C default arguments

Question

Is there a way to specify default arguments to a function in C

User · Answer

OpenCV uses something like      in the header file      ifdef   cplusplus        in case the compiler is a C   compiler         define DEFAULT VALUE value    value  else        otherwise  C compiler  do nothing         define DEFAULT VALUE value   endif  void window set size unsigned int width  DEFAULT VALUE 640                        unsigned int height DEFAULT VALUE 400      If the user doesn t know what he should write  this trick can be helpful

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I improved Jens Gustedt   s answer so that    inline functions aren   t employed defaults are computed during preprocessing modular reuseable macros possible to set compiler error that meaningfully matches the case of insufficient arguments for the allowed defaults the defaults aren   t required to form the tail of the parameter list if the argument types will remain unambiguous interopts with C11  Generic vary the function name by the number of arguments    variadic h    ifndef VARIADIC   define  NARG2  0   1   2        2  define NUMARG2       NARG2   VA ARGS    2  1  0   define  NARG3  0   1   2   3        3  define NUMARG3       NARG3   VA ARGS    3  2  1  0   define  NARG4  0   1   2   3   4        4  define NUMARG4       NARG4   VA ARGS    4  3  2  1  0   define  NARG5  0   1   2   3   4   5        5  define NUMARG5       NARG5   VA ARGS    5  4  3  2  1  0   define  NARG6  0   1   2   3   4   5   6        6  define NUMARG6       NARG6   VA ARGS    6  5  4  3  2  1  0   define  NARG7  0   1   2   3   4   5   6   7        7  define NUMARG7       NARG7   VA ARGS    7  6  5  4  3  2  1  0   define  NARG8  0   1   2   3   4   5   6   7   8        8  define NUMARG8       NARG8   VA ARGS    8  7  6  5  4  3  2  1  0   define  NARG9  0   1   2   3   4   5   6   7   8   9        9  define NUMARG9       NARG9   VA ARGS    9  8  7  6  5  4  3  2  1  0   define   VARIADIC name  num args       name         num args    VA ARGS     define  VARIADIC name  num args       name    VARIADIC name  num args    VA ARGS      define VARIADIC name  num args        VARIADIC name  num args    VA ARGS     define VARIADIC2 name  num args         VARIADIC name  num args    VA ARGS        Vary function name by number of arguments supplied  define VARIADIC NAME name  num args  name         num args     name     define NVARIADIC name  num args        VARIADIC VARIADIC NAME name  num args   num args    VA ARGS      endif   Simplified usage scenario   const uint32  uint32 frombytes uint32  out  const uint8  in  size t bytes       The output buffer defaults to NULL if not provided       include  variadic h    define uint32 frombytes 2    b  c  NULL  b  c  define uint32 frombytes 3 a  b  c     a  b  c  define uint32 frombytes      VARIADIC uint32 frombytes  NUMARG3   VA ARGS       VA ARGS      And with  Generic   const uint8  uint16 tobytes const uint16  in  uint8  out  size t bytes    const uint16  uint16 frombytes uint16  out  const uint8  in  size t bytes    const uint8  uint32 tobytes const uint32  in  uint8  out  size t bytes    const uint32  uint32 frombytes uint32  out  const uint8  in  size t bytes       The output buffer defaults to NULL if not provided  Generic function name supported on the non-uint8 type  except where said type is unavailable because the argument for output buffer was not provided       include  variadic h    define   uint16 tobytes 2 a     c  a  NULL  c  define   uint16 tobytes 3 a  b  c  a     b  c  define   uint16 tobytes      VARIADIC   uint16 tobytes  NUMARG3   VA ARGS       VA ARGS      define uint16 frombytes 2    b  c  NULL  b  c  define uint16 frombytes 3 a  b  c     a  b  c  define uint16 frombytes      VARIADIC uint16 frombytes  NUMARG3   VA ARGS       VA ARGS      define   uint32 tobytes 2 a     c  a  NULL  c  define   uint32 tobytes 3 a  b  c  a     b  c  define   uint32 tobytes      VARIADIC   uint32 tobytes  NUMARG3   VA ARGS       VA ARGS      define uint32 frombytes 2    b  c  NULL  b  c  define uint32 frombytes 3 a  b  c     a  b  c  define uint32 frombytes      VARIADIC uint32 frombytes  NUMARG3   VA ARGS       VA ARGS      define   tobytes a        Generic  a                                                                                                                                        const uint16   uint16 tobytes                                                                                                             const uint32   uint32 tobytes    VARIADIC2   uint32 tobytes  NUMARG3 a    VA ARGS     a    VA ARGS       define frombytes a        Generic  a                                                                                                                                              uint16   uint16 frombytes                                                                                                                 uint32   uint32 frombytes  VARIADIC2 uint32 frombytes  NUMARG3 a    VA ARGS     a    VA ARGS       And with variadic function name selection  which can t be combined with  Generic      winternitz   with 5 arguments is replaced with merkle lamport   on those 5 arguments    define   merkle lamport 5 a  b  c  d  e  a  b  c  d  e  define   winternitz 7 a  b  c  d  e  f  g  a  b  c  d  e  f  g  define   winternitz 5 name   merkle lamport  define   winternitz 7 name   winternitz  define   winternitz      NVARIADIC winternitz  NUMARG7   VA ARGS       VA ARGS

