[/
    Copyright (c) 2013-2017 Vinnie Falco (vinnie dot falco at gmail dot com)

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
]

[section:FieldsReader FieldsReader]

A [*FieldsReader] provides a algorithm to obtain a sequence of buffers
representing the complete serialized HTTP/1 header for a set of fields.
The implementation constructs an instance of this type when needed, and
calls into it once to retrieve the buffers.

In this table:

* `X` denotes a type that meets the requirements of [*FieldsReader].

* `F` denotes a __Fields__ where
      `std::is_same<X, F::reader>::value == `true`.

* `a` is a value of type `X`.

* `f` is a value of type `F`.

* `v` is an `unsigned` value representing the HTTP version.

* `c` is an `unsigned` representing the HTTP status-code.

* `m` is a value of type [link beast.ref.beast__http__verb `verb`].

[table FieldsReader requirements
[[expression][type][semantics, pre/post-conditions]]
[
    [`X::const_buffers_type`]
    []
    [
        A type which meets the requirements of __ConstBufferSequence__.
        This is the type of buffer returned by `X::get`.
    ]
][
    [`X(f,v,m)`]
    []
    [
        The implementation calls this constructor to indicate
        that the fields being serialized form part of an HTTP
        request. The lifetime of `f` is guaranteed
        to end no earlier than after the `X` is destroyed.
    ]
][
    [`X(f,v,c)`]
    []
    [
        The implementation calls this constructor to indicate
        that the fields being serialized form part of an HTTP
        response. The lifetime of `f` is guaranteed
        to end no earlier than after the `X` is destroyed.
    ]
][
    [`a.get()`]
    [X::const_buffers_type]
    [
        Called once after construction, this function returns
        a constant buffer sequence containing the serialized
        representation of the HTTP request or response including
        the final carriage return linefeed sequence (`"\r\n"`).
    ]
][
    [`is_fields_reader<F>`]
    [`std::true_type`]
    [
        An alias for `std::true_type` for `F`, otherwise an alias
        for `std::false_type`.
    ]
]
]

Exemplar:
```
struct FieldsReader
{
    // The type of buffers returned by `get`
    using const_buffers_type = implementation-defined;

    // Constructor for requests
    FieldsReader(F const& f, unsigned version, verb method);

    // Constructor for responses
    FieldsReader(F const& f, unsigned version, unsigned status);

    // Returns the serialized header buffers
    const_buffers_type
    get();
};
```

[endsect]