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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.
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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.
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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 <<ref_value_from>> is declared using an out-parameter, rather then returning its result. This overload is specifically designed for this use-case.
06/05/2026
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= Avoiding Physical Dependency Some users, particularly library authors, may wish to provide conversions between their types and <<ref_value>>, but at the same time would prefer to avoid having their library depend on Boost.JSON. This is possible to achieve with the help of a few forward declarations.
= Avoiding Physical Dependency Some users, particularly library authors, may wish to provide conversions between their types and <<ref_value>>, but at the same time would prefer to avoid having their library depend on Boost.JSON. This is possible to achieve with the help of a few forward declarations.
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File “doc/pages/conversion/nothrow_zh_Hans.adoc” was added. 06/05/2026
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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 <<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>>.
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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 <<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.
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The overload lets us use `ip_address` with <<ref_try_value_to>>.
The overload lets us use `ip_address` with <<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:
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``` result_for<T, value>::type tag_invoke( const try_value_to_tag< T >&, const value& ); ```
``` result_for<T, value>::type tag_invoke( const try_value_to_tag< T >&, const value& ); ```
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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:
06/05/2026
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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.
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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}.
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When an <<ref_object>> is formatted to a {std_ostream}, the result is a valid JSON. That is, the object will be output with curly braces and a comma separated list of key/value pairs, as per the JSON specification.
When an <<ref_object>> is formatted to a {std_ostream}, the result is a valid JSON. That is, the object will be output with curly braces and a comma separated list of key/value pairs, as per the JSON specification.
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Formatted Output
Formatted Output
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As with `std::pair`, the <<ref_key_value_pair>> type can be used with structured bindings in {cpp}17. Specializations of `std::tuple_size`, `std::tuple_element`, and overloads of <<ref_get>> are all provided for this purpose.
As with `std::pair`, the <<ref_key_value_pair>> type can be used with structured bindings in {cpp}17. Specializations of `std::tuple_size`, `std::tuple_element`, and overloads of <<ref_get>> are all provided for this purpose.
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For the complete listing of all available member functions and nested types, see the reference page for <<ref_object>>.
For the complete listing of all available member functions and nested types, see the reference page for <<ref_object>>.
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Internally, the container computes a hash table over the keys so that the complexity of lookups is in constant time, on average.
Internally, the container computes a hash table over the keys so that the complexity of lookups is in constant time, on average.
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Similar to the `std` counterpart, elements may be accessed directly by their key with bounds checking using <<ref_object_at>>, or without bounds checking using <<ref_object_operator_lb_rb>> which creates a null element if the key does not already exist:
Similar to the `std` counterpart, elements may be accessed directly by their key with bounds checking using <<ref_object_at>>, or without bounds checking using <<ref_object_operator_lb_rb>> which creates a null element if the key does not already exist:
06/05/2026

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