Function hash

src/ifcgeom/taxonomy.h:170–179

				virtual void reverse() { throw taxonomy::topology_error(); }
				virtual size_t calc_hash() const = 0;
				virtual size_t hash() const {
					if (computed_hash_) {
						return computed_hash_;
					}
					computed_hash_ = calc_hash();
					if (computed_hash_ == 0) {
						computed_hash_++;
					}
					return computed_hash_;
				}

				item(const IfcUtil::IfcBaseInterface* instance = nullptr) : identity_(counter_++), computed_hash_(0), instance(instance) {}