Exit Zen
12
string ID 63
type: paragraph
English
Here is our first attempt at declaring some message containers:
63/630 · 63
13
string ID 97
type: paragraph
English
These containers are capable of representing everything in the model of HTTP requests and responses described in __rfc7230__. Request and response objects are different types. The user can choose the container used to represent the fields. And the user can choose the [*Body] type, which is a concept defining not only the type of `body` member but also the algorithms used to transfer information in and out of that member when performing serialization and parsing.
466/4660 · 466
14
string ID 105
type: paragraph
English
However, a problem arises. How do we write a function which can accept an object that is either a request or a response? As written, the only obvious solution is to make the message a template type. Additional traits classes would then be needed to make sure that the passed object has a valid type which meets the requirements. These unnecessary complexities are bypassed by making each container a partial specialization: ``` /// An HTTP message template<bool isRequest, class Fields, class Body> struct message;
514/5140 · 514
15
string ID 116
type: paragraph
English
/// An HTTP request template<class Fields, class Body> struct message<true, Fields, Body> {
91/910 · 91
17
string ID 128
type: paragraph
English
/// An HTTP response template<bool isRequest, class Fields, class Body> struct message<false, Fields, Body> {
109/1090 · 109
19
string ID 141
type: paragraph
English
Now we can declare a function which takes any message as a parameter: ``` template<bool isRequest, class Fields, class Body> void f(message<isRequest, Fields, Body>& msg); ```
175/1750 · 175
20
string ID 147
type: paragraph
English
This function can manipulate the fields common to requests and responses. If it needs to access the other fields, it can use overloads with partial specialization, or in C++17 a `constexpr` expression: ``` template<bool isRequest, class Fields, class Body> void f(message<isRequest, Fields, Body>& msg) {
304/3040 · 304
22
string ID 165
type: paragraph
English
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; ```
578/5780 · 578
23
string ID 178
type: paragraph
English
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 {
470/4700 · 470
194
string ID 194
type: paragraph
English
/// An HTTP response header template<class Fields> struct header<false, Fields> : Fields {
90/900 · 90
27
string ID 203
type: paragraph
English
/// An HTTP message template<bool isRequest, class Fields, class Body> struct message : header<isRequest, Fields> {
115/1150 · 115
30
string ID 215
type: paragraph
English
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); ```
376/3760 · 376
31
string ID 224
type: paragraph
English
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.
465/4650 · 465
32
string ID 232
type: paragraph
English
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.
559/5590 · 559
33
string ID 241
type: paragraph
English
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.
575/5750 · 575
34
string ID 250
type: paragraph
English
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 {
224/2240 · 224
271
string ID 271
type: paragraph
English
/// An HTTP response header template<class Fields> struct header<false, Fields> : Fields {
90/900 · 90
39
string ID 282
type: paragraph
English
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.
266/2660 · 266
40
string ID 287
type: paragraph
English
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 {
463/4630 · 463
42
string ID 315
type: paragraph
English
Now that we've accomplished our initial goals and more, there are a few more quality of life improvements to make. Users will choose different types for `Body` far more often than they will for `Fields`. Thus, we swap the order of these types and provide a default. Then, we provide type aliases for requests and responses to soften the impact of using `bool` to choose the specialization:
389/3890 · 389