Difference between constexpr and const

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What s the difference between constexpr and const    When can I use only one of them    When can I use both and how should I choose one

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First of all  both are qualifiers in c    A variable declared const must be initialized and cannot be changed in the future  Hence generally a variable declared as a const will have a value even before compiling   But  for constexpr it is a bit different   For constexpr  you can give an expression that could be evaluated during the compilation of the program    Obviously  the variable declared as constexper cannot be changed in the future just like const

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An overview of the const and constexpr keywords In C     if a const object is initialized with a constant expression  we can use our const object wherever a constant expression is required  const int x   10  int a x     0    For example  we can make a case statement in switch  constexpr can be used with arrays  constexpr is not a type  The constexpr keyword can be used in conjunction with the auto keyword  constexpr auto x   10   struct Data        We can make a bit field element of struct         int a x       If we initialize a const object with a constant expression  the expression generated by that const object is now a constant expression as well  Constant Expression   An expression whose value can be calculated at compile time  x 5-4     This is a constant expression  For the compiler  there is no difference between typing this expression and typing 46 directly  Initialize is mandatory  It can be used for reading purposes only  It cannot be changed  Up to this point  there is no difference between the  quot const quot  and  quot constexpr quot  keywords  NOTE  We can use constexpr and const in the same declaration  constexpr const int  p   Constexpr Functions Normally  the return value of a function is obtained at runtime  But calls to constexpr functions will be obtained as a constant in compile time when certain conditions are met  NOTE   Arguments sent to the parameter variable of the function in function calls or to all parameter variables if there is more than one parameter  if C E the return value of the function will be calculated in compile time      constexpr int square  int a   return a a     constexpr int a   3  constexpr int b   5   int arr square a b 20      0     This expression is equal to int arr 35     0    In order for a function to be a constexpr function  the return value type of the function and the type of the function s parameters must be in the type category called  quot literal type quot   The constexpr functions are implicitly inline functions  An important point   None of the constexpr functions need to be called with a constant expression It is not mandatory  If this happens  the computation will not be done at compile time  It will be treated like a normal function call  Therefore  where the constant expression is required  we will no longer be able to use this expression  The conditions required to be a constexpr function are shown below  1   The types used in the parameters of the function and the type of the return value of the function must be literal type  2   A local variable with static life time should not be used inside the function  3   If the function is legal  when we call this function with a constant expression in compile time  the compiler calculates the return value of the function in compile time  4   The compiler needs to see the code of the function  so constexpr functions will almost always be in the header files  5   In order for the function we created to be a constexpr function  the definition of the function must be in the header file Thus  whichever source file includes that header file will see the function definition  Bonus Normally with Default Member Initialization  static data members with const and integral types can be initialized within the class  However  in order to do this  there must be both  quot const quot  and  quot integral types quot   If we use static constexpr then it doesn t have to be an integral type to initialize it inside the class  As long as I initialize it with a constant expression  there is no problem  class Myclass             const static int sx   15             OK          constexpr static int sy   15         OK          const static double sd   1 5         ERROR          constexpr static double sd   1 5     OK

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const applies for variables  and prevents them from being modified in your code    constexpr tells the compiler that this expression results in a compile time constant value  so it can be used in places like array lengths  assigning to const variables  etc  The link given by Oli has a lot of excellent examples    Basically they are 2 different concepts altogether  and can  and should  be used together

