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The opposite is also true: if there's a `tag_invoke` overload intended for <<ref_try_value_to>>, but not for <<ref_value_to>>, then calling <<ref_value_to>> will invoke the non-throwing overload, then construct a {ref_system_error} from the {ref_error_code} and throw it. Due to these fallbacks, it is recommended that users provide the overload from this section, rather then the other one, if they ever intend to use <<ref_try_value_to>>.
The opposite is also true: if there's a `tag_invoke` overload intended for <反之亦然：如果存在适用于&lt;<ref_try_value_to>&gt; 的 tag_invoke 重载，但没有适用于 &lt;<ref_try_value_to>>, but not for <<ref_value_to>>, then calling <<ref_value_to>> will invoke the non-throwing overload, then construct a {ref_system_error} from the {ref_error_code} and throw it. Due to these fallbacks, it is recommended that users provide the overload from this section, rather then the other one, if they ever intend to use <<&gt; 的重载，那么调用 &lt;<ref_value_to>&gt; 时会转而调用不抛异常的重载，随后根据返回的 {ref_error_code} 构造一个 {ref_system_error} 并抛出。由于存在上述回退机制，建议用户在计划使用 &lt;<ref_try_value_to>&gt; 时，优先提供本节所述的不抛异常重载，而非抛异常的重载。</ref_try_value_to></ref_value_to></ref_value_to></ref_try_value_to>>.
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If <<ref_try_value_to>> is used with a type, for which there's no `tag_invoke` overload of the form described in this section, but there is one of the form intended for <<ref_value_to>>, then the library still tries to perform the conversion. It uses the throwing overload, and attempts to convert any thrown exception into an {ref_error_code}. Note, though, that such approach will likely be slower then a dedicated overload.
If <如果将 &lt;<ref_try_value_to>> is used with a type, for which there's no `tag_invoke` overload of the form described in this section, but there is one of the form intended for <<ref_value_to>>, then the library still tries to perform the conversion. It uses the throwing overload, and attempts to convert any thrown exception into an {ref_error_code}. Note, though, that such approach will likely be slower then a dedicated overload.&gt; 与某种类型搭配使用，而该类型不存在本节中所描述的这种形式的 `tag_invoke` 重载函数，但存在用于 &lt;<ref_value_to>&gt; 形式的那种重载函数，那么库仍会尝试进行转换。它会使用抛出异常的重载函数，并尝试将任何抛出的异常转换为 {ref_error_code}。不过需要注意的是，这种方式可能比专门的重载函数要慢一些。</ref_value_to></ref_try_value_to>
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The overload lets us use `ip_address` with <<ref_try_value_to>>.
The overload lets us use该重载使我们能够将 `ip_address` with <与 &lt;<ref_try_value_to>&gt; 一起使用。</ref_try_value_to>.
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For the class `ip_address` from the section <<custom_conversions>> this overload may look like this:
For the class `ip_address` from the section <<custom_conversions>> this overload may look like this:对于 &lt;<custom_conversions>&gt; 节中所述的 `ip_address` 类，该重载可能如下所示：</custom_conversions>
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``` result_for<T, value>::type tag_invoke( const try_value_to_tag< T >&, const value& ); ```
``` result_for<Tt, value="">::type tag_invoke( const try_value_to_tag< T >&&lt; T &gt;&amp;, const value&amp; ); ```</t,>
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The library provides non-throwing conversions for all the categories of types it supports with <<ref_value_to>>. If a user wants to use it with custom types, an overload of `tag_invoke` of this form needs to be provided:
The library provides non-throwing conversions for all the categories of types it supports with <<ref_value_to>>. If a user wants to use it with custom types, an overload of `tag_invoke` of this form needs to be provided:该库为 &lt;<ref_value_to>&gt; 所支持的所有类型类别均提供了不抛异常的转换。如果用户希望将其用于自定义类型，则需要提供如下形式的 `tag_invoke` 重载：</ref_value_to>
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There's no non-throwing equivalent for <<ref_value_from>>. This is simply because we haven't yet encountered a situation where <<ref_value_from>> needed to communicate an error to the caller.
There's no non-throwing equivalent for <<ref_value_from>>. This is simply because we haven't yet encountered a situation where <<ref_value_from>> needed to communicate an error to the caller.对于 &lt;<ref_value_from>&gt;; 没有对应的不抛异常版本。这仅仅是因为我们尚未遇到需要 &lt;<ref_value_from>&gt; 向调用者报告错误的情况。</ref_value_from></ref_value_from>
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= Non-Throwing Conversions For the case where throwing exceptions is undesirable, Boost.JSON also provides a non-throwing version of <<ref_value_to>>, the function template <<ref_try_value_to>>. It returns {ref_result}, a specialised variant that either holds a value or an {ref_error_code}.
= Non-Throwing Conversions For the case where throwing exceptions is undesirable, Boost.JSON also provides a non-throwing version of <<ref_value_to>>, the function template <<ref_try_value_to>>. It returns {ref_result}, a specialised variant that either holds a value or an {ref_error_code}.不抛异常的转换 对于不希望抛出异常的场景，Boost.JSON 还提供了 &lt;<ref_value_to>&gt; 的不抛异常版本，即函数模板 &lt;<ref_try_value_to>&gt;。它返回 {ref_result}，这是一个特化的变体类型，要么持有转换得到的值，要么持有 {ref_error_code}。</ref_try_value_to></ref_value_to>
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Finally, it is worth mentioning that due to the ability to provide conversions to JSON containers without a binary dependency on the library, you don't have to push such dependency on your users. This is particularly relevant for libraries for which interoperation with Boost.JSON is only ancillary.
