122 lines
5.1 KiB
Markdown
122 lines
5.1 KiB
Markdown
# <small>nlohmann::basic_json_view::</small>materialize
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```cpp
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BasicJsonType materialize() const;
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```
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Builds the `BasicJsonType` value of this subtree: the value [`BasicJsonType::parse()`](../basic_json/parse.md) would
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have produced for the same source text, allocated for the first time by this call.
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## Return value
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The `BasicJsonType` value of this subtree, or a discarded `BasicJsonType` value (`#!cpp BasicJsonType(value_t::discarded)`)
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if the view is [discarded](is_discarded.md).
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## Exception safety
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Strong guarantee: there are no changes to the view or the document it refers to (nothing about either is mutated by
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this function).
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## Exceptions
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May throw `#!cpp std::bad_alloc` (via `BasicJsonType`'s allocator) if constructing the result fails.
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## Complexity
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Linear in the size of the subtree.
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## Notes
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`materialize()` replays the subtree through the same SAX builder [`BasicJsonType::parse()`](../basic_json/parse.md)
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uses, so the result matches it exactly — including, for an object, that a repeated key keeps only its
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last value. The replay is iterative, so it is not limited by the call stack the way a naive recursive conversion
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would be; the JSON nesting depth is limited only by available memory, as for `BasicJsonType::parse()` itself.
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With [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) enabled, the values get the source
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positions [`BasicJsonType::parse()`](../basic_json/parse.md) would give them, also for the subtree of a view (the
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positions are those in the document's whole source text), and exceptions about them name the same bytes. For values of
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an [editable document](../json_editable_document.md), positions are all-or-nothing, and [`start_pos()`](../basic_json/start_pos.md)
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and [`end_pos()`](../basic_json/end_pos.md) return `std::string::npos` for
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- a value written by an edit (a copy of an array or object of the document is also new; copies of its scalars keep
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their positions),
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- an array or object whose elements were changed (its elements keep theirs), and
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- an array or object whose last element has no positions (its end is found from that element).
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This differs from a `basic_json` value, whose positions are not updated when it is changed: they stay as they were
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parsed, even if they no longer match the value (see [`start_pos()`](../basic_json/start_pos.md)).
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Documents [loaded](../basic_json_document/load.md) from an image have the positions of the saved document. Its strings
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with escapes have positions only if the document was parsed with `JSON_DIAGNOSTIC_POSITIONS`: the position of their
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opening quote is recorded while parsing, and `save()` and `load()` keep it whether or not they are compiled with the
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macro.
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Calling `materialize()` on the same view repeatedly builds a new, independent `BasicJsonType` value each time; it
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never caches the result.
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## Examples
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??? example
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The example below skips messages that are not useful — a discarded value, or an empty array — using only
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[`is_array()`](is_array.md) and [`empty()`](empty.md), and calls `materialize()` only for the messages that are
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actually used, so no `BasicJsonType` value (and none of its per-element allocations) is ever built for the
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skipped ones.
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```cpp
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#include <array>
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#include <iostream>
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#include <nlohmann/json_view.hpp>
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using nlohmann::json_document;
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int main()
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{
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// three incoming messages; skip the ones that are not useful without ever
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// building a nlohmann::json value for them
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json_document heartbeat = json_document::parse("null");
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json_document empty_batch = json_document::parse("[]");
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json_document batch = json_document::parse(R"([{"id": 1}, {"id": 2}, {"id": 3}])");
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std::array<const json_document*, 3> messages = {{&heartbeat, &empty_batch, &batch}};
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for (const json_document* d : messages)
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{
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// is_array()/empty() only look at the flat index: a discarded
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// heartbeat or an empty batch is never turned into a nlohmann::json
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// value, so no per-element allocation happens for them
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if (!d->root().is_array() || d->root().empty())
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{
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std::cout << "skipped\n";
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continue;
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}
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// materialize() replays the subtree through the same SAX builder
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// basic_json::parse() uses, so the result is exactly what
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// basic_json::parse() would have produced for the same text
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nlohmann::json value = d->root().materialize();
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std::cout << value.dump() << '\n';
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}
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}
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```
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Output:
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```json
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skipped
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skipped
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[{"id":1},{"id":2},{"id":3}]
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```
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## See also
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- [`root`](../basic_json_document/root.md) - the view of a document's root value
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- [`BasicJsonType::parse`](../basic_json/parse.md) - build a `BasicJsonType` value directly from a JSON text
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- [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) - source positions on parsed values
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- [Zero-copy JSON views](../../features/json_view.md#getting-values-out-without-copying) - when to build a basic_json value from a view
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## Version history
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- Added in version 3.13.0.
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