Document ownership relation between layer and feature

[anandthakker]

It looks to me like vtzero::feature depends on having a pointer to its vtzero::layer object (specifically for getting property data):

vtzero/include/vtzero/layer.hpp

Lines 388 to 396 in 62d2640

	inline property feature::next_property() {
	const auto idxs = next_property_indexes();
	property p{};
	if (idxs.valid()) {
	p = {m_layer->key(idxs.key()),
	m_layer->value(idxs.value())};
	}
	return p;
	}

However, this requirement isn’t enforced--it's possible, for instance, to do:

vtzero::feature getFeature(vtzero::tile tile, std::string layer, uint64_t id) {
  return tile.get_layer_by_name(layer).get_feature_by_id(id);
}

and end up with a feature whose owning layer has been destroyed.

Since presumably the overhead of enforcing this with a shared_ptr would be undesirable, would it make sense to at least make a note of this in the documentation for feature?

[anandthakker]

in the documentation for feature

Or, alternatively, in the intro to https://github.com/mapbox/vtzero/blob/master/doc/reading.md, where a similar requirement regarding the underlying data is already discussed.

[springmeyer]

Thanks for ticketing, this has come up before and is definitely worthwhile to document better. I'll defer to @joto for where to put it, but 👍 on getting this more obvious.