Variable Number of Arguments in Macro

[Praveen.Kumar.SP]

Hi

Could anyone solve the problem for the code below

The Code:

#include "stdio.h"
#include "iostream.h"

void Temp( int a, char* str,...)
{
//code to handle the arguments
}

#define MYPRINT(_x_) printf _x_
#define MYPRINT1(_x_) Temp( 10,_x_)

int main()
{
MYPRINT(("This is a test for multiple argument %s
%d",__FILE__,__LINE__));
MYPRINT1(("This is a test for multiple argument %s
%d",__FILE__,__LINE__));

return 0;
}

Problem:

In the first macro I am able to get the result as expected from the
printf where as in the second case my parameters are not properly
passed to the function Temp. Could anyone of you tell me why i am not
abel to use the macro to pass parameter to a function with some
mandatory number of parameter and variable number of parameter?

Is there any way that i can paa the parameter as i expected?how?


Thanks
praveen

 

[Victor Bazarov]

Could anyone solve the problem for the code below

The Code:

#include "stdio.h"
#include "iostream.h"

void Temp( int a, char* str,...)
{
//code to handle the arguments
}

#define MYPRINT(_x_) printf _x_
#define MYPRINT1(_x_) Temp( 10,_x_)

int main()
{
MYPRINT(("This is a test for multiple argument %s
%d",__FILE__,__LINE__));
MYPRINT1(("This is a test for multiple argument %s
%d",__FILE__,__LINE__));

return 0;
}

Problem:

In the first macro I am able to get the result as expected from the
printf where as in the second case my parameters are not properly
passed to the function Temp. Could anyone of you tell me why i am not
abel to use the macro to pass parameter to a function with some
mandatory number of parameter and variable number of parameter?

Because when the macro MYPRINT1 is substituted you get

Temp( 10,("This is..","filename.ext",123));

The extra set of parentheses around the arguments makes it a single
expression with two operators comma instead of part of the list of
arguments to the 'Temp' function. BTW, you get 123 where 'char*'
is expected. It's most likely undefined behaviour.

Is there any way that i can paa the parameter as i expected?how?

You most likely cannot. See if your compiler supports "variadic
macros" (macros with ellipsis).

V

 

[Alf P. Steinbach]

* (e-mail address removed):

Could anyone solve the problem for the code below

"The" problem? I count a multitude of problems. Which one?

The Code:

#include "stdio.h"

Use the <headername> form instead of "headername" for standard headers.
That way you avoid picking up a header with the same name in a local
directory.

#include "iostream.h"

This is not a standard header, and won't compile with e.g. Visual C++

void Temp( int a, char* str,...)

The second argument should be declared as

char const* str

unless you want the function Temp to be able to modify the contents of
'str'.

The ellipsis '...' should generally not be used in C++ code, because
it's /dangerous/ (not typesafe) and /limited/ (no non-POD objects);
there are much better typesafe solutions.

{
//code to handle the arguments
}

#define MYPRINT(_x_) printf _x_
#define MYPRINT1(_x_) Temp( 10,_x_)

Generally it's not a good idea to use macros. See this group's FAQ and
Bjarne Stroustrup's C++ FAQ for reasons why.

int main()
{
MYPRINT(("This is a test for multiple argument %s
%d",__FILE__,__LINE__));
MYPRINT1(("This is a test for multiple argument %s
%d",__FILE__,__LINE__));

return 0;
}

Problem:

In the first macro I am able to get the result as expected from the
printf where as in the second case my parameters are not properly
passed to the function Temp. Could anyone of you tell me why i am not
abel to use the macro to pass parameter to a function with some
mandatory number of parameter and variable number of parameter?

The second macro invocation does not work because it expands to

Temp( 10, ("some text",__FILE__,__LINE__));

which is syntactically invalid.

Is there any way that i can paa the parameter as i expected?how?

No, not as you expected.

A solution depends on what you want to achieve. Obviously it's not
what's illustrated by your code, because that could be much more easily
achieved by calling Temp directly without the macro. In other words,
you have illustrated a flawed solution to some problem, instead of the
problem itself -- to get help with that problem, explain it.

I suspect, though, that it has to do with logging or tracing?

