template function unresolved external symbol [duplicate]

This question already has an answer here:

• Why can templates only be implemented in the header file? 9 answers

I have a function that I put in a separate file with my other utility functions, with a separate header

The functions is a simple array sort:

template<class TYPE1>
void sorted_index(const TYPE1* arr, int n, int* indices)
{
    int i, j;

    for (i = 0; i < n; i++)
    {
        indices[i] = i;
    }

    for (i = 0; i<n; i++)
    {
        for (j = i + 1; j<n; j++)
        {
            if (arr[indices[i]] > arr[indices[j]])
            {
                //swap
                int temp = indices[i];
                indices[i] = indices[j];
                indices[j] = temp;
            }
        }
    }
}

And I use it like this somewhere else in another file:

myfloat testvar[6] = { 2.3, 2.1, 2.2, 5, -1, -2.5 };
int indices[6];

sorted_index(testvar, 6, indices);

I get this error:

error LNK2019: unresolved external symbol "void __cdecl sorted_index(double const *,int,int *)"

If I eliminate the template definition, it compiles, so I know there isn't a dependency issue.

Why am I getting that error? Thanks