[boost] [BGL] [PropertyMap] Key param is passed by value in put function

Hi to all, I've noticed that the put function that takes a generic put_get_helper map, takes a key param. The question is: why the key param is passed by value? Is any reason for this? The key value is not modified setting the property map. I've noticed it because in my graph type the descriptors are variant, and so every time I've to set a property map, a new copy of the variant is created. I've reported the function below, for sake of comfort. It is in boost/property_map.hpp template <class PropertyMap, class Reference, class K, class V> inline void put(const put_get_helper<Reference, PropertyMap>& pa, K k, const V& v) { static_cast<const PropertyMap&>(pa)[k] = v; } Best regards, Cosimo Calabrese