107 lines
3.9 KiB
Markdown
107 lines
3.9 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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Unlike parsing with [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) enabled, the values
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produced by `materialize()` do not carry source positions. The index has the start offset of every value, but not the
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end of arrays, objects, and escaped strings, which the positions also need.
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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 json_document = 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 (not
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produced by `materialize()`)
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## Version history
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- Added in version 3.13.0.
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