Hashable struct for the GS::HashTable<Key, Value>

[Mihalcea Bogdan]

Hi,

I need to create a struct that can be used as a Key in a hashtable. 

So far I made this:

struct Item{
		GS::UInt8 parentId;
		GS::UInt8 itemId;
		const GS::UInt8 hash = this->parentId + this->itemId;

		GS::ULong Item::GenerateHashValue() {
			return GS::GenerateHashValue(this->hash);
		}
		bool operator== (const Item& other) const {
			return (this->parentId == other.parentId && this->itemId == other.itemId);
		}
	};

It gives me the error:

Does not have a GenerateHashValue member or non-member function. I am pretty sure I have GenerateHashValue.

Can you please point some directions on this matter?

[Mihalcea Bogdan]

I know how to make it work with c++ standard std::unordered_map

struct Item
{
	int parentId;
	short itemId;

	bool operator==(const Item &other) const
	{
		return (parentId == other.parentId && itemId == other.itemId);
	}
};

template <>
struct std::hash<Item>
{
	std::size_t operator()(const Item& k) const
	{
		return (k.itemId + k.parentId);
	}
};

with the above struct I can use the following:

Item itm = { 10, 20 };
Item itm2 = { 10, 20 };
std::unordered_map<Item, int> hashMap;
hashMap[itm] = 10;
hashMap[itm2] = 30;
std::cout << hashMap[itm2] << std::endl;

Output will be 30.

[Oleg]

I think you need to mplement the GenerateHashValue as non member function.

GS::ULong GenerateHashValue( const Item& item )
{
	return GS::GenerateHashValue( item.parentId + item.itemId );
}

[Oleg]

And I guess there is other variant.
Inherit your Item from GS::Hashable and implement virtual member 
virtual ULong GenerateHashValue (void) const = 0;

like this

struct Item : public GS::Hashable
{
	virtual ULong	GenerateHashValue (void) const { return this->hash; }
}

[Mihalcea Bogdan]

It says it is already declared in the .obj file. GIS is my namespace.

Error LNK2005 "unsigned int __cdecl GIS::GenerateHashValue(struct GIS::Item const &)" (?GenerateHashValue@GIS@@YAIAEBUItem@1@@Z) already defined in AddOnMain.obj AddOn \polyGIS\Build\Source.obj 

[Mihalcea Bogdan]

I can't find the GS::Hashable class. 

[Oleg]

Sorry.  I worked on old project and checked it for old API.
It seem GS::Hashable was before AC24  and implementation of the HashTable was changed.
I will try it for API 24 and 25 later.

[Mihalcea Bogdan]

I was looking at other data structures which work with the GS::hashTable, the GS::Pair<T1, T2>. It has an implementation of the  GenerateHashValue() like you said.

//This is from the GS::Pair<>
template <class Type1, class Type2>
ULong	GenerateHashValue (const Pair<Type1, Type2>& pair)
{
	return GS::GenerateHashValue (pair.first, pair.second);
}

[Oleg · Accepted solution]

Fast tests. Works for me. VisualStudio.
Using test is:

GS::HashTable<Item,int> table;
Item itm = { 10, 20 };
table.Add(itm, 2);
table[itm] = 3;

AC24 (you need const qualifier)

GS::ULong Item::GenerateHashValue() const 
{
	return GS::GenerateHashValue(this->hash);
}

AC25 (dont have time to study it )

GS::ULong Item::GenerateHashValue() const 
{
	return GS::CalculateHashValue(this->hash);
}

And just reply. Are you shure to use summary Ids for hashe generation ?
May be beter like this:

GS::ULong Item::GenerateHashValue() const 
{
	return ((ULong) parentId << 8 ) | itemId;
}

[Mihalcea Bogdan]

I was pretty sure that there will be no conflict. Now that I think more about it will not work.  Your solution works for me too, I forgot about the const, it is the same behavior as with operator overriding. Thank you very much!