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Overview   const guarantees that a program does not change an object   s value  However  const does not guarantee which type of initialization the object undergoes    Consider   const int mx   numeric limits lt int gt   max        OK  runtime initialization   The function max   merely returns a literal value  However  because the initializer is a function call  mx undergoes runtime initialization  Therefore  you cannot use it as a constant expression   int arr mx       error     constant expression required     constexpr is a new C  11 keyword that rids you of the need to create macros and hardcoded literals  It also guarantees  under certain conditions  that objects undergo static initialization  It controls the evaluation time of an expression  By enforcing compile-time evaluation of its expression  constexpr lets you define true constant expressions that are crucial for time-critical applications  system programming  templates  and generally speaking  in any code that relies on compile-time constants    Constant-expression functions  A constant-expression function is a function declared constexpr  Its body must be non-virtual and consist of a single return statement only  apart from typedefs and static asserts  Its arguments and return value must have literal types  It can be used with non-constant-expression arguments  but when that is done the result is not a constant expression   A constant-expression function is meant to replace macros and hardcoded literals without sacrificing performance or type safety   constexpr int max     return INT MAX                 OK constexpr long long max     return 2147483647        OK constexpr bool get val         bool res   false      return res        error  body is not just a return statement  constexpr int square int x    return x   x        OK  compile-time evaluation only if x is a constant expression const int res   square 5       OK  compile-time evaluation of square 5  int y   getval    int n   square y               OK  runtime evaluation of square y    Constant-expression objects  A constant-expression object is an object declared constexpr  It must be initialized with a constant expression or an rvalue constructed by a constant-expression constructor with constant-expression arguments   A constant-expression object behaves as if it was declared const  except that it requires initialization before use and its initializer must be a constant expression  Consequently  a constant-expression object can always be used as part of another constant expression   struct S       constexpr int two            constant-expression function private      static constexpr int sz      constant-expression object    constexpr int S  sz   256  enum DataPacket       Small   S  two        error  S  two   called before it was defined     Big   1024    constexpr int S  two     return sz 2    constexpr S s  int arr s two         OK  s two   called after its definition   Constant-expression constructors  A constant-expression constructor is a constructor declared constexpr  It can have a member initialization list but its body must be empty  apart from typedefs and static asserts  Its arguments must have literal types   A constant-expression constructor allows the compiler to initialize the object at compile-time  provided that the constructor   s arguments are all constant expressions   struct complex          constant-expression constructor     constexpr complex double r  double i    re r   im i          OK  empty body        constant-expression functions     constexpr double real     return re        constexpr double imag     return im    private      double re      double im     constexpr complex COMP 0 0  1 0              creates a literal complex double x   1 0  constexpr complex cx1 x  0                   error  x is not a constant expression const complex cx2 x  1                       OK  runtime initialization constexpr double xx   COMP real              OK  compile-time initialization constexpr double imaglval   COMP imag        OK  compile-time initialization complex cx3 2  4 6                           OK  runtime initialization   Tips from the book Effective Modern C   by Scott Meyers about constexpr    constexpr objects are const and are initialized with values known during compilation  constexpr functions produce compile-time results when called with arguments whose values are known during compilation  constexpr objects and functions may be used in a wider range of contexts than non-constexpr objects and functions  constexpr is part of an object   s or function   s interface    Source  Using constexpr to Improve Security  Performance and Encapsulation in C

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According to book of  The C   Programming Language 4th Editon  by Bjarne Stroustrup     const  meaning roughly       I promise not to change this value          7 5   This is used primarily to specify interfaces  so that data can be passed to functions without fear of it being modified  The compiler enforces the promise made by const      constexpr  meaning roughly       to be evaluated at compile time          10 4   This is used primarily to specify constants  to allow  For example   const int dmv   17     dmv is a named constant int var   17     var is not a constant constexpr double max1   1 4 square dmv      OK if square 17  is a constant expression constexpr double max2   1 4 square var      error   var is not a constant expression const double max3   1 4 square var     OK  may be evaluated at run time double sum const vector lt double gt  amp       sum will not modify its argument    2 2 5  vector lt double gt  v  1 2  3 4  4 5      v is not a constant const double s1   sum v      OK  evaluated at run time constexpr double s2   sum v      error   sum v  not constant expression   For a function to be usable in a constant expression  that is  in an expression that will be evaluated by the compiler  it must be defined constexpr  For example   constexpr double square double x    return x x       To be constexpr  a function must be rather simple  just a return-statement computing a value  A constexpr function can be used for non-constant arguments  but when that is done the result is not a constant expression  We allow a constexpr function to be called with non-constant-expression arguments in contexts that do not require constant expressions  so that we don   t hav e to define essentially the same function twice  once for constant expressions and once for variables  In a few places  constant expressions are required by language rules  e g   array bounds    2 2 5    7 3   case labels    2 2 4    9 4 2   some template arguments    25 2   and constants declared using constexpr   In other cases  compile-time evaluation is important for performance  Independently of performance issues  the notion of immutability  of an object with an unchangeable state  is an important design concern    10 4

