Exit Zen
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Previously a number of conversion type traits, like <<ref_is_tuple_like>> or <<ref_is_sequence_like>>, were introduced. The library tries the traits one after another and uses the implementation that corresponds to the first matching trait. In some cases, though, a type would match a trait with a higher priority, but the user intends for it to belong to a lower priority category. If this happens the user can specialize the trait that's not supposed to match for that type to be an equivalent of `std::false_type`.
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3
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That being said, if your types are default-constructible and you don't need the customisability allowed by <<ref_value_to>> and <<ref_value_from>>, then you can get a significant performance boost with direct conversions.
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3
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Iterators are ordinary pointers, and may become invalidated on insertions
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3
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The library provides non-throwing conversions for all the categories of types it supports with <<ref_value_to>>. If a user wants to use it with custom types, an overload of `tag_invoke` of this form needs to be provided:
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3
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After that the definitions of `tag_invoke` overloads should be provided. These overloads have to be templates, since <<ref_value>> is only forward-declared and hence is an incomplete type.
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3
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As the conversion result is set via an output parameter of type `value&`, the intended <<ref_storage_ptr>> is correctly propagated. But users still should take care to not create temporaries using the default {ref_memory_resource} by accident.
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4
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The library also allows changing and adding deeply nested elements. The function <<ref_value_set_at_pointer>> traverses the value similarly to <<ref_value_at_pointer>>, but additionally, it can create missing elements in certain cases:
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If the <<ref_value>> does not contain a number, or if the conversion is to an integer type `T` and the number cannot be represented exactly by `T`, the conversion will fail. Otherwise, the result is the number converted to `T` as-if by `static_cast`:
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Direct parsing is performed by the <<ref_parse_into>> family of functions. The library provides overloads that take either <<ref_string_view>> or `std::istream`, and can report errors either via throwing exceptions or setting an error code.
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and removals.
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4
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As discussed previously, we prefer to define a non-throwing overload of `tag_invoke` for <<ref_try_value_to>>, rather then the throwing overload for <<ref_value_to>>, as the latter can fallback to the former without performance degradation.
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For example, consider this alternative implementation of `tag_invoke` for `ip_address` from the section <<custom_conversions>>.
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4
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When a two element initializer list is nested within an enclosing initializer list, it is unclear whether it represents an <<ref_array>> or an <<ref_object>>:
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5
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The specific behavior is controlled by an optional parameter of type <<ref_set_pointer_options>>. For example, here's the same example with a different option:
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A sequence container of JSON values supporting dynamic size and fast, random
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5
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It exposes both a sequence API and a tuple API. But converting from <<ref_value>> to `user_ns::ip_address` would not be able to use implementation for sequences, since those are constructed empty and then populated one element at a time, while `ip_address` has a fixed size of 4. The tuple conversion would fit, though. The only problem is that <<ref_is_tuple_like>> has a lower priority than <<ref_is_sequence_like>>. In order to circumvent this, the user only needs to specialize <<ref_is_sequence_like>> to not match `ip_address`.
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5
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If you need to combine incremental parsing with direct parsing, you can resort to <<ref_parser_for>>. `parser_for<T>` is an instantiation of <<ref_basic_parser>> that parses into an object of type `T`, and is what <<ref_parse_into>> uses under the hood.
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For the class `ip_address` from the section <<custom_conversions>> this overload may look like this:
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Similar to its standard library counterpart, elements may be accessed directly by their 0-based index with bounds checking using <<ref_array_at>>, or without bounds checking using <<ref_array_operator_lb_rb>>:
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5
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<<default_memory_resource,default resource>>, an implementation-defined instance which always uses the equivalent of global operator new and delete.
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