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api/basic_json_view/materialize/index.md

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+# nlohmann::basic_json_view::materialize
+
+```
+BasicJsonType materialize() const;
+```
+
+Builds the `BasicJsonType` value of this subtree: the value [`BasicJsonType::parse()`](https://json.nlohmann.me/api/basic_json/parse/index.md) would have produced for the same source text, allocated for the first time by this call.
+
+## Return value
+
+The `BasicJsonType` value of this subtree, or a discarded `BasicJsonType` value (`BasicJsonType(value_t::discarded)`) if the view is [discarded](https://json.nlohmann.me/api/basic_json_view/is_discarded/index.md).
+
+## Exception safety
+
+Strong guarantee: there are no changes to the view or the document it refers to (nothing about either is mutated by this function).
+
+## Exceptions
+
+May throw `std::bad_alloc` (via `BasicJsonType`'s allocator) if constructing the result fails.
+
+## Complexity
+
+Linear in the size of the subtree.
+
+## Notes
+
+`materialize()` replays the subtree through the same SAX builder [`BasicJsonType::parse()`](https://json.nlohmann.me/api/basic_json/parse/index.md) uses, so the result matches it exactly -- including, for an object, that a repeated key keeps only its last value. The replay is iterative, so it is not limited by the call stack the way a naive recursive conversion would be; the JSON nesting depth is limited only by available memory, as for `BasicJsonType::parse()` itself.
+
+Unlike parsing with [`JSON_DIAGNOSTIC_POSITIONS`](https://json.nlohmann.me/api/macros/json_diagnostic_positions/index.md) enabled, the values produced by `materialize()` do not carry source positions. The index has the start offset of every value, but not the end of arrays, objects, and escaped strings, which the positions also need.
+
+Calling `materialize()` on the same view repeatedly builds a new, independent `BasicJsonType` value each time; it never caches the result.
+
+## Examples
+
+Example
+
+The example below skips messages that are not useful -- a discarded value, or an empty array -- using only [`is_array()`](https://json.nlohmann.me/api/basic_json_view/is_array/index.md) and [`empty()`](https://json.nlohmann.me/api/basic_json_view/empty/index.md), and calls `materialize()` only for the messages that are actually used, so no `BasicJsonType` value (and none of its per-element allocations) is ever built for the skipped ones.
+
+```
+#include <array>
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json_document = nlohmann::json_document;
+
+int main()
+{
+    // three incoming messages; skip the ones that are not useful without ever
+    // building a nlohmann::json value for them
+    json_document heartbeat = json_document::parse("null");
+    json_document empty_batch = json_document::parse("[]");
+    json_document batch = json_document::parse(R"([{"id": 1}, {"id": 2}, {"id": 3}])");
+    std::array<const json_document*, 3> messages = {{&heartbeat, &empty_batch, &batch}};
+
+    for (const json_document* d : messages)
+    {
+        // is_array()/empty() only look at the flat index: a discarded
+        // heartbeat or an empty batch is never turned into a nlohmann::json
+        // value, so no per-element allocation happens for them
+        if (!d->root().is_array() || d->root().empty())
+        {
+            std::cout << "skipped\n";
+            continue;
+        }
+
+        // materialize() replays the subtree through the same SAX builder
+        // basic_json::parse() uses, so the result is exactly what
+        // basic_json::parse() would have produced for the same text
+        nlohmann::json value = d->root().materialize();
+        std::cout << value.dump() << '\n';
+    }
+}
+```
+
+Output:
+
+```
+skipped
+skipped
+[{"id":1},{"id":2},{"id":3}]
+```
+
+## See also
+
+- [root](https://json.nlohmann.me/api/basic_json_document/root/index.md) - the view of a document's root value
+- [`BasicJsonType::parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) - build a `BasicJsonType` value directly from a JSON text
+- [`JSON_DIAGNOSTIC_POSITIONS`](https://json.nlohmann.me/api/macros/json_diagnostic_positions/index.md) - source positions on parsed values (not produced by `materialize()`)
+
+## Version history
+
+- Added in version 3.13.0 unreleased.