Conditional Variable Declaring

I'm trying to get a function to accept several types of data (int float, char, etc) and preform the same function on them.
I'm overloading the function by using functions that accept different types and call the main function with the type.

Code:

float mmmm(void* array, int size, int type);

float avg(int* array, int size)
  {return mmmm(array, size, 1);}

float avg(long* array, int size)
  {return mmmm(array, size, 2);}

float avg(float* array, int size)
  {return mmmm(array, size, 3);}
//etc.

My main function then asignes a pointer to the pointer it has just been given and works with the data.

Code:

float mmmm(void* array, int size, int type)  {
  void* ptr = NULL;

   switch (type) {

      case 1:
        ptr = new int*;
        *ptr = (int*)array;
        break;

      case 2:
        ptr = new long*;
        *ptr = (long*)array;
       break;

     case 3:
        ptr = new float*;
        *ptr = (float*)array;
       break;
//etc.

Then to work with the data I use indirrection twice
This fragment adds the sum of the data in the array

Code:

long double sum;

  for(int i = 0; i < size; i++)
     sum += *(*ptr + i);

Why won't the compiler compile this?
How can I get a function to accept several types of data without writing out many copies of it?

I think you want a template function.

Something like:

Code:

 template<typename T>
 T avg(T arr[], size_t size)
 {
	T sum;

	for(int i = 0; i < size; i++)
		sum += arr[i];

	return sum;
 }