Exit Zen
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= Initializer Lists Initializer lists can be used to construct or assign a <<ref_value>>:
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= Value Conversion While the <<ref_value>> container makes it easy to create ad-hoc structures, often it is necessary to convert between JSON and user-defined types or types from the standard library.
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Modifiable sequences of characters are represented using objects of type <<ref_string>>.
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When accessing a number contained within a <<ref_value>>, the function used must match the value's <<ref_kind>> exactly; no conversions will be performed. For example if `as_double` is called on a <<ref_value>> that contains a `std::uint64_t`, an exception is thrown. Similarly, the function `if_double` will return `nullptr` and calling `get_double` will result in undefined behavior:
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The elements are stored contiguously as instances of <<ref_key_value_pair>>.
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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.
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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.
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<<ref_value_to>> does not have a similar parameter, as <<ref_value>> is not created.
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An empty array may be constructed without incurring any memory allocations using the <<default_memory_resource,default memory resource>>. A <<ref_storage_ptr>> can also be explicitly specified:
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Serialization is the process where a JSON document represented in memory by a <<ref_value>> is turned into a sequence of characters. The library provides the following free functions and types for serialization:
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Simple initializer lists produce an <<ref_array>>:
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The function template <<ref_value_from>> provides an interface to construct a <<ref_value>> from a type `T`. The function template <<ref_value_to>> converts in the opposite direction, from a type `T` to <<ref_value>>. Both support a wide variety of different https://en.cppreference.com/w/cpp/language/types[fundamental types], such as `int` or `double`, standard library types, such as `std::string` or `std::vector<T>`, and can be extended to support user-defined types.
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The interface and functionality of <<ref_string>> is the same as {std_string} except that:
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In cases where you know that a <<ref_value>> contains a number but don't know its <<ref_kind>>, `value::to_number` can be used to convert the <<ref_value>> to an arithmetic type:
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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:
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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`.
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That being said, if your types are default-constructible and you don't need the customisability allowed by <<ref_value_to>> and <<ref_value_from>>, then you can get a significant performance boost with direct conversions.
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Iterators are ordinary pointers, and may become invalidated on insertions
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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:
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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.
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