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As  0x499602d2 already pointed out  const only ensures that a value cannot be changed after initialization where as constexpr  introduced in C  11  guarantees the variable is a compile time constant  Consider the following example from LearnCpp com    cout  lt  lt   Enter your age     int age  cin  gt  gt  age   const int myAge age             works constexpr int someAge age       error  age can only be resolved at runtime

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I don t think any of the answers really make it clear exactly what side effects it has  or indeed  what it is   constexpr and const at namespace file-scope are identical when initialised with a literal or expression  but with a function  const can be initialised by any function  but constexpr initialised by a non-constexpr  a function that isn t marked with constexpr or a non constexpr expression  will generate a compiler error  Both constexpr and const are implicitly internal linkage for variables  well actually  they don t survive to get to the linking stage if compiling -O1 and stronger  and static doesn t force the compiler to emit an internal  local  linker symbol for const or constexpr when at -O1 or stronger  the only time it does this is if you take the address of the variable  const and constexpr will be an internal symbol unless expressed with extern i e  extern constexpr const int i   3  needs to be used   On a function  constexpr makes the function permanently never reach the linking stage  regardless of extern or inline in the definition or -O0 or -Ofast   whereas const never does  and static and inline only have this effect on -O1 and above  When a const constexpr variable is initialised by a constexpr function  the load is always optimised out with any optimisation flag  but it is never optimised out if the function is only static or inline  or if the variable is not a const constexpr   Standard compilation  -O0    include lt iostream gt  constexpr int multiply  int x  int y       return x   y     extern const int val   multiply 10 10   int main        std  cout  lt  lt  val       compiles to  val           long   100    extra external definition supplied due to extern  main          push    rbp         mov     rbp  rsp         mov     esi  100   substituted in as an immediate         mov     edi  OFFSET FLAT  ZSt4cout         call    std  basic ostream lt char  std  char traits lt char gt   gt   operator lt  lt  int          mov     eax  0         pop     rbp         ret    static initialization and destruction 0 int  int                                      However   include lt iostream gt  const int multiply  int x  int y       return x   y     const int val   multiply 10 10     constexpr is an error int main        std  cout  lt  lt  val      Compiles to  multiply int  int           push    rbp         mov     rbp  rsp         mov     DWORD PTR  rbp-4   edi         mov     DWORD PTR  rbp-8   esi         mov     eax  DWORD PTR  rbp-4          imul    eax  DWORD PTR  rbp-8          pop     rbp         ret  main          push    rbp         mov     rbp  rsp         mov     eax  DWORD PTR val rip          mov     esi  eax         mov     edi  OFFSET FLAT  ZSt4cout         call    std  basic ostream lt char  std  char traits lt char gt   gt   operator lt  lt  int          mov     eax  0         pop     rbp         ret    static initialization and destruction 0 int  int                                            mov     esi  10         mov     edi  10         call    multiply int  int          mov     DWORD PTR val rip   eax   This clearly shows that constexpr causes the initialisation of the const constexpr file-scope variable to occur at compile time and produce no global symbol  whereas not using it causes initialisation to occur before main at runtime   Compiling using -Ofast  Even -Ofast doesn t optimise out the load  https   godbolt org z r-mhif  so you need constexpr    constexpr functions can also be called from inside other constexpr functions for the same result  constexpr on a function also prevents use of anything that can t be done at compile time in the function  for instance  a call to the  lt  lt  operator on std  cout   constexpr at block scope behaves the same in that it produces an error if initialised by a non-constexpr function  the value is also substituted in immediately    In the end  its main purpose is like C s inline function  but it is only effective when the function is used to initialise file-scope variables  which functions cannot do on C  but they can on C   because it allows dynamic initialisation of file-scope variables   except the function cannot export a global local symbol to the linker as well  even using extern static  which you could with inline on C  block-scope variable assignment functions can be inlined simply using an -O1 optimisation without constexpr on C and C

