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 · Accepted 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);

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

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

[c++] Default value of function parameter

lib.h

lib.cpp

test.cpp

lib.h

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