Variable number of arguments in C

Question

How can I write a function that accepts a variable number of arguments  Is this possible  how

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It s possible you want overloading or default parameters - define the same function with defaulted parameters   void doStuff  int a  double termstator   1 0  bool useFlag   true           stuff    void doStuff  double std termstator           assume the user always wants  1  for the a param    return doStuff  1  std termstator        This will allow you to call the method with one of four different calls   doStuff  1    doStuff  2  2 5    doStuff  1  1 0  false    doStuff  6 72           or you could be looking for the v args calling conventions from C

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Using variadic templates  example to reproduce console log as seen in JavaScript   Console console  console log  bunch    of    arguments    console warn  or some numbers    1  2  3   console error  just a prank    bro      Filename e g  js console h    include  lt iostream gt   include  lt utility gt   class Console   protected      template  lt typename T gt      void log argument T t            std  cout  lt  lt  t  lt  lt             public      template  lt typename    Args gt      void log Args amp  amp     args            int dummy       0    void  log argument std  forward lt Args gt  args   0                cout  lt  lt  endl             template  lt typename    Args gt      void warn Args amp  amp     args            cout  lt  lt   WARNING             int dummy       0    void  log argument std  forward lt Args gt  args   0                cout  lt  lt  endl             template  lt typename    Args gt      void error Args amp  amp     args            cout  lt  lt   ERROR             int dummy       0    void  log argument std  forward lt Args gt  args   0                cout  lt  lt  endl

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As others have said  C-style varargs  But you can also do something similar with default arguments

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We could also use an initializer list if all arguments are const and of the same type

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C-style variadic functions are supported in C     However  most C   libraries use an alternative idiom e g  whereas the  c  printf function takes variable arguments the c   cout object uses  lt  lt  overloading which addresses type safety and ADTs  perhaps at the cost of implementation simplicity

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in c  11 you can do   void foo const std  list lt std  string gt   amp  myArguments         do whatever you want  with all the convenience of lists    foo   arg1   arg2       list initializer FTW

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In C  11 there is a way to do variable argument templates which lead to a really elegant and type safe way to have variable argument functions   Bjarne himself gives a nice example of printf using variable argument templates in the C  11FAQ   Personally  I consider this so elegant that I wouldn t even bother with a variable argument function in C   until that compiler has support for C  11 variable argument templates

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A C  17 solution  full type safety   nice calling syntax  Since the introduction of variadic templates in C  11 and fold expressions in C  17  it is possible to define a template-function which  at the caller site  is callable as if it was a varidic function but with the advantages to    be strongly type safe  work without the run-time information of the number of arguments  or without the usage of a  stop  argument    Here is an example for mixed argument types  template lt class    Args gt  void print Args    args         std  cout  lt  lt       lt  lt  args   lt  lt    n     print 1         Hello              World       And another with enforced type match for all arguments    include  lt type traits gt     enable if  conjuction  template lt class Head  class    Tail gt  using are same   std  conjunction lt std  is same lt Head  Tail gt     gt    template lt class Head  class    Tail  class   std  enable if t lt are same lt Head  Tail    gt   value  void gt  gt  void print same type Head head  Tail    tail        std  cout  lt  lt  head       std  cout  lt  lt       lt  lt  tail   lt  lt    n     print same type  2      Hello      World          OK print same type 3         Hello      World        no matching function for call to  print same type int  const char  3   const char  8   const char  7                                                      print same type 3         Hello      World                                                                                                       More information    Variadic templates  also known as parameter pack Parameter pack since C  11  - cppreference com  Fold expressions fold expression since C  17  - cppreference com  See a full program demonstration on coliru

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int fun int n args            int  p    amp n args      int s   sizeof int      p    s   s - 1     for int i   0  i  lt  n args  i           printf  A1  d  n    p        p    2           Plain version

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The only way is through the use of C style variable arguments  as described here  Note that this is not a recommended practice  as it s not typesafe and error-prone

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If you know the range of number of arguments that will be provided  you can always use some function overloading  like  f int a       int res a  return res   f int a  int b       int res a b  return res     and so on

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spawn  allocate and initialize  a simple function  template lt typename T gt  T   spawn size t n          T   arr   new T n     va list ap    va start ap  n     for  size t i   0  i  lt  n  i        T i    va arg ap T     return arr      User writes  auto arr   spawn lt float gt   3  0 1 0 2 0 3    Semantically  this looks and feels exactly like an n-argument function   Under the hood  you might unpack it one way or the other

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Apart from varargs or overloading  you could consider to aggregate your arguments in a std  vector or other containers  std  map for example   Something like this   template  lt typename T gt  void f std  vector lt T gt  const amp    std  vector lt int gt  my args  my args push back 1   my args push back 2   f my args     In this way you would gain type safety and the logical meaning of these variadic arguments would be apparent   Surely this approach can have performance issues but you should not worry about them unless you are sure that you cannot pay the price  It is a sort of a a  Pythonic  approach to c

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You probably shouldn t  and you can probably do what you want to do in a safer  and simpler way   Technically to use variable number of arguments in C you include stdarg h   From that you ll get the va list type as well as three functions that operate on it called va start    va arg   and va end      include lt stdarg h gt   int maxof int n args             va list ap      va start ap  n args       int max   va arg ap  int       for int i   2  i  lt   n args  i              int a   va arg ap  int           if a  gt  max  max   a            va end ap       return max      If you ask me  this is a mess   It looks bad  it s unsafe  and it s full of technical details that have nothing to do with what you re conceptually trying to achieve   Instead  consider using overloading or inheritance polymorphism  builder pattern  as in operator lt  lt    in streams  or default arguments etc   These are all safer   the compiler gets to know more about what you re trying to do so there are more occasions it can stop you before you blow your leg off

