Exit Zen
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a series of member function calls to a user provided handler. This allows custom behaviors to be implemented for representing the document in memory.
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While keys are strings, the mapped type of objects and the element type of arrays is the aforementioned type <<ref_value>> which can hold any JSON element, as seen in the previous assignments. Instead of building the JSON document using a series of function calls, we can build it in one statement using an initializer list:
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which is optimized for parsing without subsequent modification. The <<ref_stream_parser>> object is reused between trials, allowing temporary memory allocated by the implementation to persist and improve performance.
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itself.
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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.
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To avoid creating such temporaries with an incorrect {ref_memory_resource}, using <<ref_value>>'s member functions <<ref_value_emplace_array>>, <<ref_value_emplace_object>>, and <<ref_value_emplace_string>> can be helpful.
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When a <<ref_value>>, <<ref_array>>, or <<ref_object>> is assigned or constructed from an initializer list, the creation of the new value happens only once. This makes initializer lists equally efficient as using the other ways to create a value. The types in this library are first-class, supporting copy and move construction and assignment:
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to a {std_ostream}.
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Initializer lists can be used to unambiguously construct or assign an <<ref_object>> or <<ref_array>>:
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An empty object 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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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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When an <<ref_array>> is formatted to a {std_ostream}, the result is a valid JSON. That is, the array will be output with square brackets and the comma separated list of values, as per the JSON specification.
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library uses the <<ref_storage_ptr>> allocator model, has the same interface on all C++ versions, and has an optimized class layout to keep the size of JSON values small. <<ref_string>> also implements an improved interface that replaces extraneous overloads with ones that use <<ref_string_view>>.
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Similarly, an initializer list for an <<ref_object>> is always interpreted as an <<ref_object>>. In such cases, the initializer list must be a list of key-value pairs. For example, the following code will not compile because `1` is not convertible to a string:
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To permit custom memory allocation strategies, these containers all allow construction with a <<ref_storage_ptr>> which is a smart pointer to a {ref_memory_resource}. The constructor signatures have the same ordering as their `std` equivalents which use {req_Allocator} parameters. Once a container is constructed its memory resource can never change. Here we create an array without performing any dynamic allocations:
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resource>>, which uses the standard C++ allocator, and is designed for general use including mutation of the document after it is parsed. The <<ref_stream_parser>> object is reused between trials, allowing temporary memory allocated by the implementation to persist and improve performance.
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The requirement for an initializer list to be interpreted as an <<ref_object>> or <<ref_array>> when initializing such an entity only applies to the outermost initializer list; subsequent nested elements will follow the usual ambiguity resolution rules.
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small buffer optimisation is guaranteed, which avoids allocating memory for
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<<ref_object>>, or <<ref_string>>.
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Boost.JSON container.
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