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api/basic_json_document/is_discarded/index.md

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+# nlohmann::basic_json_document::is_discarded
+
+```
+bool is_discarded() const noexcept;
+```
+
+Returns whether the document holds no value, either because it was default-constructed or because the last call to [`parse()`](https://json.nlohmann.me/api/basic_json_document/parse/index.md), [`parse_copy()`](https://json.nlohmann.me/api/basic_json_document/parse_copy/index.md), or [`read()`](https://json.nlohmann.me/api/basic_json_document/read/index.md) failed with `allow_exceptions` set to `false`.
+
+## Return value
+
+`true` if the document is discarded, `false` otherwise.
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+When the document is discarded, [`root()`](https://json.nlohmann.me/api/basic_json_document/root/index.md) returns a discarded view (its [`is_discarded()`](https://json.nlohmann.me/api/basic_json_view/is_discarded/index.md) is also `true`).
+
+## Examples
+
+Example
+
+```
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json_document = nlohmann::json_document;
+
+int main()
+{
+    std::cout << std::boolalpha;
+
+    // with allow_exceptions == false, a parse error produces a discarded
+    // document instead of throwing -- exactly like basic_json::parse()
+    json_document doc = json_document::parse(R"({"a": )", /* allow_exceptions */ false);
+    std::cout << doc.is_discarded() << '\n';
+    std::cout << doc.root().is_discarded() << '\n';
+
+    // a successful parse is never discarded
+    json_document ok = json_document::parse(R"({"a": 1})", false);
+    std::cout << ok.is_discarded() << '\n';
+}
+```
+
+Output:
+
+```
+true
+true
+false
+```
+
+## See also
+
+- [parse](https://json.nlohmann.me/api/basic_json_document/parse/index.md) - deserialize from a compatible input
+- [is_discarded (basic_json_view)](https://json.nlohmann.me/api/basic_json_view/is_discarded/index.md) - return whether a view is invalid
+
+## Version history
+
+- Added in version 3.13.0 unreleased.