Exit Zen
12
:77
English
Contexts we used so far were empty classes. But contexts may have data members and member functions just as any class. Implementers of conversion functions can take advantage of that to have conversions configurable at runtime or pass special objects to conversions (e.g. allocators).
93/2840 · 284
13
:82
English
Let's rewrite conversion for ``system_clock::time_point``s to allow any format supported by `std::strftime`.
64/1080 · 108
14
:90
English
This `tag_invoke` overload lets us change date conversion format at runtime. Also note, that there is no ambiguity between `as_iso_8601` overload and `date_format` overload. You can use both in your program:
91/2070 · 207
15
:99
English
Combining Contexts
5/180 · 18
16
:100
English
Often it is needed to use several conversion contexts together. For example, consider a log of remote users identified by IP addresses accessing a system. We can represent it as `std::vector<std::pair<std::chrono::system_clock::time_point, ip_address>>`. We want to serialize both ``ip_address``es and ``time_point``s as strings, but for this we need both `as_string` and `as_iso_8601` contexts. To combine several contexts just use {std_tuple}. Conversion functions will select the first element of the tuple for which a `tag_invoke` overload exists and will call that overload. As usual, `tag_invoke` overloads that don't use contexts and library-provided generic conversions are also supported. Thus, here's our example:
396/7230 · 723
17
:117
English
In this snippet `time_point` is converted using `tag_invoke` overload that takes `as_iso_8601`, `ip_address` is converted using `tag_invoke` overload that takes `as_string`, and {std_vector} is converted using a generic conversion provided by the library.
133/2550 · 255
18
:122
English
Contexts and Composite Types
8/280 · 28
19
:123
English
As was shown previously, generic conversions provided by the library forward contexts to conversions of nested objects. And in the case when you want to provide your own conversion function for a composite type enabled by a particular context, you usually also need to do that.
72/2770 · 277
20
:128
English
Consider this example. As was discussed in a previous section, <<ref_is_map_like>> requires that your key type satisfies <<ref_is_string_like>>. Now, let's say your keys are not string-like, but they do convert to <<ref_string>>. You can make such maps to also convert to objects using a context. But if you want to also use another context for values, you need a way to pass the full combined context to map elements. So, you want the following test to succeed.
252/4620 · 462
21
:141
English
For this you will have to use a different overload of `tag_invoke`. This time it has to be a function template, and it should have two parameters for contexts. The first parameter is the specific context that disambiguates that particular overload. The second parameter is the full context passed to <<ref_value_to>> or <<ref_value_from>>.
172/3390 · 339
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