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A const int var can be dynamically set to a value at runtime and once it is set to that value  it can no longer be changed  A constexpr int var cannot be dynamically set at runtime  but rather  at compile time  And once it is set to that value  it can no longer be changed  Here is a solid example  int main int argc  char argv          const int p   argc          p   69     cannot change p because it is a const        constexpr int q   argc     cannot be  bcoz argc cannot be computed at compile time      constexpr int r   2 3     this works         r   42     same as const too  it cannot be changed    The snippet above compiles fine and I have commented out those that cause it to error  The key notions here to take note of  are the notions of compile time and run time  New innovations have been introduced into C   intended to as much as possible    know    certain things at compilation time to improve performance at runtime  Any attempt of explanation which does not involve the two key notions above  is hallucination

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Basic meaning and syntax Both keywords can be used in the declaration of objects as well as functions  The basic difference when applied to objects is this   const declares an object as constant  This implies a guarantee that once initialized  the value of that object won t change  and the compiler can make use of this fact for optimizations  It also helps prevent the programmer from writing code that modifies objects that were not meant to be modified after initialization   constexpr declares an object as fit for use in what the Standard calls constant expressions  But note that constexpr is not the only way to do this    When applied to functions the basic difference is this   const can only be used for non-static member functions  not functions in general  It gives a guarantee that the member function does not modify any of the non-static data members  except for mutable data members  which can be modified anyway    constexpr can be used with both member and non-member functions  as well as constructors  It declares the function fit for use in constant expressions  The compiler will only accept it if the function meets certain criteria  7 1 5 3 4   most importantly         The function body must be non-virtual and extremely simple  Apart from typedefs and static asserts  only a single return statement is allowed  In the case of a constructor  only an initialization list  typedefs  and static assert are allowed     default and   delete are allowed  too  though   As of C  14  the rules are more relaxed  what is allowed since then inside a constexpr function  asm declaration  a goto statement  a statement with a label other than case and default  try-block  the definition of a variable of non-literal type  definition of a variable of static or thread storage duration  the definition of a variable for which no initialization is performed  The arguments and the return type must be literal types  i e   generally speaking  very simple types  typically scalars or aggregates     Constant expressions As said above  constexpr declares both objects as well as functions as fit for use in constant expressions  A constant expression is more than merely constant   It can be used in places that require compile-time evaluation  for example  template parameters and array-size specifiers    template lt int N gt    class fixed size list                   fixed size list lt X gt  mylist      X must be an integer constant expression    int numbers X       X must be an integer constant expression   But note   Declaring something as constexpr does not necessarily guarantee that it will be evaluated at compile time  It can be used for such  but it can be used in other places that are evaluated at run-time  as well   An object may be fit for use in constant expressions without being declared constexpr  Example       int main                 const int N   3         int numbers N     1  2  3       N is constant expression         This is possible because N  being constant and initialized at declaration time with a literal  satisfies the criteria for a constant expression  even if it isn t declared constexpr    So when do I actually have to use constexpr   An object like N above can be used as constant expression without being declared constexpr  This is true for all objects that are  const of integral or enumeration type and initialized at declaration time with an expression that is itself a constant expression    This is due to   5 19 2  A constant expression must not include a subexpression that involves  quot an lvalue-to-rvalue modification unless       a glvalue of integral or enumeration type       quot  Thanks to Richard Smith for correcting my earlier claim that this was true for all literal types    For a function to be fit for use in constant expressions  it must be explicitly declared constexpr  it is not sufficient for it merely to satisfy the criteria for constant-expression functions  Example   template lt int N gt   class list         constexpr int sqr1 int arg     return arg   arg      int sqr2 int arg     return arg   arg      int main         const int X   2     list lt sqr1 X  gt  mylist1      OK  sqr1 is constexpr    list lt sqr2 X  gt  mylist2      wrong  sqr2 is not constexpr       When can I   should I use both  const and constexpr together  A  In object declarations  This is never necessary when both keywords refer to the same object to be declared  constexpr implies const  constexpr const int N   5   is the same as constexpr int N   5   However  note that there may be situations when the keywords each refer to different parts of the declaration  static constexpr int N   3   int main       constexpr const int  NP    amp N     Here  NP is declared as an address constant-expression  i e  a pointer that is itself a constant expression   This is possible when the address is generated by applying the address operator to a static global constant expression   Here  both constexpr and const are required  constexpr always refers to the expression being declared  here NP   while const refers to int  it declares a pointer-to-const   Removing the const would render the expression illegal  because  a  a pointer to a non-const object cannot be a constant expression  and  b   amp N is in-fact a pointer-to-constant   B  In member function declarations  In C  11  constexpr implies const  while in C  14 and C  17 that is not the case  A member function declared under C  11 as constexpr void f     needs to be declared as constexpr void f   const   under C  14 in order to still be usable as a const function

