using only part of an array

Your function is going to need some way to determine the size of the array anyway. I suggest you make the function take begin and end iterators, in the style of the standard library algorithms, like so:

template<typename I>
void somefunction(I begin, I end);

Then, you can call it with your array like this:

somefunction(my_array, my_array + 24);
somefunction(my_array + 24, my_array + 39);
somefunction(my_array + 39, my_array + 65);

If you write the functions to operate on a pair of forward iterators rather than an array, you could just pass it like so:

somefunction1(my_array, my_array + 24);
somefunciton2(my_array + 24, my_array + 39);
somefunction3(my_array + 39, my_array + 65);

Pointers are forward iterators, and this would allow the functions to be used over parts of vectors, queues, or other STL containers as well.

The python example is making copies. If that's okay for your use case, you could do something like this (I'm swapping out your vanilla arrays for std::vector):

#include <iostream>
#include <vector>

void somefunction(std::vector<int> v) {
    std::cout << "vector has " << v.size() << " elements,"
        << " first value is " << *v.begin() << ","
        << " last value is " << *(v.end()-1) << std::endl;
}

int main() {
    std::vector<int> a;
    for (int i=0; i<65; i++) {
        a.push_back(i);
    }
    somefunction(std::vector<int>(a.begin(),a.begin()+23));
    somefunction(std::vector<int>(a.begin()+24,a.begin()+38));
    somefunction(std::vector<int>(a.begin()+39,a.begin()+65));
}

which outputs:

vector has 23 elements, first value is 0, last value is 22
vector has 15 elements, first value is 23, last value is 37
vector has 27 elements, first value is 38, last value is 64

But it sounds like you can't use std::vector, because somefunction() has a signature you can't change. Luckily, you can do similar gymnastics just manually copying parts of the array, as below:

#include <iostream>
#include <string.h>

void somefunction(int v[], int len) {
    std::cout << "vector has " << len << " elements,"
        << " first value is " << v[0] << ","
        << " last value is " << v[len-1] << std::endl;
}

int main() {
    int a[65];
    for (int i=0; i<65; i++) {
        a[i] = i;
    }
    int b[23];
    memcpy(b, a, 23*sizeof(int));
    somefunction(b, 23);
    int c[15];
    memcpy(c, a+23, 15*sizeof(int));
    somefunction(c, 15);
    int d[27];
    memcpy(d, a+38, 27*sizeof(int));
    somefunction(d, 27);
}

which again outputs:

vector has 23 elements, first value is 0, last value is 22
vector has 15 elements, first value is 23, last value is 37
vector has 27 elements, first value is 38, last value is 64