Exit Zen
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include::{entities-file}[leveloffset=+1]
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Up until now we have shown how values may be constructed from a memory resource pointer, where ownership is not transferred. In this case the caller is responsible for ensuring that the lifetime of the resource is extended for the life of the container. Sometimes you want the container to acquire shared ownership of the resource. This is accomplished with <<ref_make_shared_resource>>:
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`bool`
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Overloads of `tag_invoke` called by <<ref_value_from>> take the form:
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For the complete listing of all available member functions and nested types, see the reference page for <<ref_object>>.
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The member function <<ref_value_kind,`value::kind`>> may be used to query the kind stored in the value. Alternatively, member functions like <<ref_value_is_object,`value::is_object`>> <<ref_value_is_number,`value::is_number`>> may be used to check whether or not the value is a particular kind:
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Name
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*rapidjson*
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https://en.cppreference.com/w/cpp/types/is_arithmetic[Arithmetic type]
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Matrix
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A special variant which can hold one of any of the six standard JSON data
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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.
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Functions like <<ref_value_if_object,`value::if_object`>> actually return a pointer to the object if the value is an object, otherwise they return null. This allows them to be used both in boolean contexts as above, and in assignments or conditional expressions to capture the value of the pointer:
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of JSON texts each contained in a single contiguous character buffer, returning each result as a <<ref_value>>.
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While overloads of `tag_invoke` called by <<ref_value_to>> take the form:
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After a <<ref_value>> is constructed, its kind can change depending on what is assigned to it, or by calling functions such as <<ref_value_emplace_array,`value::emplace_array`>> or <<ref_value_emplace_bool,`value::emplace_bool`>>. If the assignment is successful, in other words it completes without any exceptions then the value is replaced. Otherwise, the value is unchanged. All operations which can fail to modify a value offer the strong exception safety guarantee.
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The <<ref_serialize>> function converts a <<ref_value>> into a {std_string}:
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These containers are explored in-depth in the sections that follow.
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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:
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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.
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