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Both const and constexpr can be applied to variables and functions  Even though they are similar to each other  in fact they are very different concepts   Both const and constexpr mean that their values can t be changed after their initialization  So for example   const int x1 10  constexpr int x2 10   x1 20     ERROR  Variable  x1  can t be changed  x2 20     ERROR  Variable  x2  can t be changed    The principal difference between const and constexpr is the time when their initialization values are known  evaluated   While the values of const variables can be evaluated at both compile time and runtime  constexpr are always evaluated at compile time  For example   int temp rand       temp is generated by the the random generator at runtime   const int x1 10     OK - known at compile time  const int x2 temp     OK - known only at runtime  constexpr int x3 10     OK - known at compile time  constexpr int x4 temp     ERROR  Compiler can t figure out the value of  temp  variable at compile time so  constexpr  can t be applied here    The key advantage to know if the value is known at compile time or runtime is the fact that compile time constants can be used whenever compile time constants are needed  For instance  C   doesn t allow you to specify C-arrays with the variable lengths   int temp rand       temp is generated by the the random generator at runtime   int array1 10      OK  int array2 temp      ERROR    So it means that   const int size1 10     OK - value known at compile time  const int size2 temp     OK - value known only at runtime  constexpr int size3 10     OK - value known at compile time    int array3 size1      OK - size is known at compile time  int array4 size2      ERROR - size is known only at runtime time  int array5 size3      OK - size is known at compile time    So const variables can define both compile time constants like size1 that can be used to specify array sizes and runtime constants like size2 that are known only at runtime and can t be used to define array sizes  On the other hand constexpr always define compile time constants that can specify array sizes   Both const and constexpr can be applied to functions too  A const function must be a member function  method  operator  where application of const keyword means that the method can t change the values of their member  non-static  fields  For example   class test      int x      void function1              x 100     OK           void function2   const            x 100     ERROR  The const methods can t change the values of object fields            A constexpr is a different concept  It marks a function  member or non-member  as the function that can be evaluated at compile time if compile time constants are passed as their arguments  For example you can write this   constexpr int func constexpr int X  int Y        return X Y      int func int X  int Y        return X Y      int array1 func constexpr 10 20       OK - func constexpr   can be evaluated at compile time  int array2 func 10 20       ERROR - func   is not a constexpr function   int array3 func constexpr 10 rand         ERROR - even though func constexpr   is the  constexpr  function  the expression  constexpr 10 rand     can t be evaluated at compile time    By the way the constexpr functions are the regular C   functions that can be called even if non-constant arguments are passed  But in that case you are getting the non-constexpr values   int value1 func constexpr 10 rand        OK  value1 is non-constexpr value that is evaluated in runtime  constexpr int value2 func constexpr 10 rand        ERROR  value2 is constexpr and the expression func constexpr 10 rand    can t be evaluated at compile time    The constexpr can be also applied to the member functions  methods   operators and even constructors  For instance   class test2       static constexpr int function int value                return value 1              void f                 int x function 10                 A more  crazy  sample   class test3       public       int value          constexpr const method - can t chanage the values of object fields and can be evaluated at compile time      constexpr int getvalue   const               return value              constexpr test3 int Value            value Value                   constexpr test3 x 100      OK  Constructor is constexpr   int array x getvalue        OK  x getvalue   is constexpr and can be evaluated at compile time

[c++] Difference between `constexpr` and `const`

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