Re: how to have user defined hash for unordered_map ?

abir <>
Fri, 11 Jul 2008 11:48:47 -0700 (PDT)
On Jul 11, 8:17 pm, James Kanze <> wrote:

On Jul 11, 7:05 am, abir <> wrote:

  I want a user defined key for tr1 unordered_map.

Pete Becker has given the reply to your question, but...

Previously i used unordered_map from boost, which automatically takes
hash_value friend to compute hash, so i thought it is in the tr1
Now this works for tr1,
unordered_map<WORKER,int,boost::hash<WORKER>> m;
but for boost, this even works.
unordered_map<WORKER,int> m;
 friend std::size_t hash_value(const self_type& self){
        return self.pos() + self.w().count;
I am still not sure why i cant just have a overloaded hash or some
other function, and unordered_map automatically finds it like boost.
Why i have to specify hash function for my class while i don't need
for int, float ot string!
There was some problem of const correctness in previous post, actually
pos() & w() have to be const member function.

template<typename W>
class worker{
    typedef worker<W> self_type;
    worker(W& w,int pos) : w_(&w),pos_(pos){}
    bool operator== (const self_type& other) const {
        assert(w_ == other.w_);

Do you mean this? If so, the simplest way of ensuring it is to
make w_ static.

        return pos_ == other.pos_;

Yes i mean this.
w_ can't be static, as two worker can do different work.
Two worker doing different work are not comparable and so are not
allowed to form hash key for same container.

    W* w_;
    int pos_;
    friend std::size_t hash(const self_type& self){
        return self.pos_ + w_->count;

James Kanze (GABI Software) email:james.ka...@gma=

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