Default value of function parameter

Question

1   int Add  int a  int b   3   int Add  int a  int b         2   int Add  int a  int b   int Add  int a  int b   3         Both work  which is the standard way and why

User · Answer

The first way would be preferred to the second.

This is because the header file will show that the parameter is optional and what its default value will be. Additionally, this will ensure that the default value will be the same, no matter the implementation of the corresponding .cpp file.

In the second way, there is no guarantee of a default value for the second parameter. The default value could change, depending on how the corresponding .cpp file is implemented.

User · Answer

On thing to remember here is that the default param must be the last param in the function definition  Following code will not compile  void fun int first  int second   10  int third    Following code will compile  void fun int first  int second  int third   10

User · Answer

Default arguments must be specified with the first occurrence of the function name   typically  in the function prototype  If the function prototype is omitted because the function definition also serves as the prototype  then the default arguments should be specified in the function header

User · Answer

In C   the requirements imposed on default arguments with regard to their location in parameter list are as follows     Default argument for a given parameter has to be specified no more than once  Specifying it more than once  even with the same default value  is illegal  Parameters with default arguments have to form a contiguous group at the end of the parameter list    Now  keeping that in mind  in C   you are allowed to  grow  the set of parameters that have default arguments from one declaration of the function to the next  as long as the above requirements are continuously satisfied   For example  you can declare a function with no default arguments  void foo int a  int b     In order to call that function after such declaration you ll have to specify both arguments explicitly    Later  further down  in the same translation unit  you can re-declare it again  but this time with one default argument  void foo int a  int b   5     and from this point on you can call it with just one explicit argument   Further down you can re-declare it yet again adding one more default argument  void foo int a   1  int b     and from this point on you can call it with no explicit arguments   The full example might look as follows  void foo int a  int b    int main       foo 2  3      void foo int a  int b   5      redeclare   foo 8      OK  calls  foo 8  5      void foo int a   1  int b      redeclare again   foo       OK  calls  foo 1  5      void foo int a  int b                 As for the code in your question  both variants are perfectly valid  but they mean different things  The first variant declares a default argument for the second parameter right away  The second variant initially declares your function with no default arguments and then adds one for the second parameter   The net effect of both of your declarations  i e  the way it is seen by the code that follows the second declaration  is exactly the same  the function has default argument for its second parameter  However  if you manage to squeeze some code between the first and the second declarations  these two variants will behave differently  In the second variant the function has no default arguments between the declarations  so you ll have to specify both arguments explicitly

User · Answer

If you put the declaration in a header file  and the definition in a separate  cpp file  and  include the header from a different  cpp file  you will be able to see the difference   Specifically  suppose   lib h  int Add int a  int b     lib cpp  int Add int a  int b   3               test cpp   include  lib h   int main         Add 4       The compilation of test cpp will not see the default parameter declaration  and will fail with an error   For this reason  the default parameter definition is usually specified in the function declaration   lib h  int Add int a  int b   3

[c++] Default value of function parameter

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