properties  static constexpr    amp static constexpr var  addr offset        if  counter          static constexpr value counter - 1         final test call this method from main   void test static const         constexpr short counter   2       const value counter       std  cout  lt  lt  std  endl       static value counter       std  cout  lt  lt  std  endl       static const value counter       std  cout  lt  lt  std  endl       constexpr value counter       std  cout  lt  lt  std  endl       static constexpr value counter       std  cout  lt  lt  std  endl      Possible program output   value   address of const is               1 564 value   address of const is               2 3D4 value   address of const is               3 244  value   address of static is              1 C58 value   address of static is              1 C58 value   address of static is              1 C58  value   address of static const is        1 C64 value   address of static const is        1 C64 value   address of static const is        1 C64  value   address of constexpr is           0 564 value   address of constexpr is           0 3D4 value   address of constexpr is           0 244  value   address of static constexpr is    0 EA0 value   address of static constexpr is    0 EA0 value   address of static constexpr is    0 EA0   As you can see yourself constexpr is initilized multiple times  address is not the same  while static keyword ensures that initialization is performed only once

[c++] Does static constexpr variable inside a function make sense?

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