Implementation of radix sort in c++

radix_sort.cpp

#include <queue>

int select_digit (int number, int place)
{ return number/place % 10; }


/*
	Create an array of 10 buckets, each storing an empty queue (use ArrayQueue)
	for every place (6 digit numbers: 1s, 10s, 100s, 1,000s, 10,000s, and 100,000s)
		for every value in array a
			put it in the correct bucket using select_digit (for the current place)
		for every bucket (in the order 0 to 9)
			move each of the values, from its queue, back into the array a (leaving all the queues empty)
*/
void radix_sort(int a[], int length) {

	const int MAX_DIGIT = 6;
	const int NUM_BINS = 10;
	const int RADIX = 10;
	std::queue<int> bins[NUM_BINS];

	for (int d = 1, place = 1; d <= MAX_DIGIT; place *= RADIX, ++d) {
		for (int i = 0; i < length; ++i) {
			bins[select_digit(a[i], place)].push(a[i]);
		}

		int a_pos = 0;
		for (int i = 0; i < NUM_BINS; ++i) {
			while (!bins[i].empty()) {
				a[a_pos++] = bins[i].front();
				bins[i].pop();
			}
		}
	}
}