Re: How to pass the field name of a struct as a parameter?

From:

Markus Schoder <a3vr6dsg-usenet@yahoo.de>

Newsgroups:

comp.lang.c++

Date:

Thu, 29 Jun 2006 03:05:41 +0200

Message-ID:

<44a32766$0$29128$9b4e6d93@newsread4.arcor-online.net>

Jellicle wrote:

There are some sturct arrays (or struct vectors), such as:

struct S1_t
{
        double keyField ;

      ..// other fields
}

S1_t s1[100];
std::vector<S1_t> vecS1;

I need to find the index of the item in array "s1" or vector "vecS1"
which
has the minimal value in field "keyField".

Some codes could be:
//----------------------------------------------

   double minvalue = s1[0];
   for (int i=0; i< 100; i++ ) // for (int i=0; i< vecS1.size(); i++)
...
   {
          if (s1[i].keyField < minvalue )
           {
                   minvalue =s1[i].keyField;
                   index = i;
           }
   }
//----------------------------------------------

However, since there are many such arrays(vectors) with different
struct type
(so, with different "field" name)

How can I define a function (or MACRO?) to get the index
of the minumal item (by comparing the the value of a key field)?

such as MIN(struct_array_variable, field_name)

e.g.

    index = MIN(s1, keyField);

to get the index of array "s1" with a minimal value of "keyField".

A STL style solution using iterators would be:

#include <iterator>

template<class Iterator, typename T> Iterator minfield(Iterator start,
  const Iterator &end, T std::iterator_traits<Iterator>::value_type::*p)
{
  T acc = (*start).*p;
  Iterator min_i(start);
  ++start;
  while(start != end)
  {
    const T &r = (*start).*p;
    if(r < acc)
    {
      acc = r;
      min_i = start;
    }
    ++start;
  }
  return min_i;
}

Usage would be:

index = minfield(s1, s1 + 100, &S1_t::keyfield) - s1;

or

index = minfield(vecS1.begin(), vecS1.end(), &S1_t::keyfield) -
  vecS1.begin();

Note that the function assumes that there is at least one value in the range
[start, end).