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= Non-Throwing Conversions For the case where throwing exceptions is undesirable, Boost.JSON also provides a non-throwing version of <<ref_value_to>>, the function template <<ref_try_value_to>>. It returns {ref_result}, a specialised variant that either holds a value or an {ref_error_code}.
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= Avoiding Physical Dependency Some users, particularly library authors, may wish to provide conversions between their types and <<ref_value>>, but at the same time would prefer to avoid having their library depend on Boost.JSON. This is possible to achieve with the help of a few forward declarations.
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= Allocation Control As <<ref_value_from>> creates a <<ref_value>> object, users may want to control the way memory is allocated for it. For this reason the function has an optional <<ref_storage_ptr>> parameter, that is used to set the {ref_memory_resource} for the result.
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= `value` JSON documents are represented in memory as instances of <<ref_value>>: a {req_Regular} type which satisfies {req_DefaultConstructible}, {req_CopyConstructible}, {req_CopyAssignable}, {req_MoveConstructible}, {req_MoveAssignable}, and many of the requirements of allocator-aware containers. It is implemented as a https://en.wikipedia.org/wiki/Tagged_union[__variant__] internally, and can dynamically store any of the six defined JSON value types:
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= `array` A <<ref_value>> stores an instance of <<ref_array>> as the underlying representation for a JSON array. Instances of the __array__ type function identically to a {std_vector} of <<ref_value>>. Additionally, all values inserted into the container will use the same <<ref_storage_ptr>> as the container itself.
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= `storage_ptr` Variable-length containers in this library all use dynamically allocated memory to store their contents. Callers can gain control over the strategy used for allocation by specifying a <<ref_storage_ptr>> in select constructors and function parameter lists. A <<ref_storage_ptr>> has these properties:
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= Uses-allocator Construction To support code bases which are already using polymorphic allocators, the containers in this library support {std_uses_allocator} construction. For <<ref_array>>, <<ref_object>>, <<ref_string>>, and <<ref_value>>:
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= Input/Output The library provides parsing and serialization algorithms to transform JSON to and from the <<ref_value>> container as needed. This is accomplished through free functions and classes, described as follows.
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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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