 

[Victor Bazarov]

* (e-mail address removed):

[..]

The second macro invocation does not work because it expands to

Temp( 10, ("some text",__FILE__,__LINE__));

Really? You mean __FILE__ and __LINE__ do not get substituted?
Why? Have you tried it?

which is syntactically invalid.

Why is it invalid?

V

 

[Alf P. Steinbach]

* Victor Bazarov:

* (e-mail address removed):

[..]

The second macro invocation does not work because it expands to

Temp( 10, ("some text",__FILE__,__LINE__));
Really?

Yep.


You mean __FILE__ and __LINE__ do not get substituted?

No, I haven't written that; the result is churned once more through the
macro substitution machinery, and so on.

Understanding this becomes important when you have code like

#include <iostream>

#define VB( x ) #x

int main()
{
std::cout << VB(__FILE__) << std::endl;
}

where the macro invocation expands to

#__FILE__

which in the next round becomes

"__FILE__"

which results in the output of the string "__FILE__", not the source
code file name.

Why?

Because that's how macros work; look it up in your favorite C++ textbook.

Have you tried it?
No.



Why is it invalid?

There you caught me. ;-) It's not syntactically but semantically
invalid; sorry for the typo. The type of the comma expression is 'int',
whereas the Temp function requires a char* as second argument.

 

[Victor Bazarov]

* Victor Bazarov:

* (e-mail address removed):
[..]
The second macro invocation does not work because it expands to

Temp( 10, ("some text",__FILE__,__LINE__));

Really?

Yep.


[...about using the # operator...]

Why?

Because that's how macros work; look it up in your favorite C++
textbook.

Have you tried it?

No.

Well, do, then.

#define M(x) printf("%s", x)
#include <stdio.h>
int main()
{
M((__FILE__));
}

And then turn to *your* favourite C++ textbook.

There you caught me. ;-) It's not syntactically but semantically
invalid; sorry for the typo. The type of the comma expression is
'int', whereas the Temp function requires a char* as second argument.

V

 

[Alf P. Steinbach]

* Victor Bazarov:

* Victor Bazarov:

Alf P. Steinbach wrote:
* (e-mail address removed):
[..]
The second macro invocation does not work because it expands to

Temp( 10, ("some text",__FILE__,__LINE__));
Really?

Yep.


[...about using the # operator...]

No, what you completely snipped was about macro expansion.

Well, do, then.

#define M(x) printf("%s", x)
#include <stdio.h>
int main()
{
M((__FILE__));
}

And then turn to *your* favourite C++ textbook.

That's not an example of anything discussed previously, and, since I
don't think you don't know that: I resent that kind of discussion technique.

It doesn't remove the egg on your face.

For that you need to employ a strong egg-remover, not a context-remover.

 

[Ron Natalie]

Hi

Could anyone solve the problem for the code below

C now has varadic macros. C++ doesn't (nor does it
support overloading of macros).

 

[Frederick Gotham]

Ron Natalie posted:

C now has varadic macros. C++ doesn't (nor does it
support overloading of macros).

Not really, given that nobody uses C99.

 

[Ian Collins]

Ron Natalie posted:






Not really, given that nobody uses C99.

I do, so does my choice of OS.

 

[ax]

Hi
Could anyone solve the problem for the code below

The Code:

#include "stdio.h"
#include "iostream.h"

void Temp( int a, char* str,...)
{
//code to handle the arguments
}

#define MYPRINT(_x_) printf _x_
#define MYPRINT1(_x_) Temp( 10,_x_)

int main()
{
MYPRINT(("This is a test for multiple argument %s
%d",__FILE__,__LINE__));
MYPRINT1(("This is a test for multiple argument %s
%d",__FILE__,__LINE__));

return 0;
}

it seems easy in assembly
xxxx:
xxx xx x, x

_MYPRINT1:
xxx xxxxxxxxxxx
xxxx xx
xxxx _Temp
xxxx xxxxxxxxxxx
xxx

to call in this way
MYPRINT1("This is a test for multiple argument %s%d",
__FILE__,__LINE__);
but it is OT and so i have write 'x' where there are alphabeticals
letters (don't counting _Temp)

but hey, here *you* are all the smart guys so find some other solution
in C++