flecs - Query for relationship example

main.cpp

#include <iostream>

#include "flecs.h"

struct Position
{
	int x, y, z;
};

struct Tag1 {};
struct Tag2 {};

int main(int argc, char* argv[])
{
	{
		flecs::world ecs(argc, argv);

		std::cout << "Tag1" << std::endl;

		// Tag1 is a regular tag, this means pairs with this tag will be stored per table.
		// Each unique combination of (Tag1, *) makes a new table.
		ecs.component<Tag1>();


		ecs.system()
			.with<Tag1>(flecs::Wildcard) // Query for anything with relation (Tag1, *)
			.iter([](flecs::iter& it) {
				auto ecs = it.world();
				auto second = it.pair(1).second();

				// All of these entities have (Tag1, second)
				std::cout << "iter: " << second.name() << std::endl;
				for (auto i : it) {
					auto entity = it.entity(i);
					std::cout << "- " << entity.name() << std::endl;
				}
			});

		// Create entities for example
		auto e1 = ecs.entity("e1")
			.set<Position>({ 0, 1, 0 });

		auto e2 = ecs.entity("e2")
			.set<Position>({ 0, 0, 1 });

		ecs.entity("t1-1")
			.add<Tag1>(e1);
		ecs.entity("t1-2")
			.add<Tag1>(e2);

		ecs.progress();
	}

	std::cout << std::endl;

	{
		flecs::world ecs(argc, argv);

		std::cout << "Tag2" << std::endl;

		// Tag2 is a Union tag, this means entities with different combinations of (Tag2, *) can share the same
		// table. The table column will store the entity id for the second part of the relation.
		ecs.component<Tag2>()
			.add(flecs::Union);

		ecs.system()
			.with<Tag2>(flecs::Wildcard) // Query for anything with relation (Tag2, *)
			.iter([](flecs::iter& it) {
				auto ecs = it.world();
				auto secondColumn = it.field<flecs::entity_t>(1);

				std::cout << "iter" << std::endl;
				for (auto i : it) {
					auto entity = it.entity(i);
					auto second = ecs.entity(secondColumn[i]);
					// This entity has (Tag2, second)
					std::cout << "- " << entity.name() << ", " << second.name() << std::endl;
				}
			});

		// Create entities for example
		auto e1 = ecs.entity("e1")
			.set<Position>({ 0, 1, 0 });

		auto e2 = ecs.entity("e2")
			.set<Position>({ 0, 0, 1 });

		ecs.entity("t2-1")
			.add<Tag2>(e1);
		ecs.entity("t2-2")
			.add<Tag2>(e2);

		ecs.progress();
	}

	return 0;
}

output.txt

Tag1
iter: e1
- t1-1
iter: e2
- t1-2

Tag2
iter
- t2-1, e1
- t2-2, e2