The API of the ReadSource concept in buffers/read_source.hpp has a read function template which accepts any mutable buffer sequence. We should offer a helper free function that models of ReadSource can use to invoke a lambda accepting a buffers::mutable_buffer and system::error_code& and returning the number of bytes read , where the lambda is invoked repeatedly and the helper integrates the results, with special care to handle error::eof. The implementation of any_read_source here https://github.com/cppalliance/buffers/blob/develop/include/boost/buffers/any_buffers.hpp can be used for guidance.