C++ :: Calculate The Size Of Values?

I'm trying to calculate the size of values I've dynamically allocated memory to, i.e.

col_ind = new long[nz];
row_ptr = new long[inrows+1];
val = new double[nz];

from pointers. Now I've created the function,

int getmemsize(int i) {
int sizes[3];
sizes[0] = sizeof val;
sizes[1] = sizeof col_ind;
sizes[2] = sizeof row_ptr;
return sizes[i];
}

but each one is returning just the value 4? My arrays (one example) look like:

val = [ 5 2 5 9 8 3 9 10 9 ]
col_ind = [ 0 1 1 ]
row_ptr = [ 0 2 3 ]