Re: [Boost-users] Using hash_range with polymorphism

Subject: Re: [Boost-users] Using hash_range with polymorphism
From: Kelvin Chung (kelvSYC_at_[hidden])
Date: 2011-12-01 09:26:58

On 2011-12-01 14:05:07 +0000, Daniel James said:

> On 1 December 2011 07:48, Kelvin Chung <kelvSYC_at_[hidden]> wrote:
>> I'm having a hard time reconciling two distinct notions. Â Suppose I have a
>> class hierarchy with root class Foo, and subclasses Foo1, Foo2, etc. (all of
>> which are leaf classes) Â Now, suppose that hash_value() is defined for all
>> the subclasses but not Foo itself (say, if Foo was abstract).
>> 
>> Now suppose I have this:
>> 
>> struct FooList {
>> Â  Â  Â  Â std::vector<boost::shared_ptr<Foo>> foos;
>> }
>> 
>> std::size_t hash_value(const FooList& list)
> 
> You should probably be including something representing the dynamic
> type in the hash value, but that depends on how you implement
> 'operator==' for FooList. If you don't then defining 'hash_value'
> doesn't make much sense.

That's what I am trying to do: I'm trying to use the hash_value() for 
the actual type of the Foos in foos (eg. go through foos, and call 
hash_value(const Foo1&) if the pointer in foos is a pointer to a Foo1 
upcasted to Foo, and so on), but all I am apparently doing is slicing 
down to Foo, and as hash_value(const Foo&) does not exist, I get a 
compile-time error.

FooList::operator==() is fairly straightforward, I'd imagine: iterate 
through the two foos and call a polymorphic equals() function to 
determine equality.

I could do a polymorphic Foo::hash() and then redirect hash_value() for 
the leaf classes to hash():

class Foo {
protected:
    virtual std::size_t hash() = 0;
};

std::size_t Foo1::hash() { … }	// public in Foo1

// It would be nice to have a template that does this so I don't have 
to do this for every leaf class
std::size_t hash_value(const Foo1& foo1) { return foo1.hash(); }

But it does seem like reinventing the wheel.  If this is in fact the 
preferred solution to the conundrum,