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It is possible now   using boost any and templates In this case  arguments type can be mixed   include  lt boost any hpp gt   include  lt iostream gt    include  lt vector gt  using boost  any cast   template  lt typename T  typename    Types gt   void Alert T var1 Types    var2           std  vector lt boost  any gt  a    var1 var2            for  int i   0  i  lt  a size   i               if  a i  type      typeid int                 std  cout  lt  lt   int     lt  lt  boost  any cast lt int gt   a i    lt  lt  std  endl            if  a i  type      typeid double                 std  cout  lt  lt   double     lt  lt  boost  any cast lt double gt   a i    lt  lt  std  endl            if  a i  type      typeid const char                  std  cout  lt  lt   char     lt  lt  boost  any cast lt const char  gt   a i    lt  lt std  endl               etc             void main         Alert  something  0 0 0 3

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There is no standard C   way to do this without resorting to C-style varargs         There are of course default arguments that sort of  look  like variable number of arguments depending on the context   void myfunc  int i   0  int j   1  int k   2        other code     myfunc    myfunc  2    myfunc  2  1    myfunc  2  1  0      All four function calls call myfunc with varying number of arguments  If none are given  the default arguments are used  Note however  that you can only omit trailing arguments  There is no way  for example to omit i and give only j

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In C  11 you have two new options  as the Variadic functions reference page in the Alternatives section states         Variadic templates can also be used to create functions that take variable number of    arguments  They are often the better choice because they do not impose restrictions on   the types of the arguments  do not perform integral and floating-point promotions  and   are type safe   since C  11    If all variable arguments share a common type  a std  initializer list provides a    convenient mechanism  albeit with a different syntax  for accessing variable arguments        Below is an example showing both alternatives  see it live     include  lt iostream gt   include  lt string gt   include  lt initializer list gt   template  lt typename T gt  void func T t         std  cout  lt  lt  t  lt  lt  std  endl      template lt typename T  typename    Args gt  void func T t  Args    args     recursive variadic function       std  cout  lt  lt  t  lt  lt std  endl        func args          template  lt class T gt  void func2  std  initializer list lt T gt  list         for  auto elem   list                 std  cout  lt  lt  elem  lt  lt  std  endl            int main         std  string         str1   Hello             str2   world          func 1 2 5  a  str1        func2   10  20  30  40          func2   str1  str2            If you are using gcc or clang we can use the PRETTY FUNCTION magic variable to display the type signature of the function which can be helpful in understanding what is going on  For example using   std  cout  lt  lt    PRETTY FUNCTION    lt  lt        lt  lt  t  lt  lt std  endl     would results int following for variadic functions in the example  see it live    void func T  Args      T   int  Args    lt double  char  std  basic string lt char gt  gt    1 void func T  Args      T   double  Args    lt char  std  basic string lt char gt  gt    2 5 void func T  Args      T   char  Args    lt std  basic string lt char gt  gt    a void func T   T   std  basic string lt char gt    Hello   In Visual Studio you can use FUNCSIG   Update Pre C  11  Pre C  11 the alternative for std  initializer list would be std  vector or one of the other standard containers    include  lt iostream gt   include  lt string gt   include  lt vector gt   template  lt class T gt  void func1  std  vector lt T gt  vec         for  typename std  vector lt T gt   iterator iter   vec begin     iter    vec end      iter                 std  cout  lt  lt   iter  lt  lt  std  endl            int main         int arr1      10  20  30  40        std  string arr2        hello    world           std  vector lt int gt  v1  arr1  arr1 4         std  vector lt std  string gt  v2  arr2  arr2 2          func1  v1         func1  v2         and the alternative for variadic templates would be variadic functions although they are not type-safe and in general error prone and can be unsafe to use but the only other potential alternative would be to use default arguments  although that has limited use  The example below is a modified version of the sample code in the linked reference    include  lt iostream gt   include  lt string gt   include  lt cstdarg gt   void simple printf const char  fmt             va list args      va start args  fmt        while   fmt      0             if   fmt     d                 int i   va arg args  int               std  cout  lt  lt  i  lt  lt    n             else if   fmt     s                 char   s   va arg args  char                std  cout  lt  lt  s  lt  lt    n                       fmt             va end args       int main         std  string         str1   Hello             str2   world          simple printf  dddd   10  20  30  40        simple printf  ss   str1 c str    str2 c str            return 0        Using variadic functions also comes with restrictions in the arguments you can pass which is detailed in the draft C   standard in section 5 2 2 Function call paragraph 7      When there is no parameter for a given argument  the argument is passed in such a way that the receiving function can obtain the value of the argument by invoking va arg  18 7   The lvalue-to-rvalue  4 1   array-to-pointer  4 2   and function-to-pointer  4 3  standard conversions are performed on the argument expression  After these conversions  if the argument does not have arithmetic  enumeration  pointer  pointer to member  or class type  the program is ill-formed  If the argument has a non-POD class type  clause 9   the behavior is undefined

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