When should you use constexpr capability in C 11

Question

It seems to me that having a  function that always returns 5  is breaking or diluting the meaning of  calling a function    There must be a reason  or a need for this capability or it wouldn t be in C  11   Why is it there       preprocessor   define MEANING OF LIFE 42     constants  const int MeaningOfLife   42      constexpr-function  constexpr int MeaningOfLife      return 42      It seems to me that if I wrote a function that return a literal value  and I came up to a code-review  someone would tell me  I should then  declare a constant value instead of writing return 5

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Take std  numeric limits lt T gt   max    for whatever reason  this is a method  constexpr would be beneficial here   Another example  you want to declare a C-array  or a std  array  that is as big as another array  The way to do this at the moment is like so   int x 10   int y sizeof x   sizeof x 0      But wouldn   t it be better to be able to write   int y size of x      Thanks to constexpr  you can   template  lt typename T  size t N gt  constexpr size t size of T   amp   N         return N

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Introduction  constexpr was not introduced as a way to tell the implementation that something can be evaluated in a context which requires a constant-expression  conforming implementations has been able to prove this prior to C  11   Something an implementation cannot prove is the intent of a certain piece of code    What is it that the developer want to express with this entity  Should we blindly allow code to be used in a constant-expression  just because it happens to work      What would the world be without constexpr   Let s say you are developing a library and realize that you want to be able to calculate the sum of every integer in the interval  0 N    int f  int n      return n  gt  0   n   f  n-1    n      The lack of intent  A compiler can easily prove that the above function is callable in a constant-expression if the argument passed is known during translation  but you have not declared this as an intent - it just happened to be the case   Now someone else comes along  reads your function  does the same analysis as the compiler   Oh  this function is usable in a constant-expression    and writes the following piece of code   T arr f 10       freakin  magic   The optimization  You  as an  awesome  library developer  decide that f should cache the result when being invoked  who would want to calculate the same set of values over and over   int func  int n       static std  map lt int  int gt   cached     if   cached find  n      cached end           cached n    n  gt  0   n   func  n-1    n     return  cached n       The result  By introducing your silly optimization  you just broke every usage of your function that happened to be in a context where a constant-expression was required   You never promised that the function was usable in a constant-expression  and without constexpr there would be no way of providing such promise      So  why do we need constexpr   The primary usage of constexpr is to declare intent   If an entity isn t marked as constexpr - it was never intended to be used in a constant-expression  and even if it is  we rely on the compiler to diagnose such context  because it disregards our intent

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Your basic example serves he same argument as that of constants themselves  Why use  static const int x   5  int arr x     over  int arr 5     Because it s way more maintainable  Using constexpr is much  much faster to write and read than existing metaprogramming techniques

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From Stroustrup s speech at  Going Native 2012    template lt int M  int K  int S gt  struct Unit      a unit in the MKS system        enum   m M  kg K  s S        template lt typename Unit gt     a magnitude with a unit  struct Value          double val       the magnitude         explicit Value double d    val d        construct a Value from a double      using Speed   Value lt Unit lt 1 0 -1 gt  gt       meters second type using Acceleration   Value lt Unit lt 1 0 -2 gt  gt       meters second second type using Second   Unit lt 0 0 1 gt       unit  sec using Second2   Unit lt 0 0 2 gt      unit  second second   constexpr Value lt Second gt  operator   s long double d        a f-p literal suffixed by    s        return Value lt Second gt   d           constexpr Value lt Second2 gt  operator   s2 long double d       a f-p literal  suffixed by    s2         return Value lt Second2 gt   d       Speed sp1   100m 9 8s     very fast for a human  Speed sp2   100m 9 8s2     error  m s2 is acceleration    Speed sp3   100 9 8s     error  speed is m s and 100 has no unit   Acceleration acc   sp1 0 5s     too fast for a human

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Have just started switching over a project to c  11 and came across a perfectly good situation for constexpr which cleans up alternative methods of performing the same operation   The key point here is that you can only place the function into the array size declaration when it is declared constexpr   There are a number of situations where I can see this being very useful moving forward with the area of code that I am involved in   constexpr size t GetMaxIPV4StringLength         return   sizeof   255 255 255 255          void SomeIPFunction         char szIPAddress  GetMaxIPV4StringLength          SomeIPGetFunction  szIPAddress

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constexpr functions are really nice and a great addition to c     However  you are right in that most of the problems it solves can be inelegantly worked around with macros   However  one of the uses of constexpr has no C  03 equivalent  typed constants      This is bad for obvious reasons   define ONE 1      This works most of the time but isn t fully typed  enum   TWO   2        This doesn t compile enum   pi   3 1415f        This is a file local lvalue masquerading as a global    rvalue   It works most of the time   But May subtly break    with static initialization order issues  eg pi   0 for some files  static const float pi   3 1415f      This is a true constant rvalue constexpr float pi   3 1415f      Haven t you always wanted to do this     constexpr std  string awesome    oh yeah         UPDATE  sadly std  string lacks a constexpr ctor  struct A      static const int four   4     static const int five   5     constexpr int six   6      int main         amp A  four     linker error     amp A  six     compiler error        EXTREMELY subtle linker error    int i   rand    A  four  A  five        It not safe use static const class variables with the ternary operator       Adding this to any cpp file would fix the linker error    int A  four    int A  six

