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Function "Overload" Question


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#1 PeterF   Members   -  Reputation: 562

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Posted 17 June 2012 - 05:00 PM

This is a long shot, but is there any way, in C++, to overload a function such that it takes an arbitrary number of parameters (without explicitly overloading it a thousand times) and can evaluate various control structures and/or output the "parameter number" (ie foo(a, b, c), where "a" is parameter 0, "b" is parameter 1 etc? Obviously each of the arbitrary number of parameters would have to be the same type. An example of my ideal solution...:

[source lang="cpp"]foo(any number of std::strings){ std::string str; cin >> str; if (str == any of the input strings){ return parameter_number_of_the_string_it's_equal_to; } // if} // foo[/source]

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#2 BattleMetalChris   Members   -  Reputation: 157

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Posted 17 June 2012 - 06:33 PM

Could you not just pass a reference to a vector of the parameters? This also lets you use templates if you plan on having several versions for different parameter types.

typedef std::vector<std::string> ParamsVec;



int foo(ParamsVec& params)

{

	 std::string str;

	 cin >> str;



	 int count = 0;

	 for (ParamsVec::iterator it = params.begin(); it != params.end(); ++it)

	 {

	 	 if ((*it) == str) return count;

	 	 count++;

	 }

}




EDIT: aargh, stupid source tags, why do they cut off angle brackets if I specify a language type?...

Edited by BattleMetalChris, 17 June 2012 - 06:50 PM.


#3 PeterF   Members   -  Reputation: 562

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Posted 17 June 2012 - 06:51 PM

That works, thanks. I was just wondering if there was actually something like what I described with the arbitrary number of separate parameters, as it would cut out the step of creating the vector. Otherwise I can live with that.

#4 SiCrane   Moderators   -  Reputation: 9338

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Posted 17 June 2012 - 09:11 PM

If using a C++11 compiler you can use variadic templates.

#5 krippy2k8   Members   -  Reputation: 642

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Posted 18 June 2012 - 02:24 AM

I'd just like to add that you don't necessarily need to use the same type for your parameters. You can use a variant type to wrap different parameter types and pass them in a vector to your function. Look at Boost.Any for a very simple, elegant and fairly efficient variant type. Definitely faster than doing things like converting strings to integers or floats.

You can also roll out a bunch of template functions to help automate the process of creating the vectors to make the code easier to use. i.e:

void foo( const std::vector<boost::any>& params )
{
   //iterate params and work!
}

template<typename T1>
void foo( T1 p1 )
{
   std::vector<boost::any> params;
   params.push_back(p1);
   foo(params);
}

template<typename T1 p1, typename T2 p2>
{
   std::vector<boost::any> params;
   params.push_back(p1);
   params.push_back(p2);
   foo(params);
}
///etc up to Tn/Pn


Then you just need to put your control logic in the first function, and be able to make calls like this:

foo( 1, 0.5f, "whatever", new SomeUserType() );

You could also use some C++ macros/metaprogramming to generate as many template functions as you need without having to create them all explicitly.

Not quite as handy as variadic templates, but should work on any(most?) C++03 compiler.

#6 Eigen   Members   -  Reputation: 519

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Posted 18 June 2012 - 04:10 AM

Variable argument list?

#include <stdarg.h>

void vaFunc(int stringCount, ...)
{
  va_list vl;
  va_start(vl, stringCount);

  for(int i = 0; i < stringCount; ++i)
  {
	printf("arg%d = %s\n", i, va_arg(vl, char *));
  }

  va_end(vl);
}

vaFunc(2, "hello", "world");



This function expects all arguments to be char *, but if you want them to be to be of different type, you need to differate between them somehow. Take a look at this for an idea: http://msdn.microsof...9(v=vs.80).aspx

Edited by Eigen, 18 June 2012 - 04:16 AM.


#7 Eigen   Members   -  Reputation: 519

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Posted 18 June 2012 - 04:36 AM

Scrap my previous post, I misread the topic. What you actually need, is this:


int countArguments(const char * arg, ...)
{
  va_list vl;
  va_start(vl, arg);

  printf("arg0 = %s\n", arg);

  int i = 1;
  while((arg = va_arg(vl, const char *)) != NULL)
  {
	printf("arg%d = %s\n", i, arg);
	i++;
  }

  va_end( vl );

  return i;
}

printf("Argument count: %d\n", countArguments("Hello", "world", NULL); // Pass 0 or NULL as the last parameters, to detect the end of argument list


#8 ChaosEngine   Crossbones+   -  Reputation: 2106

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Posted 18 June 2012 - 07:49 PM

"Basically whenever you invoke the dread ellipses construct you leave the happy world of type safety." — SiCrane



:)


if you think programming is like sex, you probably haven't done much of either.-------------- - capn_midnight

#9 Ryan_001   Prime Members   -  Reputation: 1207

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Posted 19 June 2012 - 04:08 AM

There's always the option of using macros to define how ever many versions of the function you want. Its very messy but it does work. I suggest using the boost pre-processor library to help.




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