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boost::log::filter
// In header: <boost/log/expressions/filter.hpp>
class filter {
public:
  // types
  typedef bool result_type;  // Result type. 
  // member classes/structs/unions
  // Default filter, always returns true. 
  struct default_filter {
    // types
    typedef bool result_type;
    // public member functions
    result_type operator()(attribute_value_set const &) const;
  };
  // construct/copy/destruct
  filter();
  filter(filter const &);
  filter(filter &&) noexcept;
  template<typename FunT> filter(FunT &&);
  filter & operator=(filter &&) noexcept;
  filter & operator=(filter const &);
  template<typename FunT> filter & operator=(FunT const &);
  // public member functions
  result_type operator()(attribute_value_set const &) const;
  void reset();
  void swap(filter &) noexcept;
};Log record filter function wrapper.
filter 
        public
       construct/copy/destructfilter();
Default constructor. Creates a filter that always returns true. 
filter(filter const & that);
Copy constructor
filter(filter && that) noexcept;
Move constructor. The moved-from filter is left in an unspecified state.
template<typename FunT> filter(FunT && fun);
Initializing constructor. Creates a filter which will invoke the specified function object.
filter & operator=(filter && that) noexcept;
Move assignment. The moved-from filter is left in an unspecified state.
filter & operator=(filter const & that);
Copy assignment.
template<typename FunT> filter & operator=(FunT const & fun);
Initializing assignment. Sets the specified function object to the filter.
filter public member functionsresult_type operator()(attribute_value_set const & values) const;
Filtering operator.
| Parameters: | 
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| Returns: | 
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void reset();
Resets the filter to the default. The default filter always returns true. 
void swap(filter & that) noexcept;
Swaps two filters