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It s useful for something like     constants  const int MeaningOfLife   42      constexpr-function  constexpr int MeaningOfLife      return 42     int some arr MeaningOfLife       Tie this in with a traits class or the like and it becomes quite useful

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All of the other answers are great  I just want to give a cool example of one thing you can do with constexpr that is amazing  See-Phit  https   github com rep-movsd see-phit blob master seephit h  is a compile time HTML parser and template engine  This means you can put HTML in and get out a tree that is able to be manipulated  Having the parsing done at compile time can give you a bit of extra performance   From the github page example    include  lt iostream gt   include  seephit h  using namespace std     int main       constexpr auto parser       R         lt span  gt       lt p  color  red  height  10   gt   name   is a   profession   in   city   lt  p   gt       lt  span gt          html     spt  tree spt tree parser      spt  template dict dct    dct  name      Mary     dct  profession      doctor     dct  city      London      spt tree root render cerr  dct     cerr  lt  lt  endl     dct  city      New York     dct  name      John     dct  profession      janitor      spt tree root render cerr  dct     cerr  lt  lt  endl

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Another use  not yet mentioned  is constexpr constructors  This allows creating compile time constants which don t have to be initialized during runtime   const std  complex lt double gt  meaning of imagination 0  42      Pair that with user defined literals and you have full support for literal user defined classes   3 14D   42 i

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When to use constexpr    whenever there is a compile time constant

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It can enable some new optimisations   const traditionally is a hint for the type system  and cannot be used for optimisation  e g  a const member function can const cast and modify the object anyway  legally  so const cannot be trusted for optimisation    constexpr means the expression really is constant  provided the inputs to the function are const   Consider   class MyInterface   public      int GetNumber   const   0       If this is exposed in some other module  the compiler can t trust that GetNumber   won t return different values each time it s called - even consecutively with no non-const calls in between - because const could have been cast away in the implementation    Obviously any programmer who did this ought to be shot  but the language permits it  therefore the compiler must abide by the rules    Adding constexpr   class MyInterface   public      constexpr int GetNumber   const   0       The compiler can now apply an optimisation where the return value of GetNumber   is cached and eliminate additional calls to GetNumber    because constexpr is a stronger guarantee that the return value won t change

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Suppose it does something a little more complicated  constexpr int MeaningOfLife   int a  int b     return a   b     const int meaningOfLife   MeaningOfLife  6  7     Now you have something that can be evaluated down to a constant while maintaining good readability and allowing slightly more complex processing than just setting a constant to a number  It basically provides a good aid to maintainability as it becomes more obvious what you are doing   Take max  a  b   for example  template lt  typename Type  gt  constexpr Type max  Type a  Type b     return a  lt  b   b   a     Its a pretty simple choice there but it does mean that if you call max with constant values it is explicitly calculated at compile time and not at runtime  Another good example would be a DegreesToRadians function   Everyone finds degrees easier to read than radians   While you may know that 180 degrees is 3 14159265  Pi  in radians it is much clearer written as follows  const float oneeighty   DegreesToRadians  180 0f     Lots of good info here  http   en cppreference com w cpp language constexpr

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There used to be a pattern with metaprogramming   template lt unsigned T gt  struct Fact       enum Enum           VALUE   Fact lt T-1 gt  T             template lt  gt  struct Fact lt 1u gt        enum Enum           VALUE   1                Fact lt 10 gt   VALUE is known be a compile-time constant   I believe constexpr was introduced to let you write such constructs without the need for templates and weird constructs with specialization  SFINAE and stuff - but exactly like you d write a run-time function  but with the guarantee that the result will be determined in compile-time   However  note that   int fact unsigned n        if  n  1  return 1      return fact n-1  n     int main         return fact 10       Compile this with g   -O3 and you ll see that fact 10  is indeed evaulated at compile-time   An VLA-aware compiler  so a C compiler in C99 mode or C   compiler with C99 extensions  may even allow you to do   int main         int tab fact 10        int tab2 std  max 20 30        But that it s non-standard C   at the moment - constexpr looks like a way to combat this  even without VLA  in the above case   And there s still the problem of the need to have  formal  constant expressions as template arguments

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From what I ve read  the need for constexpr comes from an issue in metaprogramming   Trait classes may have constants represented as functions  think  numeric limits  max     With constexpr  those types of functions can be used in metaprogramming  or as array bounds  etc etc   Another example off of the top of my head would be that for class interfaces  you may want derived types define their own constants for some operation   Edit   After poking around on SO  it looks like others have come up with some examples of what might be possible with constexprs

[c++] When should you use constexpr capability in C++11?

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