Comparing C and C++ data structure impl https://godbolt.org/z/hnEvd8Wx1

fenwick_tree.cpp

#include <vector>
#include <iostream>

struct fenwick_tree {
    size_t len; // 0-based len
    std::vector<long long> t; // 1-based tree, indexes [1:len]

    fenwick_tree(std::vector<long long> const &a) 
    {
        len = a.size();
        t.assign(len + 1, 0);
        for (size_t i = 0; i < len; ++i)
            add_to(i, a[i]);
    }

    long long sum_to(size_t r) 
    {
        long long s = 0;
        for (++r; r > 0; r -= r & -r)
            s += t[r];
        return s;
    }

    void add_to(size_t i, long long delta) 
    {
        for (++i; i <= len; i += i & -i)
            t[i] += delta;   
    }
};


int main() 
{
    std::vector<long long> a(10, 1);
    fenwick_tree ft(a);
    for (int i = 0; i < 10; ++i)
        std::cout << ft.sum_to(i) << " ";

}

fenwick_tree_array.c

#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>

typedef long long* fenwick_tree; // plain array data structure

long long fenwick_sum_to(fenwick_tree ft, size_t r) 
{
    long long s = 0;
    for (++r; r > 0; r -= r & -r)
        s += ft[r];
    return s;
}

// len is the length of the original 0-based array
void fenwick_add_to(fenwick_tree ft, size_t len, size_t i, long long delta) 
{
    for (++i; i <= len; i += i & -i)
        ft[i] += delta;
}

// array can init globally if size is known 
fenwick_tree fenwick_init(long long a[], size_t len) 
{
    fenwick_tree ft = calloc(len + 1, sizeof(long long));
    for (size_t i = 0; i < len; ++i) 
        fenwick_add_to(ft, len, i, a[i]);
    return ft;
}

int main() 
{
    long long a[10] = {1, 1, 1, 1, 1, 1, 1, 1, 1, 1};
    fenwick_tree ft = fenwick_init(a, 10);

    for (int i = 0; i < 10; ++i)
        printf("%lld\n", fenwick_sum_to(ft, i));
}

fenwick_tree_struct.c

#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>

typedef struct {
    size_t len; // 0-based len
    long long* t; // 1-based tree, indexes [1:len]
} fenwick_tree;


long long fenwick_sum_to(fenwick_tree ft, size_t r) 
{
    long long s = 0;
    for (++r; r > 0; r -= r & -r)
        s += ft.t[r];
    return s;
}

// len is the length of the original 0-based array
void fenwick_add_to(fenwick_tree ft, size_t i, long long delta) 
{
    for (++i; i <= ft.len; i += i & -i)
        ft.t[i] += delta;
}

fenwick_tree fenwick_init(long long a[], size_t len) 
{
    fenwick_tree ft;
    ft.len = len;
    ft.t = calloc(len + 1, sizeof(long long));
    for (size_t i = 0; i < len; ++i)
        fenwick_add_to(ft, i, a[i]);
    return ft;
}


int main() 
{
    long long a[10] = {1, 1, 1, 1, 1, 1, 1, 1, 1, 1};

    fenwick_tree ft = fenwick_init(a, 10);
    for (int i = 0; i < 10; ++i)
        printf("%lld\n", fenwick_sum_to(ft, i));

}