Variable number of parameters and function template

Question

C++ / C#

I have a template class that implements a matrix (cells can contain data of different types).
to fill the matrix, the function is used:

void setItems(int _row, int _col, ...){

    fieldInitialization(_row, _col);

    int argumentNumber = _row * _col;
    va_list arguments;
    va_start(arguments, argumentNumber);
    for (int i = 0 ; i < _row ; i++){
      for (int j = 0 ; j < _col ; j++){
        tab[i][j] = va_arg(arguments, T);
      }
    }
    va_end(arguments);
  }

The compiler warns:

second parameter of ‘va_start’ not last named argument [enabled by default]

and

- ‘char’ is promoted to ‘int’ when passed through ‘...’ [enabled by default]
  - (so you should pass ‘int’ not ‘char’ to ‘va_arg’)
  - if this code is reached, the program will abort

When filling with ints, everything is OK,
as soon as I switch to char, the program stops as the compiler warned :(
I ask for tips / help on how to implement the extraction of arguments in advance without knowing the type.
ps The task is educational, therefore, for example, you can’t use specialization.

Answer 1

It seems to me that there is an error here:
va_start(arguments, argumentNumber);
The second argument to the va_start macro is the last known argument in the function. The compiler even tells you about it.
For example:

int magicFunction(int a, int b, int c, ...) {
    va_list args;
    va_start(args, c);
   //...
}

Therefore, you should write it like this:
By the way, if you have the opportunity to use a debugger, look at the stack when calling this function, and compare the addresses where argumentNumber and _col are located. So the first option (c int) is also non-working.