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### basic_streambuf

Automatically resizable buffer class based on std::streambuf.

    template<
        typename Allocator = std::allocator<char>>
    class basic_streambuf :
      noncopyable

##### Types

Name | Description
--- | ---
const_buffers_type | The type used to represent the input sequence as a list of buffers.
mutable_buffers_type | The type used to represent the output sequence as a list of buffers.

##### Member Functions

Name | Description
--- | ---
basic_streambuf | Construct a basic_streambuf object.
commit | Move characters from the output sequence to the input sequence.
consume | Remove characters from the input sequence.
data | Get a list of buffers that represents the input sequence.
max_size | Get the maximum size of the basic_streambuf.
prepare | Get a list of buffers that represents the output sequence, with the given size.
size | Get the size of the input sequence.

##### Protected Member Functions

Name | Description
--- | ---
overflow | Override std::streambuf behaviour.
reserve |
underflow | Override std::streambuf behaviour.

The `basic_streambuf` class is derived from `std::streambuf` to associate the streambuf's input and output sequences with one or more character arrays. These character arrays are internal to the `basic_streambuf` object, but direct access to the array elements is provided to permit them to be used efficiently with I/O operations. Characters written to the output sequence of a `basic_streambuf` object are appended to the input sequence of the same object.

The `basic_streambuf` class's public interface is intended to permit the following implementation strategies:

- A single contiguous character array, which is reallocated as necessary to accommodate changes in the size of the character sequence. This is the implementation approach currently used in Asio.
- A sequence of one or more character arrays, where each array is of the same size. Additional character array objects are appended to the sequence to accommodate changes in the size of the character sequence.
- A sequence of one or more character arrays of varying sizes. Additional character array objects are appended to the sequence to accommodate changes in the size of the character sequence.

The constructor for basic_streambuf accepts a `size_t` argument specifying the maximum of the sum of the sizes of the input sequence and output sequence. During the lifetime of the `basic_streambuf` object, the following invariant holds:

    size() <= max_size()

Any member function that would, if successful, cause the invariant to be violated shall throw an exception of class `std::length_error`.

The constructor for `basic_streambuf` takes an Allocator argument. A copy of this argument is used for any memory allocation performed, by the constructor and by all member functions, during the lifetime of each `basic_streambuf` object.

##### Examples

Writing directly from an streambuf to a socket:

    boost::asio::streambuf b;
    std::ostream os(&b);
    os << "Hello, World!\n";

    // try sending some data in input sequence
    size_t n = sock.send(b.data());

    b.consume(n); // sent data is removed from input sequence

Reading from a socket directly into a streambuf:

    boost::asio::streambuf b;

    // reserve 512 bytes in output sequence
    boost::asio::streambuf::const_buffers_type bufs = b.prepare(512);

    size_t n = sock.receive(bufs);

    // received data is "committed" from output sequence to input sequence
    b.commit(n);

    std::istream is(&b);
    std::string s;
    is >> s;