Re: hash_set: how to handle collisions?

James Kanze <>
Fri, 11 Jul 2008 02:54:57 -0700 (PDT)
On Jul 11, 3:20 am, Markus Dehmann <> wrote:

Do I have to handle hash collisions in a hash_set myself?

Which hash_set? There is not, and almost certainly never will
be, a hash_set in C++.

Which is, of course, a technicality. There will be an
unordered_set, and most existing hash_set are only slightly
different (but in different and incompatible ways).

I did a test in which I use find() to look for objects in a
hash_set. These objects are definitely not contained, but
find() sometimes finds them anyway. See this code:

#include <iostream>
#include <stdexcept>
#include <ctime>
#include <set>
// also include header that defines gnu_namespace and includes
// hash_set

If you're asking specifics about the GNU implementation, you
should probably ask an implementation specific group. But
supposing that the issue is more general, and concerns what
unordered_set will guarantee in the end.

class MyContainer {
  std::vector<int> v;
  std::size_t hashcode() const {
    std::size_t hash = 0;
    for(unsigned i=0; i<v.size(); ++i){ // sdbm function gives
collisions sometimes
      hash = v[i] + (hash << 6) + (hash << 16) - hash;
    return hash;
  void add(int i){v.push_back(i);}

struct eqPtrs {
  bool operator()(const MyContainer* h1, const MyContainer* h2) const
    return h1->hashcode() == h2->hashcode(); // problematic b/c of

You've defined two objects to be equal if their hash codes are
equal. Is that really what you want?

namespace gnu_namespace {
  template<> struct hash<const MyContainer*> {
    inline size_t operator()(const MyContainer* d) const {
      return d->hashcode();

int getRand(int min, int max){
  return ((rand() % (max-min+1)) + min);

int main(int argc, char** argv){
  typedef hash_set<const MyContainer*, hash<const MyContainer*>,
eqPtrs> MyMap;
  MyMap myMap;
  int repeat = 100000;
  int size = 10;
  for(int i=0; i<repeat; ++i){
    MyContainer* h = new MyContainer();
    for(int j=0; j<size; ++j){
      h->add(getRand(0, 1000));
  for(int i=0; i<repeat; ++i){
    MyContainer* h = new MyContainer();
    for(int j=0; j<size; ++j){
      h->add(getRand(2000, 3000));
    MyMap::const_iterator found = myMap.find(h);
    assert(found == myMap.end()); // aborts!

I'm not sure I understand. You create a random object, and
assert that it is in the container. Why should you expect it to
be in the container?

  // TODO: finally delete elements in myMap
  return EXIT_SUCCESS;}


The solution seems to be to adapt the equality condition in
eqPtrs to also test for actual equality of the members, not
just equality of the hash codes:

struct eqPtrs {
  bool operator()(const MyContainer* h1, const MyContainer* h2) const
    return h1->hashcode() == h2->hashcode() && haveSameElements(*h1,
*h2); // added condition

Is that the solution, or am I doing something wrong in general?

I don't know. It's up to you to define what equality should
mean. The only requirement is that if two elements are equal,
they have the same hash code.

James Kanze (GABI Software)
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