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Note, that if there is no dedicated `tag_invoke` overload for a given type and a given context, the implementation falls back to overloads without context. Thus it is easy to combine contextual conversions with conversions provided by the library:
Note, that if there is no dedicated `tag_invoke` overload for a given type and a given context, the implementation falls back to overloads without context. Thus it is easy to combine contextual conversions with conversions provided by the library:请注意，如果对于给定类型和给定上下文没有专用的`tag_invoke`重载，实现将回退到不带上下文的重载。因此，可以轻松地将上下文相关的转换与库提供的转换结合起来：
06/05/2026
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These `tag_invoke` overloads take an extra `as_string` parameter, which disambiguates this specific representation of `ip_address` from all other potential representations. In order to take advantage of them one needs to pass an `as_string` object to <<ref_value_from>> or <<ref_value_to>> as the last argument:
These 这些`tag_invoke` overloads take an extra `as_string` parameter, which disambiguates this specific representation of `ip_address` from all other potential representations. In order to take advantage of them one needs to pass an `as_string` object to <重载接受一个额外的`as_string`参数，用于将`ip_address`的这种特定表示形式与其他所有可能的表示形式区分开来。要使用这些重载，需要将`as_string`对象作为最后一个参数传递给 &lt;<ref_value_from>> or <&gt; 或 &lt;<ref_value_to>> as the last argument:&gt;：</ref_value_to></ref_value_from>
06/05/2026
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Let's implement conversion from `user_ns::ip_address` to a JSON string:
Let's implement conversion from 我们来实现从`user_ns::ip_address` to a JSON string:到JSON字符串的转换：
06/05/2026
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= Contextual Conversions Previously in this section we've been assuming that there is a particular fitting JSON representation for a type. But this is not always the case. Often one needs to represent particular value with JSON of certain format in one situation and with another format in a different situation. This can be achieved with Boost.JSON by providing an extra argument---context.
= Contextual Conversions Previously in this section we've been assuming that there is a particular fitting JSON representation for a type. But this is not always the case. Often one needs to represent particular value with JSON of certain format in one situation and with another format in a different situation. This can be achieved with Boost.JSON by providing an extra argument---context.上下文相关的转换 此前在本节中，我们一直假定某个类型存在一种特定且合适的 JSON 表示形式。但实际情况并非总是如此。很多时候，同一个值在不同场景下需要以不同的 JSON 格式来表示。在 Boost.JSON 中，可以通过提供一个额外的参数——上下文（context）——来实现这一点。
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File “doc/pages/conversion/context_zh_Hans.adoc” was added. 06/05/2026
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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>>.
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>>.
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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.
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.
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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.
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.
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Contexts and Composite Types
Contexts and Composite Types
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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.
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.
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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:
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:
06/05/2026
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Combining Contexts
Combining Contexts
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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:
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:
06/05/2026
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Let's rewrite conversion for ``system_clock::time_point``s to allow any format supported by `std::strftime`.
Let's rewrite conversion for ``system_clock::time_point``s to allow any format supported by `std::strftime`.
06/05/2026
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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).
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).
06/05/2026
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Passing Data to Conversion Functions
Passing Data to Conversion Functions
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One particular use case that is enabled by contexts is adaptor libraries that define JSON conversion logic for types from a different library.
One particular use case that is enabled by contexts is adaptor libraries that define JSON conversion logic for types from a different library.
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The new context is used in a similar fashion to `as_string` previously in this section.
The new context is used in a similar fashion to `as_string` previously in this section.
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Reverse conversion is left out for brevity.
Reverse conversion is left out for brevity.
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As an example, let's implement conversion for https://en.cppreference.com/w/cpp/chrono/system_clock[``std::chrono::system_clock::time_point``s] using https://en.wikipedia.org/wiki/ISO_8601[ISO 8601] format.
As an example, let's implement conversion for https://en.cppreference.com/w/cpp/chrono/system_clock[``std::chrono::system_clock::time_point``s] using https://en.wikipedia.org/wiki/ISO_8601[ISO 8601] format.
06/05/2026

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