User · Answer

No

User · Answer

Yes you can do somthing simulair  here you have to know the different argument lists you can get but you have the same function to handle then all   typedef enum   my input set1   0  my input set2  my input set3  INPUT SET   typedef struct      INPUT SET type      char  text    input set1   typedef struct      INPUT SET type      char  text      int var    input set2   typedef struct      INPUT SET type      int text    input set3   typedef union       INPUT SET type      input set1 set1      input set2 set2      input set3 set3    MY INPUT   void my func MY INPUT input        switch input type                case my input set1          break          case my input set2          break          case my input set3          break          default             unknown input         break

User · Answer

Yes   -  But not in a way you would expect   int f1 int arg1  double arg2  char  name  char  opt    int f2 int arg1  double arg2  char  name      return f1 arg1  arg2  name   Some option        Unfortunately  C doesn t allow you to overload methods so you d end up with two different functions  Still  by calling f2  you d actually be calling f1 with a default value  This is a  Don t Repeat Yourself  solution  which helps you to avoid copying pasting existing code

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Another trick using macros    include  lt stdio h gt    define func      FUNC   VA ARGS    15  0   define FUNC a  b       func a  b   int  func  int a  int b        return a   b     int main void        printf   d n   func 1        printf   d n   func 1  2        return 0      If only one argument is passed  b receives the default value  in this case 15

User · Answer

No   Not even the very latest C99 standard supports this

User · Answer

Generally no  but in gcc You may make the last parameter of funcA   optional with a macro   In funcB   i use a special value  -1  to signal that i need the default value for the  b  parameter    include  lt stdio h    int funcA  int a  int b         return a b     define funcA  a        funcA  a      VA ARGS    8      int funcB  int a  int b      if  b    -1   b   8    return a b     int main void     printf  funcA 1 2    i n   funcA 1 2       printf  funcA 1      i n   funcA 1          printf  funcB 1  2    i n   funcB 1  2       printf  funcB 1 -1    i n   funcB 1 -1

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No  but you might consider using a set of functions  or macros  to approximate using default args      No default args int foo3 int a  int b  int c        return            Default 3rd arg int foo2 int a  int b        return foo3 a  b  0       default c       Default 2nd and 3rd args int foo1 int a        return foo3 a  1  0       default b and c

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Probably the best way to do this  which may or may not be possible in your case depending on your situation  is to move to C   and use it as  a better C   You can use C   without using classes  templates  operator overloading or other advanced features   This will give you a variant of C with function overloading and default parameters  and whatever other features you chose to use   You just have to be a little disciplined if you re really serious about using only a restricted subset of C     A lot of people will say it s a terrible idea to use C   in this way  and they might have a point   But s it s just an opinion  I think it s valid to use features of C   that you re comfortable with without having to buy into the whole thing  I think a significant part of the reason for the sucess of C   is that it got used by an awful lot of programmers in it s early days in exactly this way

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Yet another option uses structs   struct func opts     int    arg1    char   arg2    int    arg3      void func int arg  struct func opts  opts        int arg1   0  arg3   0      char  arg2    Default       if opts                  if opts- gt arg1              arg1   opts- gt arg1          if opts- gt arg2              arg2   opts- gt arg2          if opts- gt arg3              arg3   opts- gt arg3                 do stuff       call with defaults func 3  NULL       also call with defaults struct func opts opts    0   func 3   amp opts       set some arguments opts arg3   3  opts arg2    Yes   func 3   amp opts

User · Answer

Yes  with features of C99 you may do this  This works without defining new data structures or so and without the function having to decide at runtime how it was called  and without any computational overhead   For a detailed explanation see my post at  http   gustedt wordpress com 2010 06 03 default-arguments-for-c99   Jens

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Not really   The only way would be to write a varargs function and manually fill in default values for arguments which the caller doesn t pass

