Exit Zen
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In the following example a JSON text is parsed from standard input a line at a time. Error codes are used instead. The function <<ref_stream_parser_finish>> is used to indicate the end of the input:
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Type satisfying <<ref_is_path_like>>.
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The function <<ref_get_null_resource>> returns a global instance of the null resource. This resource offers a simple invariant: all calls to allocate will throw the exception `std::bad_alloc`. An instance of the null resource can be used to make parsing guarantee that allocations from the heap are never made. This is explored in more detail in a later section.
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`std::getline` use (see the warning in <<non_standard_json>> section).
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The containers <<ref_array>>, <<ref_object>>, and <<ref_value>> all propagate the memory resource they were constructed with to child elements:
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After default construction, or after <<ref_stream_parser_reset>> is called with no arguments, the <<ref_value>> produced after a successful parse operation uses the default memory resource. To use a different memory resource, call `reset` with the resource to use. Here we use a <<ref_monotonic_resource>>, which is optimized for parsing but not subsequent modification:
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Here, the map is converted into an <<ref_object>>, since it matches <<ref_is_map_like>>. Each of its keys is converted into a <<ref_string>>, as `std::string` matches <<ref_is_string_like>>, and each of its values is converted into an <<ref_array>>, as `std::pair` matches <<ref_is_tuple_like>>. Finally, elements of pairs are converted into a `std::int64_t` number and a `bool`.
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It is important to note that <<ref_storage_ptr>> supports both shared-ownership and reference lifetime models. Construction from a memory resource pointer does not transfer ownership:
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Shared ownership is achieved using the function <<ref_make_shared_resource>>, which creates a new, reference-counted memory resource using a dynamic memory allocation and returns it as a <<ref_storage_ptr>>:
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Users who wish to implement custom parsing strategies may create their own handler to use with an instance of <<ref_basic_parser>>. The handler implements the function signatures required by SAX event interface. In <<examples_validate>> example we define the "null" parser, which throws out the parsed results, to use in the implementation of a function that determines if a JSON text is valid.
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^|&mdash; |<<ref_value_try_as_null>> ^|&mdash; ^|&mdash;
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The emplace members of <<ref_value>> return a typed reference to the underlying representation. For example, the call to <<ref_value_emplace_string,`value::emplace_string`>> in the previous example returns a <<ref_string,`string&`>>. This table shows the underlying type for each kind:
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interface similar to {std_unordered_map}, that remembers insertion order.
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If the <<ref_kind>> of a <<ref_value>> is known, functions such as <<ref_value_as_bool,`value::as_bool`>> or <<ref_value_as_string,`value::as_string`>> may be used to obtain a reference to the underlying representation without changing the existing value:
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However, as shown above these functions throw an exception if the kind in the <<ref_value>> does not match the kind denoted by the function signature. This can be used as a concise form of validation: access values as if they were the right type, but handle the resulting exception indicating if the schema of the JSON is not valid.
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We can query a value for its underlying representation of a particular kind in a way that does not throw exceptions by requesting a pointer which may be null, instead of a reference. Here we use <<ref_value_if_string,`value::if_string`>> to conditionally perform an assignment without using exceptions:
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When a <<ref_value>> is formatted to a {std_ostream}, the result is serialized JSON as if by calling <<ref_serialize>>.
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