Indexed Tree with template

#include <cstdio>
#include <functional>

using namespace std;

template <typename _T>
class IndexedTree {
private:
	size_t m;
	_T *A;
	function<_T(const _T &, const _T &)> F;
public:
	IndexedTree(size_t n, function<_T(const _T &, const _T &)> f) {
		for (m = 1; m < n; m <<= 1);
		A = new _T[m << 1];
		F = f;
	}
	void Update(size_t idx, _T val) {
		idx |= m;
		A[idx] = val;
		while (idx >>= 1) A[idx] = F(A[idx << 1], A[idx << 1 | 1]);
	}
	_T Query(size_t L, size_t R) {
		_T Lval = _T(), Rval = _T();
		L |= m;
		R |= m;
		while (L <= R) {
			if (L & 1) Lval = F(Lval, A[L++]);
			if (!(R & 1)) Rval = F(A[R--], Rval);
			L >>= 1;
			R >>= 1;
		}
		return F(Lval, Rval);
	}
};

int a[10] = { 6, 2, 3, 8, 4, 5, 1, 9, 7, 10 };

int main() {
	IndexedTree<int> IT(10, [](auto u, auto v) { return u + v; });
	for (int i = 0; i < 10; i++) IT.Update(i, a[i]);
	printf("%d %d : %d\n", 2, 6, IT.Query(2, 6));
	printf("%d %d : %d\n", 3, 8, IT.Query(3, 8));
	return 0;
}