Boost Json Translation (zh_Hans)/Doc / Pages / Conversion / Custom (adoc)

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	English = Custom Conversions Boost.JSON uses two mechanisms to customize conversion between <<ref_value>> and user types. One mechanism involves specializing type traits. The other one is more powerful and requires defining overloads of `tag_invoke`. Both mechanisms will be further explained in this section. 301/10000 · 301
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	English Conversion Traits 17/10000 · 17
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	English Previously a number of conversion type traits, like <<ref_is_tuple_like>> or <<ref_is_sequence_like>>, were introduced. The library tries the traits one after another and uses the implementation that corresponds to the first matching trait. In some cases, though, a type would match a trait with a higher priority, but the user intends for it to belong to a lower priority category. If this happens the user can specialize the trait that's not supposed to match for that type to be an equivalent of `std::false_type`. 518/10000 · 518
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	English Consider this type: 19/10000 · 19
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	English It exposes both a sequence API and a tuple API. But converting from <<ref_value>> to `user_ns::ip_address` would not be able to use implementation for sequences, since those are constructed empty and then populated one element at a time, while `ip_address` has a fixed size of 4. The tuple conversion would fit, though. The only problem is that <<ref_is_tuple_like>> has a lower priority than <<ref_is_sequence_like>>. In order to circumvent this, the user only needs to specialize <<ref_is_sequence_like>> to not match `ip_address`. 533/10000 · 533
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	English `tag_invoke` Overloads 22/10000 · 22
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	English The second, more powerful approach, is to provide the conversion implementation yourself. With Boost.JSON this is done by defining an overload of `tag_invoke` function (the benefits of this mechanism are outlined in http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1895r0.pdf[C++ proposal P1895]. In essence, `tag_invoke` provides a uniform interface for defining customization points by using argument-dependent lookup to find a viable overload from the point at which it is called. As the name suggests, a tag type is passed as an argument in order to: 562/10000 · 562
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	English discard candidates that are unrelated to that particular 56/10000 · 56
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	English customization point, and 24/10000 · 24
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	English embed the user-defined type into the arguments list (e.g. by using a tag type 77/10000 · 77
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	English template such as `value_to_tag<T>`) so that its http://eel.is/c++draft/basic.lookup.argdep#2[associated namespaces and entities] are examined when name lookup is performed. 172/10000 · 172
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	English This has the effect of finding user-provided `tag_invoke` overloads, even if they are declared (lexically) after the definition of the calling function. 152/10000 · 152
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	English Overloads of `tag_invoke` called by <<ref_value_from>> take the form: 69/10000 · 69
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	English ``` void tag_invoke( const value_from_tag&, value&, T ); ``` 60/10000 · 60
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	English While overloads of `tag_invoke` called by <<ref_value_to>> take the form: 73/10000 · 73
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	English ``` T tag_invoke( const value_to_tag< T >&, const value& ); ``` 63/10000 · 63
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	English If we implemented conversion for `user_ns::ip_address` manually with this approach, it would look like this: 108/10000 · 108
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	English Since the type being converted is embedded into the function's signature, user-provided overloads are visible to argument-dependent lookup and will be candidates when a conversion is performed: 193/10000 · 193
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	English Users can freely combine types with custom conversions with types with library-provided conversions. The library handles them correctly: 136/10000 · 136
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