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3. Doc / Qbk / 08 Design / 1 Http Message (qbk)
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29 string ID 213
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — ``` 3/10000 · 3
30 string ID 215
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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); ``` 376/10000 · 376
31 string ID 224
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 465/10000 · 465
32 string ID 232
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 559/10000 · 559
33 string ID 241
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 575/10000 · 575
34 string ID 250
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 { 224/10000 · 224
35 string ID 267
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — private: 8/10000 · 8
269 string ID 269
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — }; 2/10000 · 2
271 string ID 271
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — /// An HTTP response header template<class Fields> struct header<false, Fields> : Fields { 90/10000 · 90
279 string ID 279
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — }; ``` 6/10000 · 6
39 string ID 282
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 266/10000 · 266
40 string ID 287
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 { 463/10000 · 463
312 string ID 312
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — }; ``` 6/10000 · 6
42 string ID 315
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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: 389/10000 · 389
43 string ID 322
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — ``` /// An HTTP header template<bool isRequest, class Body, class Fields = fields> struct header; 97/10000 · 97
44 string ID 327
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — /// An HTTP message template<bool isRequest, class Body, class Fields = fields> struct message; 95/10000 · 95
45 string ID 331
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — /// An HTTP request template<class Body, class Fields = fields> using request = message<true, Body, Fields>; 108/10000 · 108
46 string ID 335
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — /// An HTTP response template<class Body, class Fields = fields> using response = message<false, Body, Fields>; ``` 115/10000 · 115
47 string ID 340
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — This allows concise specification for the common cases, while allowing for maximum customization for edge cases: ``` request<string_body> req; 142/10000 · 142
48 string ID 345
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	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — response<file_body> res; ``` 28/10000 · 28