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## use_default

### Overview

The header <boost/core/use_default.hpp> provides the type `boost::use_default` which is used by other Boost libraries as a sentinel type in templates to indicate defaults.

### Example

    template<class Derived, class Base,
       class Value = boost::use_default,
       class CategoryOrTraversal = boost::use_default,
       class Reference = boost::use_default,
       class Difference = boost::use_default>
    class iterator_adaptor;

    template<class Value>
    class node_iterator
        : public iterator_adaptor<node_iterator<Value>, Value*,
            boost::use_default, boost::forward_traversal_tag>;

### Reference

    namespace boost {
      struct use_default { };
    }