Function prototype with optional parameters?

[Joerg Toellner]

Hi Group,

sorry, its again me with a dumb question.

If i have a function with the optional parameter "mode" like this:

void myfunc(char *blah, int mode = NULL)
{
if(mode == NULL)
do_something();
else
do_something_else();
}

and i want to have a prototype for this function in my header-file and so i
write:

void myfunc(char *blah, int mode = NULL);

I get a compile error: redefenition of mode

If i write:

void myfunc(char *blah, int mode);

and i use the function with a function call like this:

myfunc("Hello World");

I got a compile error: function does not take 1 Parameter.

But i want to have the mode-param. optional. So you can call the function
with two parameters or with one and using the default value for "mode".

How have i to do this to keep the compiler (and me :-)) ) happy?

Someone out there who is willing to enlighten me a little bit? Thanks a
loooot!
CU
Joerg Toellner

[Jakob Bieling]

"Joerg Toellner" <to******@oss-gmbh.de> wrote in message
news:bf*************@news.t-online.com...

If i have a function with the optional parameter "mode" like this:

void myfunc(char *blah, int mode = NULL)
{
if(mode == NULL)
do_something();
else
do_something_else();
}

and i want to have a prototype for this function in my header-file and so i write:

void myfunc(char *blah, int mode = NULL);

I get a compile error: redefenition of mode

Right, because the default value for the optional argument shall onl
appear once. If you have a function prototype, the default argument goes
there, and only there. Like this:

void myfunc(char *blah, int mode = NULL);

void myfunc(char *blah, int mode)
{
if(mode == NULL)
do_something();
else
do_something_else();
}

hth
--
jb

(replace y with x if you want to reply by e-mail)

[Jan Rendek]

>

If i have a function with the optional parameter "mode" like this:

void myfunc(char *blah, int mode = NULL)
{
if(mode == NULL)
do_something();
else
do_something_else();
}

and i want to have a prototype for this function in my header-file and so i
write:

void myfunc(char *blah, int mode = NULL);

Define the default param value in the header:

..hpp: void myfunc(char *blah, int mode = NULL);

but NOT in your source file:

..cpp: void myfunc(char *blah, int mode) { /* WHATEVER */ }
Take no offence, but by the look of your questions, you ought to get
a decent C++ book. Everything you asked so far should be in the first
chapters.

regards,

Jan

--
Jan Rendek
INRIA Lorraine
r e n d e k @ l o r i a . f r

[Joerg Toellner]

Hi Jakob,

AAAAAAAH! I seeeee!
*SomeoneSwitchesABigLightBulbOverMyHeadToOnState*

Thanks a lot, that makes sense!
Why haven't not thought about that by myself? *ShameRedChangingFaceColor*

Thx. again...and have a nice day!
CU
Joerg Toellner

"Jakob Bieling" <ne*****@gmy.net> schrieb im Newsbeitrag
news:bf*************@news.t-online.com...

Right, because the default value for the optional argument shall onl
appear once. If you have a function prototype, the default argument goes
there, and only there. Like this:

void myfunc(char *blah, int mode = NULL);

void myfunc(char *blah, int mode)
{
if(mode == NULL)
do_something();
else
do_something_else();
}

hth
--
jb

[Joerg Toellner]

Hi Peter,

Wow! What an Idea with the function overloading. I know function overloading
and used it where i have different Inputs for the same function. But never
thought of this usage for defaulting parameters. But it make sense.

I'll look, if i can get the recommended book.

TYVM
Joerg Toellner

"Peter van Merkerk" <me*****@deadspam.com> schrieb im Newsbeitrag
news:bf************@ID-133164.news.uni-berlin.de...

See above. Alternatively you could create an overloaded variant of the
function.

--------foo.h-----------------
void myfunc(char *blah);
void myfunc(char *blah, int mode);

--------foo.cpp-----------------
void myfunc(char *blah)
{
do_something();
}

void myfunc(char *blah, int mode)
{
do_something_else();
}

Item 24 of the book "Effective C++" from Scott Meyers (highly
recommended) discusses how to choose beteen function overloading and
parameter defaulting.