Re: [Boost-users] Flyweights: compute a hash from them

Subject: Re: [Boost-users] Flyweights: compute a hash from them
From: JOAQUIN M. LOPEZ MUÑOZ (joaquin_at_[hidden])
Date: 2013-03-05 15:15:10

Ell 05/03/2013, a las 18:15, "Akim Demaille" <akim_at_[hidden]> escribió:

> Hi all,
>
> My parser is building string_t = flyweight<std::string> for identifiers,
> and it works very well.  Now, I would like to make an unordered_map
> <string_t, foo_t>, but I have to define a hash function for string_t.
> I can do it but since the whole point of flyweight is to make unique
> instances, it would be a pity to hash the value.  So I hash the address
> of the result of get().
>
> Is this ok?  So far everything is fine, but I might have missed something.

This is absolutely correct, and in fact the optimum way to hash flyweights.

> In particular, I don't understand why their is no standard hash function
> defined for flyweights.

No particular reason other than that it is not customary to provide this definition in advance for most non-primitive types provided by Boost.

Joaquín M López Muñoz
Telefónica Digital

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