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boost::intrusive::link_mode_type
// In header: <boost/intrusive/link_mode.hpp> enum link_mode_type { normal_link, safe_link, auto_unlink };
This enumeration defines the type of value_traits that can be defined for Boost.Intrusive containers
normal_link
If this linking policy is specified in a value_traits class as the link_mode, containers configured with such value_traits won't set the hooks of the erased values to a default state. Containers also won't check that the hooks of the new values are default initialized.
safe_link
If this linking policy is specified in a value_traits class as the link_mode, containers configured with such value_traits will set the hooks of the erased values to a default state. Containers also will check that the hooks of the new values are default initialized.
auto_unlink
Same as "safe_link" but the user type is an auto-unlink type, so the containers with constant-time size features won't be compatible with value_traits configured with this policy. Containers also know that the a value can be silently erased from the container without using any function provided by the containers.