Exit Zen
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With the removal of overloads that specify parameters for a substring, a member function `subview` that returns a <<ref_string_view>> is provided to facilitate cheap substring operations:
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We can define a conversion from the user-defined type to a <<ref_value>> by defining an overload of `tag_invoke` in the same namespace. This maps `customer` to a JSON object:
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A <<ref_string>> may be constructed using the <<default_memory_resource,default memory resource>> without incurring any memory allocations. Alternatively, a <<ref_storage_ptr>> can be provided explicitly:
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Unless otherwise specified, the parser in this library is strict. It recognizes only valid, standard JSON. The parser can be configured to allow certain non-standard extensions by filling in a <<ref_parse_options>> structure and passing it by value. By default all extensions are disabled:
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This allows us to use the library function <<ref_value_from>> to produce a <<ref_value>> from our type:
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When building with {cpp}20 or later, the use of https://en.cppreference.com/w/cpp/language/aggregate_initialization#Designated_initializers[designated initializers] with <<ref_parse_options>> is possible:
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To go from JSON to a user-defined type we use <<ref_value_to>>, which uses another overload of `tag_invoke`. This converts a JSON value to a `customer`. It throws an exception if the contents of the value do not match what is expected:
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<<ref_string>> directly supports access to its storage in the range `[size(), capacity())`. This can be used for efficient assembly of a string from several parts. After the string is assembled, use the member function <<ref_string_grow>> to update the string's size and insert the null terminator. For example:
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The library's generic algorithms recognize when you are converting a <<ref_value>> to a container which resembles an array or object, so if you wanted to turn a JSON array into a vector of customers you might write:
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Instances of <<ref_parser>> and <<ref_stream_parser>> offer functionality beyond what is available when using the <<ref_parse>> free functions:
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The default memory resource uses the global `operator new` and `operator delete` to allocate memory. This resource is not reference counted and has a non-trivial deallocate function. All default-constructed <<ref_storage_ptr>> objects reference the same memory resource:
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To use the <<ref_parser>>, declare an instance. Then call <<ref_parser_write>> once with the buffer containing representing the input JSON. Finally, call <<ref_parser_release>> to take ownership of the resulting <<ref_value>> upon success. This example persists the parser instance in a class member to reuse across calls:
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Sometimes a protocol may have a JSON text followed by data that is in a different format or specification. The JSON portion can still be parsed by using the function <<ref_parser_write_some>>. Upon success, the return value will indicate the number of characters consumed from the input, which will exclude the non-JSON characters:
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The <<ref_stream_parser>> implements a https://en.wikipedia.org/wiki/Online_algorithm[__streaming algorithm__]; it allows incremental processing of large JSON inputs using one or more contiguous character buffers. The entire input JSON does not need to be loaded into memory at once. A network server can use the streaming interface to process incoming JSON in fixed-size amounts, providing these benefits:
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The resource to use when constructing values may be specified in calls to <<ref_parse>> as shown here:
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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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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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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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