Exit Zen
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} ```
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Often, in non-trivial HTTP applications, we want to read the HTTP header and examine its contents before choosing a type for [*Body]. To accomplish this, there needs to be a way to model the header portion of a message. And we'd like to do this in a way that allows functions which take the header as a parameter, to also accept a type representing the whole message (the function will see just the header part). This suggests inheritance, by splitting a new base class off of the message: ``` /// An HTTP message header template<bool isRequest, class Fields> struct header; ```
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Code which accesses the fields has to laboriously mention the `fields` member, so we'll not only make `header` a base class but we'll make a quality of life improvement and derive the header from the fields for notational convenience. In order to properly support all forms of construction of [*Fields] there will need to be a set of suitable constructor overloads (not shown): ``` /// An HTTP request header template<class Fields> struct header<true, Fields> : Fields {
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};
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/// An HTTP response header template<class Fields> struct header<false, Fields> : Fields {
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};
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/// An HTTP message template<bool isRequest, class Fields, class Body> struct message : header<isRequest, Fields> {
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};
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```
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Note that the `message` class now has a constructor allowing messages to be constructed from a similarly typed `header`. This handles the case where the user already has the header and wants to make a commitment to the type for [*Body]. A function can be declared which accepts any header: ``` template<bool isRequest, class Fields> void f(header<isRequest, Fields>& msg); ```
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string ID 224
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Until now we have not given significant consideration to the constructors of the `message` class. But to achieve all our goals we will need to make sure that there are enough constructor overloads to not only provide for the special copy and move members if the instantiated types support it, but also allow the fields container and body container to be constructed with arbitrary variadic lists of parameters. This allows the container to fully support allocators.
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The solution used in the library is to treat the message like a `std::pair` for the purposes of construction, except that instead of `first` and `second` we have the `Fields` base class and `message::body` member. This means that single-argument constructors for those fields should be accessible as they are with `std::pair`, and that a mechanism identical to the pair's use of `std::piecewise_construct` should be provided. Those constructors are too complex to repeat here, but interested readers can view the declarations in the corresponding header file.
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string ID 241
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There is now significant progress with our message container but a stumbling block remains. There is no way to control the allocator for the `std::string` members. We could add an allocator to the template parameter list of the header and message classes, use it for those strings. This is unsatisfying because of the combinatorial explosion of constructor variations needed to support the scheme. It also means that request messages could have [*four] different allocators: two for the fields and body, and two for the method and target strings. A better solution is needed.
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string ID 250
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To get around this we make an interface modification and then add a requirement to the [*Fields] type. First, the interface change: ``` /// An HTTP request header template<class Fields> struct header<true, Fields> : Fields {
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private:
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};
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/// An HTTP response header template<class Fields> struct header<false, Fields> : Fields {
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}; ```
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The start-line data members are replaced by traditional accessors using non-owning references to string buffers. The method is stored using a simple integer instead of the entire string, for the case where the method is recognized from the set of known verb strings.
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Now we add a requirement to the fields type: management of the corresponding string is delegated to the [*Fields] container, which can already be allocator aware and constructed with the necessary allocator parameter via the provided constructor overloads for `message`. The delegation implementation looks like this (only the response header specialization is shown): ``` /// An HTTP response header template<class Fields> struct header<false, Fields> : Fields {
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