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### buffer

The boost::asio::buffer function is used to create a buffer object to represent raw memory, an array of POD elements, a vector of POD elements, or a std::string.

    mutable_buffers_1 buffer(
        const mutable_buffer & b);

    mutable_buffers_1 buffer(
        const mutable_buffer & b,
        std::size_t max_size_in_bytes);

    const_buffers_1 buffer(
        const const_buffer & b);

    const_buffers_1 buffer(
        const const_buffer & b,
        std::size_t max_size_in_bytes);

    mutable_buffers_1 buffer(
        void * data,
        std::size_t size_in_bytes);

    const_buffers_1 buffer(
        const void * data,
        std::size_t size_in_bytes);

    template<
        typename PodType,
        std::size_t N>
    mutable_buffers_1 buffer(
        PodType & data);

    template<
        typename PodType,
        std::size_t N>
    mutable_buffers_1 buffer(
        PodType & data,
        std::size_t max_size_in_bytes);

    template<
        typename PodType,
        std::size_t N>
    const_buffers_1 buffer(
        const PodType & data);

    template<
        typename PodType,
        std::size_t N>
    const_buffers_1 buffer(
        const PodType & data,
        std::size_t max_size_in_bytes);

    template<
        typename PodType,
        std::size_t N>
    mutable_buffers_1 buffer(
        boost::array< PodType, N > & data);

    template<
        typename PodType,
        std::size_t N>
    mutable_buffers_1 buffer(
        boost::array< PodType, N > & data,
        std::size_t max_size_in_bytes);

    template<
        typename PodType,
        std::size_t N>
    const_buffers_1 buffer(
        boost::array< const PodType, N > & data);

    template<
        typename PodType,
        std::size_t N>
    const_buffers_1 buffer(
        boost::array< const PodType, N > & data,
        std::size_t max_size_in_bytes);

    template<
        typename PodType,
        std::size_t N>
    const_buffers_1 buffer(
        const boost::array< PodType, N > & data);

    template<
        typename PodType,
        std::size_t N>
    const_buffers_1 buffer(
        const boost::array< PodType, N > & data,
        std::size_t max_size_in_bytes);

    template<
        typename PodType,
        typename Allocator>
    mutable_buffers_1 buffer(
        std::vector< PodType, Allocator > & data);

    template<
        typename PodType,
        typename Allocator>
    mutable_buffers_1 buffer(
        std::vector< PodType, Allocator > & data,
        std::size_t max_size_in_bytes);

    template<
        typename PodType,
        typename Allocator>
    const_buffers_1 buffer(
        const std::vector< PodType, Allocator > & data);

    template<
        typename PodType,
        typename Allocator>
    const_buffers_1 buffer(
        const std::vector< PodType, Allocator > & data,
        std::size_t max_size_in_bytes);

    const_buffers_1 buffer(
        const std::string & data);

    const_buffers_1 buffer(
        const std::string & data,
        std::size_t max_size_in_bytes);

A buffer object represents a contiguous region of memory as a 2-tuple consisting of a pointer and size in bytes. A tuple of the form `{void*, size_t}` specifies a mutable (modifiable) region of memory. Similarly, a tuple of the form `{const void*, size_t}` specifies a const (non-modifiable) region of memory. These two forms correspond to the classes mutable_buffer and const_buffer, respectively. To mirror C++'s conversion rules, a mutable_buffer is implicitly convertible to a const_buffer, and the opposite conversion is not permitted.

The simplest use case involves reading or writing a single buffer of a specified size:

    sock.send(boost::asio::buffer(data, size));

In the above example, the return value of boost::asio::buffer meets the requirements of the ConstBufferSequence concept so that it may be directly passed to the socket's write function. A buffer created for modifiable memory also meets the requirements of the MutableBufferSequence concept.

