kinda fast std::vector combinations function c++

vector_combinations.cpp

#include <stdint.h>
#include <stdlib.h>
#include <vector>

#define uint32 uint32_t

std::vector<std::vector<uint32>> combinations(std::vector<uint32> src, uint32 r) {
  std::vector<std::vector<uint32>> cs;
  if (r == 1) {
    for (auto i = 0; i < src.size(); i++) {
      std::vector<uint32> c;
      c.push_back(src[i]);
      cs.push_back(c);
    }
    return cs;
  }
  uint32* places = (uint32*)malloc(sizeof(uint32) * r);
  for (auto i = 0; i < r; i++) places[i] = i;
  while (true) {
    // push_back another combination
    std::vector<uint32> c; c.reserve(r);
    for (auto i = 0; i < r; i++) {
      c.push_back(src[places[i]]);
    }
    cs.push_back(c);
    // update places
    for (auto i = 0; i < r-1; i++) {
      places[i]++;
      if (places[i+1] == places[i]) {
	places[i] = i;
	if (i == r-2) places[r-1]++;
	continue;
      }
      break;
    }
    if (places[r-1] == src.size()) break;
  }
  free(places);
  return cs;
}

thanks mayne