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api/basic_json_view/materialize.md

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+# <small>nlohmann::basic_json_view::</small>materialize
+
+```cpp
+BasicJsonType materialize() const;
+```
+
+Builds the `BasicJsonType` value of this subtree: the value [`BasicJsonType::parse()`](../basic_json/parse.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 (`#!cpp BasicJsonType(value_t::discarded)`)
+if the view is [discarded](is_discarded.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 `#!cpp 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()`](../basic_json/parse.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`](../macros/json_diagnostic_positions.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()`](is_array.md) and [`empty()`](empty.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.
+
+    ```cpp
+    #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:
+
+    ```json
+    skipped
+    skipped
+    [{"id":1},{"id":2},{"id":3}]
+    
+    ```
+
+## See also
+
+- [root](../basic_json_document/root.md) - the view of a document's root value
+- [`BasicJsonType::parse`](../basic_json/parse.md) - build a `BasicJsonType` value directly from a JSON text
+- [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) - source positions on parsed values (not
+  produced by `materialize()`)
+
+## Version history
+
+- Added in version 3.13.0.