## Struct fallback_to_none

boost::log::fallback_to_none

## Synopsis

    // In header: <boost/log/attributes/fallback_policy.hpp>

    struct fallback_to_none {

      enum @0 { guaranteed_result = = false };

      // public static functions
      template<typename FunT> static bool apply_default(FunT &);
      template<typename FunT> static bool apply_default(FunT const &);
      static void on_invalid_type(typeindex::type_index const &);
      static void on_missing_value();
    };

## Description

The `fallback_to_none` policy results in returning an empty value reference if the attribute value cannot be extracted.

### `fallback_to_none` public static functions

1.     template<typename FunT> static bool apply_default(FunT &);

   The method is called in order to apply a function object to the default value.

2.     template<typename FunT> static bool apply_default(FunT const &);

   The method is called in order to apply a function object to the default value.

3.     static void on_invalid_type(typeindex::type_index const &);

   The method is called when value extraction failed because the attribute value has different type than requested.

4.     static void on_missing_value();

   The method is called when value extraction failed because the attribute value was not found.