How to pass optional arguments to a method in C

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How to pass optional arguments to a method in C     Any code snippet

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An important rule with respect to default parameter usage:
Default parameters should be specified at right most end, once you specify a default value parameter you cannot specify non default parameter again. ex:

int DoSomething(int x, int y = 10, int z) -----------> Not Allowed

int DoSomething(int x, int z, int y = 10) -----------> Allowed

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Use default parameters  template  lt typename T gt  void func T a  T b   T          std  cout  lt  lt  a  lt  lt  b      int main         func 1 4      a   1  b   4     func 1        a   1  b   0      std  string x    Hello       std  string y    World        func x y       a    Hello   b   World      func x         a    Hello   b            Note   The following are ill-formed  template  lt typename T gt  void func T a   T    T b    template  lt typename T gt  void func T a  T b   a

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It might be interesting to some of you that in case of multiple default parameters   void printValues int x 10  int y 20  int z 30        std  cout  lt  lt   Values     lt  lt  x  lt  lt       lt  lt  y  lt  lt       lt  lt  z  lt  lt    n       Given the following function calls   printValues 1  2  3   printValues 1  2   printValues 1   printValues      The following output is produced   Values  1 2 3 Values  1 2 30 Values  1 20 30 Values  10 20 30   Reference  http   www learncpp com cpp-tutorial 77-default-parameters

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To follow the example given here  but to clarify syntax with the use of header files  the function forward declaration contains the optional parameter default value   myfile h  void myfunc int blah  int mode   0     myfile cpp  void myfunc int blah  int mode     mode   0          if  mode    0          do something         else         do something else

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Typically by setting a default value for a parameter   int func int a  int b   -1         std  cout  lt  lt   a      lt  lt  a      if  b    -1                  std  cout  lt  lt     b      lt  lt  b      std  cout  lt  lt    n      int main          func 1  2       prints  a 1  b 2 n      func 3          prints  a 3 n      return 0

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With the introduction of std  optional in C  17 you can pass optional arguments    include  lt iostream gt   include  lt string gt   include  lt optional gt   void myfunc const std  string amp  id  const std  optional lt std  string gt  amp  param   std  nullopt        std  cout  lt  lt   id    lt  lt  id  lt  lt     param         if  param          std  cout  lt  lt   param  lt  lt  std  endl      else         std  cout  lt  lt    lt parameter not set gt    lt  lt  std  endl     int main          myfunc  first        myfunc  second     something        Output   id first param  lt parameter not set gt  id second param something   See https   en cppreference com w cpp utility optional

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Here is an example of passing mode as optional parameter   void myfunc int blah  int mode   0        if  mode    0          do something         else         do something else        you can call myfunc in both ways and both are valid  myfunc 10          Mode will be set to default 0 myfunc 10  1       Mode will be set to 1

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With commas separating them  just like parameters without default values   int func  int x   0  int y   0     func       doesn t pass optional parameters  defaults are used  x   0 and y   0  func 1  2      provides optional parameters  x   1 and y   2

[c++] How to pass optional arguments to a method in C++?

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