Finally, it is worth mentioning that due to the ability to provide conversions to JSON containers without a binary dependency on the library, you don't have to push such dependency on your users. This is particularly relevant for libraries for which interoperation with Boost.JSON is only ancillary.最后值得一提的是，由于可以在不引入对库的二进制依赖的前提下提供到 JSON 容器的转换，因此您无需将此类依赖强加给您的用户。这一点对于那些仅需与 Boost.JSON 进行辅助性互操作的库尤为重要。
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If your users are external, they and not you decide whether throwing exceptions is acceptable. So, in this case it is better to use non-throwing `tag_invoke` overloads. In addition, for customising conversion of composite types, always use `tag_invoke` overload with 2 context parameters. This will allow correct context propagation to elements of composites. This will also allow propagation of exceptions from conversion of elements.
If your users are external, they and not you decide whether throwing exceptions is acceptable. So, in this case it is better to use non-throwing `tag_invoke` overloads. In addition, for customising conversion of composite types, always use `tag_invoke` overload with 2 context parameters. This will allow correct context propagation to elements of composites. This will also allow propagation of exceptions from conversion of elements.如果您的用户是外部的，那么是否允许抛出异常是由他们而非您来决定的。因此，在这种情况下，最好使用不抛异常的 `tag_invoke` 重载。此外，在自定义复合类型的转换时，应始终使用带有两个上下文参数的 `tag_invoke` 重载。这将确保上下文能正确传递给复合类型的各个元素，同时也支持从元素转换中传播异常。
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The next thing to consider is whether your conversions are intended for internal use, or whether your users are not members of your team. If your users are external, then they will ultimately determine the conditions in which these conversions will be used. Conversely, for internal libraries and applications, you have the full control of usage conditions.
The next thing to consider is whether your conversions are intended for internal use, or whether your users are not members of your team. If your users are external, then they will ultimately determine the conditions in which these conversions will be used. Conversely, for internal libraries and applications, you have the full control of usage conditions.接下来需要考虑的是：您的转换是仅供内部使用，还是面向团队以外的用户。如果您的用户属于外部人员，那么他们最终将决定这些转换的使用条件；反之，对于内部库和应用程序，您可以完全掌控其使用条件。
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If library-provided conversions are suitable for you, you have the option to use direct conversions. This also puts the requirement of being default constructible on many of your types.
If library-provided conversions are suitable for you, you have the option to use direct conversions. This also puts the requirement of being default constructible on many of your types.如果库提供的转换对你适用，你可以选择使用直接转换。但这同时也要求你的许多类型必须是可默认构造的。
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The first advice is to use one of the library-provided conversions, rather then providing a custom one, unless the resulting format is undesirable. If the library deduces the wrong conversion category, you can opt out by specialising the relevant trait to inherit from `std::false_type`.
The first advice is to use one of the library-provided conversions, rather then providing a custom one, unless the resulting format is undesirable. If the library deduces the wrong conversion category, you can opt out by specialising the relevant trait to inherit from首要建议是：除非生成的格式不符合需求，否则应优先使用库提供的转换，而非自行实现自定义转换。如果库错误地推断了转换类别，你可以通过将相关特征特化为继承自 `std::false_type`. 来排除该转换。
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= Conversion Customization Guidelines With so many options, it can be hard to choose the best way to customise conversion for your type. In this section, we will discuss those options and suggest which to choose when.
= Conversion Customization Guidelines With so many options, it can be hard to choose the best way to customise conversion for your type. In this section, we will discuss those options and suggest which to choose when.转换自定义指南 由于选项众多，为类型选择最合适的转换自定义方式可能并不容易。本节将讨论这些选项，并给出不同场景下的选用建议。
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Forward declarations of contextual conversions are done very similarly:
Forward declarations of contextual conversions are done very similarly:上下文相关转换的前向声明方式与此非常相似：
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As discussed previously, we prefer to define a non-throwing overload of `tag_invoke` for <<ref_try_value_to>>, rather then the throwing overload for <<ref_value_to>>, as the latter can fallback to the former without performance degradation.
As discussed previously, we prefer to define a non-throwing overload of `tag_invoke` for <<ref_try_value_to>>, rather then the throwing overload for <<ref_value_to>>, as the latter can fallback to the former without performance degradation.如前所述，我们更倾向于为 &lt;<ref_try_value_to>&gt; 定义不抛异常的 `tag_invoke` 重载，而不是为 &lt;<ref_value_to>&gt; 定义抛异常的重载，因为后者可以在无性能损失的情况下回退到前者。</ref_value_to></ref_try_value_to>
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After that the definitions of `tag_invoke` overloads should be provided. These overloads have to be templates, since <<ref_value>> is only forward-declared and hence is an incomplete type.
After that the definitions of `tag_invoke` overloads should be provided. These overloads have to be templates, since <<ref_value>> is only forward-declared and hence is an incomplete type.之后应提供 `tag_invoke` 重载的定义。这些重载必须是模板，因为 &lt;<ref_value>&gt; 仅是前向声明的，因此是一个不完整类型。</ref_value>
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Note that <<ref_value_from>> is declared using an out-parameter, rather then returning its result. This overload is specifically designed for this use-case.
Note that <请注意，&lt;<ref_value_from>> is declared using an out-parameter, rather then returning its result. This overload is specifically designed for this use-case.&gt; 是通过输出参数声明的，而非返回其结果。该重载正是为这一使用场景而设计的。</ref_value_from>
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