An individual buffer may be created from a builtin array, std::vector or boost::array of POD elements. This helps prevent buffer overruns by automatically determining the size of the buffer:

    char d1[128];
    size_t bytes_transferred = sock.receive(boost::asio::buffer(d1));

    std::vector<char> d2(128);
    bytes_transferred = sock.receive(boost::asio::buffer(d2));

    boost::array<char, 128> d3;
    bytes_transferred = sock.receive(boost::asio::buffer(d3));

In all three cases above, the buffers created are exactly 128 bytes long. Note that a vector is never automatically resized when creating or using a buffer. The buffer size is determined using the vector's `size()` member function, and not its capacity.

##### Accessing Buffer Contents

The contents of a buffer may be accessed using the boost::asio::buffer_size and boost::asio::buffer_cast functions:

    boost::asio::mutable_buffer b1 = ...;
    std::size_t s1 = boost::asio::buffer_size(b1);
    unsigned char* p1 = boost::asio::buffer_cast<unsigned char*>(b1);

    boost::asio::const_buffer b2 = ...;
    std::size_t s2 = boost::asio::buffer_size(b2);
    const void* p2 = boost::asio::buffer_cast<const void*>(b2);

The boost::asio::buffer_cast function permits violations of type safety, so uses of it in application code should be carefully considered.

##### Buffer Invalidation

A buffer object does not have any ownership of the memory it refers to. It is the responsibility of the application to ensure the memory region remains valid until it is no longer required for an I/O operation. When the memory is no longer available, the buffer is said to have been invalidated.

For the boost::asio::buffer overloads that accept an argument of type std::vector, the buffer objects returned are invalidated by any vector operation that also invalidates all references, pointers and iterators referring to the elements in the sequence (C++ Std, 23.2.4)

For the boost::asio::buffer overloads that accept an argument of type std::string, the buffer objects returned are invalidated according to the rules defined for invalidation of references, pointers and iterators referring to elements of the sequence (C++ Std, 21.3).

##### Buffer Arithmetic

Buffer objects may be manipulated using simple arithmetic in a safe way which helps prevent buffer overruns. Consider an array initialised as follows:

    boost::array<char, 6> a = { 'a', 'b', 'c', 'd', 'e' };

A buffer object `b1` created using:

    b1 = boost::asio::buffer(a);

represents the entire array, `{ 'a', 'b', 'c', 'd', 'e' }`. An optional second argument to the boost::asio::buffer function may be used to limit the size, in bytes, of the buffer:

    b2 = boost::asio::buffer(a, 3);

such that `b2` represents the data `{ 'a', 'b', 'c' }`. Even if the size argument exceeds the actual size of the array, the size of the buffer object created will be limited to the array size.

An offset may be applied to an existing buffer to create a new one:

    b3 = b1 + 2;

where `b3` will set to represent `{ 'c', 'd', 'e' }`. If the offset exceeds the size of the existing buffer, the newly created buffer will be empty.

Both an offset and size may be specified to create a buffer that corresponds to a specific range of bytes within an existing buffer:

    b4 = boost::asio::buffer(b1 + 1, 3);

so that `b4` will refer to the bytes `{ 'b', 'c', 'd' }`.

##### Buffers and Scatter-Gather I/O

To read or write using multiple buffers (i.e. scatter-gather I/O), multiple buffer objects may be assigned into a container that supports the MutableBufferSequence (for read) or ConstBufferSequence (for write) concepts:

    char d1[128];
    std::vector<char> d2(128);
    boost::array<char, 128> d3;

    boost::array<mutable_buffer, 3> bufs1 = {
      boost::asio::buffer(d1),
      boost::asio::buffer(d2),
      boost::asio::buffer(d3) };
    bytes_transferred = sock.receive(bufs1);

    std::vector<const_buffer> bufs2;
    bufs2.push_back(boost::asio::buffer(d1));
    bufs2.push_back(boost::asio::buffer(d2));
    bufs2.push_back(boost::asio::buffer(d3));
    bytes_transferred = sock.send(bufs2);