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

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".

    And ,

     index = MIN(s2,f2) get that of "s2" (defined as follows:)

struct s2_t
{
   int f2;

   // other fields

}

s2_t s2[100]

A similar function is FIND(struct_array, field, value)
to find the index from array "struct_array", where

struct_array[index].field == value;

I think that the sticking point is that how to pass the field name
as a parameter? isn't it?

Could anyone help me?

Thank you very much!