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YES  Through macros  3 Parameters     define my func2      my func3   VA ARGS    0 5   define my func1      my func2   VA ARGS    10   define VAR FUNC  1   2   3  NAME       NAME  define my func      VAR FUNC   VA ARGS    my func3  my func2  my func1    VA ARGS     void my func3 char a  int b  float c     b 10  c 0 5       printf  a  c  b  d  c  f n   a  b  c       If you want 4th argument  then an extra my func3 needs to be added  Notice the changes in VAR FUNC  my func2 and my func  4 Parameters     define my func3      my func4   VA ARGS     default       lt    New function added  define my func2      my func3   VA ARGS     float 1 2   define my func1      my func2   VA ARGS    10   define VAR FUNC  1   2   3   4  NAME       NAME  define my func      VAR FUNC   VA ARGS    my func4  my func3  my func2  my func1    VA ARGS     void my func4 char a  int b  float c  const char  d     b 10  c 0 5  d  default        printf  a  c  b  d  c  f  d  s n   a  b  c  d       Only exception that float variables cannot be given default values  unless if it is the last argument as in the 3 parameters case   because they need period        which is not accepted within macro arguments  But can figure out a work around as seen in my func2 macro  of 4 parameters case   Program  int main void        my func  a        my func  b   20       my func  c   200  10 5       my func  d   2000  100 5   hello         return 0      Output   a a  b 10  c 0 500000  d default                                                                                                                                                   a b  b 20  c 0 500000  d default                                                                                                                                                   a c  b 200  c 10 500000  d default                                                                                                                                                 a d  b 2000  c 100 500000  d hello

User · Answer

Why can t we do this   Give the optional argument a default value  In that way  the caller of the function don t necessarily need to pass the value of  the argument  The argument takes the default value  And easily that argument becomes optional for the client   For e g   void foo int a  int b   0    Here b is an optional argument

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We can create functions which use named parameters  only  for default values  This is a continuation of bk  s answer    include  lt stdio h gt                                                                  struct range   int from  int to  int step      define range      range  struct range   from 1  to 10  step 1    VA ARGS            use parentheses to avoid macro subst                 void  range  struct range r                                                             for  int i   r from  i  lt   r to  i    r step                                           printf   d    i                                                               puts                                                                                                                                                             int main                                                                              range                                                                            range  from 2   to 4                                                             range  step 2                                                                        The C99 standard defines that later names in the initialization override previous items  We can also have some standard positional parameters as well  just change the macro and function signature accordingly  The default value parameters can only be used in named parameter style   Program output   1 2 3 4 5 6 7 8 9 10  2 3 4  1 3 5 7 9

User · Answer

No  that s a C   language feature

User · Answer

Short answer  No   Slightly longer answer  There is an old  old workaround where you pass a string that you parse for optional arguments   int f int arg1  double arg2  char  name  char  opt     where opt may include  name value  pair or something  and which you would call like  n   f 2 3 0  foo   plot yes save no      Obviously this is only occasionally useful  Generally when you want a single interface to a family of functionality     You still find this approach in particle physics codes that are written by professional programs in c    like for instance ROOT   It s main advantage is that it may be extended almost indefinitely while maintaining back compatibility

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Wow  everybody is such a pessimist around here   The answer is yes    It ain t trivial  by the end  we ll have the core function  a supporting struct  a wrapper function  and a macro around the wrapper function  In my work I have a set of macros to automate all this  once you understand the flow it ll be easy for you to do the same   I ve written this up elsewhere  so here s a detailed external link to supplement the summary here  http   modelingwithdata org arch 00000022 htm  We d like to turn  double f int i  double x    into a function that takes defaults  i 8  x 3 14   Define a companion struct   typedef struct       int i      double x    f args    Rename your function f base  and define a wrapper function that sets defaults and calls the base   double var f f args in       int i out   in i   in i   8      double x out   in x   in x   3 14      return f base i out  x out       Now add a macro  using C s variadic macros  This way users don t have to know they re actually populating a f args struct and think they re doing the usual    define f      var f  f args    VA ARGS        OK  now all of the following would work   f 3  8          i 3  x 8 f  i 1  2 3     i 1  x 2 3 f 2             i 2  x 3 14 f  x 9 2        i 8  x 9 2   Check the rules on how compound initializers set defaults for the exact rules   One thing that won t work  f 0   because we can t distinguish between a missing value and zero  In my experience  this is something to watch out for  but can be taken care of as the need arises---half the time your default really is zero   I went through the trouble of writing this up because I think named arguments and defaults  really do make coding in C easier and even more fun  And C is awesome for being so simple and still having enough there to make all this possible

[c] C default arguments

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