Variable Number of Arguments in Macro

Discussion in 'C++' started by Praveen.Kumar.SP@gmail.com, Jun 29, 2006.

  1. Guest

    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
    , Jun 29, 2006
    #1
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  2. wrote:
    > 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
    --
    Please remove capital 'A's when replying by e-mail
    I do not respond to top-posted replies, please don't ask
    Victor Bazarov, Jun 29, 2006
    #2
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  3. * :
    >
    > 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++
    7.1 or better. Use <iostream> instead. <iostream> is a standard header.


    > 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?

    --
    A: Because it messes up the order in which people normally read text.
    Q: Why is it such a bad thing?
    A: Top-posting.
    Q: What is the most annoying thing on usenet and in e-mail?
    Alf P. Steinbach, Jun 29, 2006
    #3
  4. Alf P. Steinbach wrote:
    > * :
    >> [..]

    >
    > 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
    --
    Please remove capital 'A's when replying by e-mail
    I do not respond to top-posted replies, please don't ask
    Victor Bazarov, Jun 29, 2006
    #4
  5. * Victor Bazarov:
    > Alf P. Steinbach wrote:
    >> * :
    >>> [..]

    >> 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.


    >> which is syntactically invalid.

    >
    > 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.


    --
    A: Because it messes up the order in which people normally read text.
    Q: Why is it such a bad thing?
    A: Top-posting.
    Q: What is the most annoying thing on usenet and in e-mail?
    Alf P. Steinbach, Jun 29, 2006
    #5
  6. Alf P. Steinbach wrote:
    > * Victor Bazarov:
    >> Alf P. Steinbach wrote:
    >>> * :
    >>>> [..]
    >>> 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.

    >>> which is syntactically invalid.

    >>
    >> 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.


    V
    --
    Please remove capital 'A's when replying by e-mail
    I do not respond to top-posted replies, please don't ask
    Victor Bazarov, Jun 29, 2006
    #6
  7. * Victor Bazarov:
    > Alf P. Steinbach wrote:
    >> * Victor Bazarov:
    >>> Alf P. Steinbach wrote:
    >>>> * :
    >>>>> [..]
    >>>> 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.


    >>> 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.


    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.


    --
    A: Because it messes up the order in which people normally read text.
    Q: Why is it such a bad thing?
    A: Top-posting.
    Q: What is the most annoying thing on usenet and in e-mail?
    Alf P. Steinbach, Jun 29, 2006
    #7
  8. Ron Natalie Guest

    wrote:
    > 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).
    Ron Natalie, Jun 30, 2006
    #8
  9. 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.


    --

    Frederick Gotham
    Frederick Gotham, Jun 30, 2006
    #9
  10. Ian Collins Guest

    Frederick Gotham wrote:
    > 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.
    >

    I do, so does my choice of OS.

    --
    Ian Collins.
    Ian Collins, Jun 30, 2006
    #10
  11. ax Guest

    On 29 Jun 2006 07:05:00 -0700, wrote:
    >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++ :)
    ax, Jun 30, 2006
    #11
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