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### Fields

An instance of Fields is a container for holding HTTP header fields and their values. The implementation also calls upon the container to store the request target and non-standard strings for method and obsolete reason phrase as needed. Types which meet these requirements can always be serialized.

##### Associated Types

- `is_fields`
- FieldsWriter

##### Requirements

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

In this table:

- `F` denotes a type that meets the requirements of Fields.
- `W` denotes a type meeting the requirements of FieldsWriter.
- `a` denotes a value of type `F`.
- `c` denotes a (possibly const) value of type `F`.
- `b` is a value of type `bool`
- `n` is a value of type `boost::optional<std::uint64_t>`.
- `s` is a value of type `string_view`.
- `v` is a value of type `unsigned int` representing the HTTP-version.

Table 1.32. Valid expressions

Expression | Type | Semantics, Pre/Post-conditions
--- | --- | ---
`F::writer` | `W` | A type which meets the requirements of FieldsWriter.
`c.get_method_impl()` | `string_view` | Returns the method text. The implementation only calls this function for request headers when retrieving the method text previously set with a call to `set_method_impl` using a non-empty string.
`c.get_target_impl()` | `string_view` | Returns the target string. The implementation only calls this function for request headers.
`c.get_reason_impl()` | `string_view` | Returns the obsolete request text. The implementation only calls this for response headers when retrieving the reason text previously set with a call to `set_reason_impl` using a non-empty string.
`c.get_chunked_impl()` | `bool` | Returns `true` if the Transfer-Encoding field value indicates that the payload is chunk encoded. Both of these conditions must be true: The Transfer-Encoding field is present in the message. The last item the value of the field is "chunked".
`c.get_keep_alive_impl(v)` | `bool` | Returns `true` if the semantics of the Connection field and version indicate that the connection should remain open after the corresponding response is transmitted or received: If `(v < 11)` the function returns `true` if the "keep-alive" token is present in the Connection field value. Otherwise the function returns `false`. If `(v == 11)`, the function returns `false` if the "close" token is present in the Connection field value. Otherwise the function returns `true`.
`c.has_content_length()` | `bool` | Returns `true` if the Content-Length field is present.
`a.set_method_impl(s)` |  | Stores a copy of `s` as the method text, or erases the previously stored value if `s` is empty. The implementation only calls this function for request headers. This function may throw `std::invalid_argument` if the operation is not supported by the container.
`a.set_target_impl(s)` |  | Stores a copy of `s` as the target, or erases the previously stored value if `s` is empty. The implementation only calls this function for request headers. This function may throw `std::invalid_argument` if the operation is not supported by the container.
`a.set_reason_impl(s)` |  | Stores a copy of `s` as the reason text, or erases the previously stored value of the reason text if `s` is empty. The implementation only calls this function for request headers. This function may throw `std::invalid_argument` if the operation is not supported by the container.
`a.set_chunked_impl(b)` |  | Adjusts the Transfer-Encoding field value as follows: If `b` is `true`, the "chunked" token is appended to the list of encodings if it does not already appear last in the list. If the Transfer-Encoding field is absent, the field will be inserted to the container with the value "chunked". If `b` is `false, the "chunked" token is removed from the list of encodings if it appears last in the list. If the result of the removal leaves the list of encodings empty, the Transfer-Encoding field shall not appear when the associated FieldsWriter serializes the fields. If the result of adjusting the field value produces an empty string, the field is removed from the container.
`a.set_content_length_impl(n)` |  | Adjusts the Content-Length field value as follows: If `n` contains a value, the Content-Length field will be set to the text representation of the value. Any previous Content-Length fields are removed from the container. If `n` does not contain a value, any present Content-Length fields are removed from the container.
`a.set_keep_alive_impl(v,b)` |  | Adjusts the Connection field value depending on the values of `v` and `b`. The field value is treated as connection-option (rfc7230). If `(v < 11 && b)`, then all "close" tokens present in the value are removed, and the "keep-alive" token is added to the value if it is not already present. If `(v < 11 && ! b)`, then all "close" and "keep-alive" tokens present in the value are removed. If `(v == 11 && b)`, then all "keep-alive" and "close" tokens present in the value are removed. If `(v == 11 && ! b)`, then all "keep-alive" tokens present in the value are removed, and the "close" token is added to the value if it is not already present. If the result of adjusting the field value produces an empty string, the field is removed from the container.

##### Exemplar

    class Fields
    {
    public:
        /// Constructed as needed when fields are serialized
        struct writer;

    protected:
        /** Returns the request-method string.

            @note Only called for requests.
        */
        string_view
        get_method_impl() const;

        /** Returns the request-target string.

            @note Only called for requests.
        */
        string_view
        get_target_impl() const;

        /** Returns the response reason-phrase string.

            @note Only called for responses.
        */
        string_view
        get_reason_impl() const;

        /** Returns the chunked Transfer-Encoding setting
        */
        bool
        get_chunked_impl() const;

        /** Returns the keep-alive setting
        */
        bool
        get_keep_alive_impl(unsigned version) const;

        /** Returns `true` if the Content-Length field is present.
        */
        bool
        has_content_length_impl() const;

        /** Set or clear the method string.

            @note Only called for requests.
        */
        void
        set_method_impl(string_view s);

        /** Set or clear the target string.

            @note Only called for requests.
        */
        void
        set_target_impl(string_view s);

        /** Set or clear the reason string.

            @note Only called for responses.
        */
        void
        set_reason_impl(string_view s);

        /** Sets or clears the chunked Transfer-Encoding value
        */
        void
        set_chunked_impl(bool value);

        /** Sets or clears the Content-Length field
        */
        void
        set_content_length_impl(boost::optional<std::uint64_t>);

        /** Adjusts the Connection field
        */
        void
        set_keep_alive_impl(unsigned version, bool keep_alive);
    };

    static_assert(is_fields<Fields>::value,
        "Fields requirements not met");

##### Models

- `basic_fields`
- `fields`