Where to put default parameter value in C

Question

What s the place for the default parameter value  Just in function definition  or declaration  or both places

User · Answer

C   places the default parameter logic in the calling side  this means that if the default value expression cannot be computed from the calling place  then the default value cannot be used   Other compilation units normally just include the declaration so default value expressions placed in the definition can be used only in the defining compilation unit itself  and after the definition  i e  after the compiler sees the default value expressions    The most useful place is in the declaration   h  so that all users will see it   Some people like to add the default value expressions in the implementation too  as a comment    void foo int x   42           int y   21    void foo int x      42              int y      21                  However  this means duplication and will add the possibility of having the comment out of sync with the code  what s worse than uncommented code  code with misleading comments

User · Answer

Although this is an  old  thread  I still would like to add the following to it   I ve experienced the next case    In the header file of a class  I had    int SetI2cSlaveAddress  UCHAR addr  bool force        In the source file of that class  I had     int CI2cHal  SetI2cSlaveAddress  UCHAR addr  bool force   false                 As one can see  I had put the default value of the parameter  force  in the class source file  not in the class header file   Then I used that function in a derived class as follows  derived class inherited the base class in a public way       SetI2cSlaveAddress  addr      assuming it would take the  force  parameter as  false   for granted    However  the compiler  put in c  11 mode  complained and gave me the following compiler error    home     mystuff domoproject lib i2cdevs max6956io cpp  In member function  void CMax6956Io  Init unsigned char  unsigned char  unsigned int     home     mystuff domoproject lib i2cdevs max6956io cpp 26 30  error  no matching function for call to  CMax6956Io  SetI2cSlaveAddress unsigned char amp     home     mystuff domoproject lib i2cdevs max6956io cpp 26 30  note  candidate is  In file included from  home geertvc mystuff domoproject lib i2cdevs       include i2cdevs max6956io h 35 0                   from  home geertvc mystuff domoproject lib i2cdevs max6956io cpp 1   home     mystuff domoproject lib i2cdevs       include i2chal i2chal h 65 9  note  int CI2cHal  SetI2cSlaveAddress unsigned char  bool   home     mystuff domoproject lib i2cdevs       include i2chal i2chal h 65 9  note    candidate expects 2 arguments  1 provided make 2        lib i2cdevs CMakeFiles i2cdevs dir max6956io cpp o  Error 1 make 1        lib i2cdevs CMakeFiles i2cdevs dir all  Error 2 make       all  Error 2   But when I added the default parameter in the header file of the base class      int SetI2cSlaveAddress  UCHAR addr  bool force   false      and removed it from the source file of the base class      int CI2cHal  SetI2cSlaveAddress  UCHAR addr  bool force     then the compiler was happy and all code worked as expected  I could give one or two parameters to the function SetI2cSlaveAddress      So  not only for the user of a class it s important to put the default value of a parameter in the header file  also compiling and functional wise it apparently seems to be a must

User · Answer

You may do in either  according to standard   but remember  if your code is seeing the declaration without default argument s  before the definition that contains default argument  then compilation error can come   For example  if you include header containing function declaration without default argument list  thus compiler will look for that prototype as it is unaware of your default argument values and hence prototype won t match   If you are putting function with default argument in definition  then include that file but I won t suggest that

User · Answer

One more point I haven t found anyone mentioned   If you have virtual method  each declaration can have its own default value   It depends on the interface you are calling which value will be used   Example on ideone  struct iface       virtual void test int a   0    std  cout  lt  lt  a        struct impl   public iface       virtual void test int a   5  override   std  cout  lt  lt  a        int main         impl d      d test        iface  a    amp d      a- gt test        It prints 50  I strongly discourage you to use it like this

User · Answer

Default parameter values must appear on the declaration  since that is the only thing that the caller sees   EDIT  As others point out  you can have the argument on the definition  but I would advise writing all code as if that wasn t true

User · Answer

Good question    I find that coders typically use the declaration to declare defaults  I ve been held to one way  or warned  or the other too based on the compiler  void testFunct int nVal1  int nVal2 500   void testFunct int nVal1  int nVal2        using namespace std      cout  lt  lt  nVal1  lt  lt   lt  lt  nVal2  lt  lt  endl

User · Answer

If the functions are exposed - non-member  public or protected - then the caller should know about them  and the default values must be in the header   If the functions are private and out-of-line  then it does make sense to put the defaults in the implementation file because that allows changes that don t trigger client recompilation  a sometimes serious issue for low-level libraries shared in enterprise scale development    That said  it is definitely potentially confusing  and there is documentation value in presenting the API in a more intuitive way in the header  so pick your compromise - though consistency s the main thing when there s no compelling reason either way

User · Answer

Adding one more point  Function declarations with default argument should be ordered from right to left and from top to bottom   For example in the below function declaration if you change the declaration order then the compiler gives you a missing default parameter error   Reason the compiler allows you to separate the function declaration with default argument within the same scope but it should be in order from RIGHT to LEFT  default arguments  and from TOP to BOTTOM order of function declaration default argument       declaration void function char const  msg  bool three  bool two  bool one   false   void function char const  msg  bool three   true  bool two  bool one      Error  void function char const  msg  bool three  bool two   true  bool one      OK   void function char const  msg  bool three   true  bool two  bool one      OK  int main         function  Using only one Default Argument   false  true       function  Using Two Default Arguments   false       function  Using Three Default Arguments        return 0       definition void function char const  msg  bool three  bool two  bool one         std  cout lt  lt msg lt  lt     lt  lt three lt  lt     lt  lt two lt  lt     lt  lt one lt  lt std  endl

User · Answer

You can do either  but never both  Usually you do it at function declaration and then all callers can use that default value  However you can do that at function definition instead and then only those who see the definition will be able to use the default value

User · Answer

the declaration is generally the most  useful   but that depends on how you want to use the class   both is not valid

[c++] Where to put default parameter value in C++?

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