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### FieldsWriter

A FieldsWriter 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.

##### Associated Types

- FieldsWriter

##### Requirements

| Warning
These requirements may undergo non-backward compatible changes in subsequent versions.

In this table:

- `W` denotes a type that meets the requirements of FieldsWriter.
- `F` denotes a Fields where `std::is_same<W, F::writer>::value == true`.
- `a` is a value of type `W`.
- `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 `verb`.

Table 1.41. Valid expressions

expression | type | semantics, pre/post-conditions
--- | --- | ---
`W::const_buffers_type` |  | A type which meets the requirements of ConstBufferSequence. This is the type of buffer returned by `W::get`.
`W{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 `W` is destroyed.
`W{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 `W` is destroyed.
`W{f}` |  | The implementation calls this constructor to indicate that the fields being serialized form part of a chunked encoding final-chunk trailer. The lifetime of `f` is guaranteed to end no earlier than after the `W` is destroyed.

`a.get()`

`W::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"`).

Copies may be made of the returned sequence, but the underlying memory is still owned by the writer. The implementation will destroy all copies of the buffer sequence before destroying `a`.

##### Exemplar

    struct FieldsWriter
    {
        // The type of buffers returned by `get`
        struct const_buffers_type;

        // Constructor for requests
        FieldsWriter(Fields const& f, unsigned version, verb method);

        // Constructor for responses
        FieldsWriter(Fields const& f, unsigned version, unsigned status);

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

##### Models

- `basic_fields::writer`