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The type of the function's first argument.
typedef see_below first_argument_type;
          The type of first_argument_type
          is based on the type T
          of the wrapper's target object:
        
T is a pointer to
              a function type accepting two arguments, first_argument_type
              is a synonym for the return type of T;
            T is a class type
              with a member type first_argument_type,
              then first_argument_type
              is a synonym for T::first_argument_type;
            first_argument_type
              is not defined.
            
          Header: asio/bind_allocator.hpp
        
          Convenience header: asio.hpp