Property Tree as a Container

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## Property Tree as a Container

Every property tree node models the ReversibleSequence concept, providing access to its immediate children. This means that iterating over a `ptree` (which is the same as its root node - every `ptree` node is also the subtree it starts) iterates only a single level of the hierarchy. There is no way to iterate over the entire tree.

It is very important to remember that the property sequence is not ordered by the key. It preserves the order of insertion. It closely resembles a std::list. Fast access to children by name is provided via a separate lookup structure. Do not attempt to use algorithms that expect an ordered sequence (like binary_search) on a node's children.

The property tree exposes a second container-like interface, called the associative view. Its iterator type is the nested type assoc_iterator (and its const counterpart const_assoc_iterator). You can get an ordered view of all children by using ordered_begin() and ordered_end().

The associative view also provides find() and equal_range() members, which return assoc_iterators, but otherwise have the same semantics as the members of std::map of the same name.

You can get a normal iterator from an assoc_iterator by using the to_iterator() member function. Converting the other way is not possible.