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### Mutable buffer sequence requirements

A mutable buffer sequence represents a set of memory regions that may be used to receive the output of an operation, such as the `receive` operation of a socket.

A type `X` meets the `MutableBufferSequence` requirements if it satisfies the requirements of `Destructible` (C++Std [destructible]) and `CopyConstructible` (C++Std [copyconstructible]), as well as the additional requirements listed below.

In the table below, `x` denotes a (possibly const) value of type `X`, and `u` denotes an identifier.

Table 21. MutableBufferSequence requirements

expression | return type | assertion/note pre/post-condition
--- | --- | ---
`boost::asio::buffer_sequence_begin(x)` `boost::asio::buffer_sequence_end(x)` | An iterator type meeting the requirements for bidirectional iterators (C++Std [bidirectional.iterators]) whose value type is convertible to `mutable_buffer`. |
    X u(x);
|  | post:
    equal(
      boost::asio::buffer_sequence_begin(x),
      boost::asio::buffer_sequence_end(x),
      boost::asio::buffer_sequence_begin(u),
      boost::asio::buffer_sequence_end(u),
      [](const mutable_buffer& b1,
         const mutable_buffer& b2)
       {
         return b1.data() == b2.data()
             && b1.size() == b2.size();
       })