Is there optional parameter of function in C/C++?

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Hi everyone,
I want to use an optional parameter of a function (in C/C++), how to do this? Thanks.

Posted 17-Aug-11 17:42pm

Andrewpeter

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7 solutions

Solution 6

I see from previous solutions (they are quite right) you want to use optional parameters for a call into a dll function. for these functions you should use struct types as parameter:

typedef struct
{
  unsigned int cbSize;
  unsigned int fValid;
  int          param1;
  int          param2;
 // and so on
} DLL_PARAM;
void ExportFunction(DLL_PARAM* param)
{
  if(!param || (sizeof(DLL_PARAM)!=param->cbSize)) return;
  if(1 & param->fValid)
  {
    // param1 is valid
  }
  if(2 & param->fValid)
  {
    // param2 is valid
  }
  // and so on
}

for the caller

void usedll()
{
  DLL_PARAM  param={ sizeof(DLL_PARAM),0, };
  param.param1 = 100; param.fValid |= 1;
  // optional
  // param.param2 = 333; param.fValid |= 2;
  dllfunction(¶m);
}

Good luck.

Posted 18-Aug-11 2:53am

mbue

Comments

Andrewpeter 20-Aug-11 22:04pm

Thanks, my 5 for your post.

Solution 4

Best Example of function with optional variable is
printf
scanf
sprintf

Posted 17-Aug-11 21:42pm

Mohibur Rashid

Solution 5

Another possibility is this:

C++

void func( int iFirst, int iOptionalSecond = 0);

In reality, this isn't "optional" parameters, but it can be called without the second parameter, in which case it defaults to 0. So this could be called as either of these:

C++

func(5);
func(5, 20);

Hope that helps.

Posted 18-Aug-11 0:43am

krmed

Updated 18-Aug-11 5:02am

v2

Solution 7

Hi everyone, thank you for your supports, but I want to use C++ language to create a dll file with optional parameters and use it in VB6, e.g:

C++

int _stdcall f(int x, int option1, int option2)

and use it in VB6:

VB

Private Declare Function f Lib "abc.dll"(Byval x As Integer, Byval Option1 As Integer, Byval Option2 As Integer)   ' Declare function
Private Sub Form_Load()
MsgBox f(2)
' Or
MsgBox f(2,1)
' Or
MsgBox f(2,1,3)
End Sub

Hope you help me. Thanks.

Posted 20-Aug-11 16:06pm

Andrewpeter

Comments

Philippe Mori 25-Aug-11 19:57pm

I doubt that you will be able to uses default defined in C++ from VB. I think, you would have to make a wrapper on VB side or uses overloadding. You might be able to duplicate default values in your VB declaration (but if you change the default in C++, you will have to update in in VB also).

Solution 8

yes you can create.
As one of the friend here hav already mentioned the use of default parameters.
you can use that concept.

create a dll as follows:

int func(int a, int b =0; bool b = false);

and while calling these functions in the vb.
if required u pass the parameters else call it with one mandatory parameters as f olows:

1) func(a = 20);
2) func(a= 20,b= 30);
3) func(a= 20,b= 30, true);

but you have to pass the mandatory parameter(parameter 1) in all the cases.

Thanks,
Arun P.

Posted 22-Aug-11 2:03am

Arun Parthasarathy

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Prerak Patel · Accepted Answer · 2011-08-17T17:57:00

Solution 1

C++ has following approach.

Function overloading

C++

void func( int required );
void func( int required, int optional );

Default parameters

C++

void func( int required, int optional = 0 );

Posted 17-Aug-11 17:57pm

Prerak Patel

Comments

Andrewpeter 18-Aug-11 0:31am

Thanks, voted.

Prerak Patel 18-Aug-11 1:07am

OP:Thank Prerak Patel, but if I write a dll file using function overloading and export these functions, how to do in this case?

Prerak Patel 18-Aug-11 1:08am

It's not clear to me, but did you try making a dll like this? What problem do you face? If the answer has helped you, you should mark it as answer.

axel sch. · Accepted Answer · 2011-08-17T20:54:00

Another possiblity is to use the variable argument list for C functions.

See http://msdn.microsoft.com/en-us/library/kb57fad8.aspx[^] for a detailed description of the commands and the usage.

It is used also in the known command printf() where it is possible to have more than one argument given to the function.

The function declaration would look like

int func(int iArg, ...);

Regards
Axel

Is there optional parameter of function in C/C++?

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