function call argument count

function call argument count

[don fisher]

Hello,

Is there a call that can be made from within a function to determine 
how many arguments have been passed to it? I feel such a call would be 
of great assistance when used in conjunction with variable length 
argument lists. In the "old days", VMS pushed a parameter count as 
part of the call frame. You could even determine which variables were 
null, since they always did call by reference and null parameters were 
indicated by a zero address.

Sorry if this is well known, or an invalid request for some reason.
thanks
don
-- 
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|    Don Fisher				  dfisher@as.arizona.edu  |
|    Steward Observatory		  			  |
|    933 N. Cherry Ave.    		  VOICE: (520)621-7647	  |
|    University of Arizona		  FAX:   (520)621-9843    |
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Re: function call argument count

[Eljay Love-Jensen]

Hi Don,

There's not an easy way that is platform agnostic to determine the argument 
count.  You can do it yourself as a convention:

void MyFunc(int argcount, ...);

Then call it with the explicit argument count, via:

int a=1, b=2, c=3;
MyFunc(3, a, b, c);

I recommend that variable arguments be passed in as a std::vector<> or 
std::map<> or std::set<> or whatever kind of container is appropriate.  Or 
better yet, as generic begin() and end() iterators to the aforementioned 
containers.  By "generic" I mean using templates.

HTH,
--Eljay

Re: function call argument count

[Chris Wolstenholme]

Of course using a vector, map, etc. only works if all the parameters are of
the same type. Using the explicit count is a better option for different
argument types (if the type order is known).

I sometimes use some defined value before each variable to say there is
another parameter and to give a clue to the type, and then use a different
value to define the end of the list. These can be #defines or an enumerated
type. For example:

enum Var_Types
{
    End = 0, Int, Double, String
};

void myFunc(Var_Types more,...);

Then:

myFunc(End);

would mean no parameters and

myFunc(Int, 1, Int, 3, Double, 5.8, String, "Hello", String, "Goodbye",
End);

You then test the Var_Types value, and the next parameter according to it.
You could use something similar for a parameter list with all the same type
if you have a value you know won't be passed in and can use it as a
terminator - e.g. expecting positive integers could end with a value of -1.

This was just in case you're interested in lists with different types... :-)

Chris

----- Original Message ----- 
From: "Eljay Love-Jensen" <eljay@adobe.com>
To: "don fisher" <dfisher@as.arizona.edu>; <gcc-help@gcc.gnu.org>
Sent: Monday, May 24, 2004 1:30 PM
Subject: Re: function call argument count


> Hi Don,
>
> There's not an easy way that is platform agnostic to determine the
argument
> count.  You can do it yourself as a convention:
>
> void MyFunc(int argcount, ...);
>
> Then call it with the explicit argument count, via:
>
> int a=1, b=2, c=3;
> MyFunc(3, a, b, c);
>
> I recommend that variable arguments be passed in as a std::vector<> or
> std::map<> or std::set<> or whatever kind of container is appropriate.  Or
> better yet, as generic begin() and end() iterators to the aforementioned
> containers.  By "generic" I mean using templates.
>
> HTH,
> --Eljay
>
>

Re: function call argument count

[Justinas]

On Sun, 23 May 2004 15:01:04 -0700
don fisher <dfisher@as.arizona.edu> wrote:

> Hello,
> 
> Is there a call that can be made from within a function to determine 
> how many arguments have been passed to it? I feel such a call would be 
> of great assistance when used in conjunction with variable length 
> argument lists. In the "old days", VMS pushed a parameter count as 
> part of the call frame. You could even determine which variables were 
> null, since they always did call by reference and null parameters were 
> indicated by a zero address.
> 
> Sorry if this is well known, or an invalid request for some reason.
> thanks
> don
> -- 
> -------------------------------------------------------------------
> |    Don Fisher				  dfisher@as.arizona.edu  |
> |    Steward Observatory		  			  |
> |    933 N. Cherry Ave.    		  VOICE: (520)621-7647	  |
> |    University of Arizona		  FAX:   (520)621-9843    |
> |    Tucson, AZ  85721                				  |
> -------------------------------------------------------------------
> 

hi, one idea: try to compare esp, a stack pointer(x86, i'm not sure how ir would be on other platforms). Sure, this works when you know the size of variable, and they should be same type...



Justinas.