Question

There is inconsistency between this example and async_read manual. According to the manual async_read handler expects 2 arguments: error code and how many bytes were transferred, but in the example the handler accepts only error code. What is going on here?

Answer 1

The resulting type from boost::bind still meets the requirements of ReadHandler. Boost.Bind‘s documentation states that extra arguments are silently ignored.

For example, in the internals of Boost.Asio, the ReadHandler call chain may look something like:

handler( error, bytes_transferred ) 
`-- binder::operator()( error, bytes_transferred )
    `-- bound_function( error )

For a better illustration and explanation of bind, consider reading this blog entry. One of its illustration specifically shows the case of surplus arguments.