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api/basic_json_view/is_discarded.md

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+# <small>nlohmann::basic_json_view::</small>is_discarded
+
+```cpp
+bool is_discarded() const noexcept;
+```
+
+Returns whether this view is invalid, i.e. does not refer to a value. This is the case for a default-constructed
+view (see [(constructor)](basic_json_view.md)), and for [`root()`](../basic_json_document/root.md) of a document
+that is itself [discarded](../basic_json_document/is_discarded.md) -- in particular, the root of a failed
+[`parse()`](../basic_json_document/parse.md) with `allow_exceptions` set to `#!cpp false`.
+
+A discarded view is also what [`operator[]`](operator[].md) returns for a missing key, an index out of range, or a
+JSON pointer that cannot be resolved, and for any access on a view that is itself discarded (so a chain such as
+`#!cpp v["a"]["b"]` is safe). [`at`](at.md) throws instead.
+
+## Return value
+
+`#!cpp true` if the view or its document is discarded, `#!cpp false` otherwise.
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+A `basic_json_view` is not convertible to `#!cpp bool`: such a conversion would mean "refers to a value", whereas
+`basic_json` converts to the `#!cpp bool` it holds, so the same code would silently behave differently. Test
+`#!cpp !v.is_discarded()` explicitly.
+
+## Examples
+
+??? example
+
+    The example below classifies several parsed documents by the type of their root value, without materializing any
+    of them into a `BasicJsonType` value.
+
+    ```cpp
+    #include <iostream>
+    #include <nlohmann/json_view.hpp>
+    
+    using json_document = nlohmann::json_document;
+    
+    int main()
+    {
+        std::cout << std::boolalpha;
+    
+        // several incoming messages, one json_document per message. type() and
+        // is_*() only look at the flat index built by parse(); no nlohmann::json
+        // tree exists yet, and none is built unless materialize() is called
+        json_document d_null = json_document::parse("null");
+        json_document d_bool = json_document::parse("true");
+        json_document d_int = json_document::parse("-42");
+        json_document d_unsigned = json_document::parse("42");
+        json_document d_float = json_document::parse("4.2");
+        json_document d_string = json_document::parse(R"("hi")");
+        json_document d_array = json_document::parse("[1, 2, 3]");
+        json_document d_object = json_document::parse(R"({"a": 1})");
+    
+        std::cout << d_null.root().is_null() << '\n';
+        std::cout << d_bool.root().is_boolean() << '\n';
+        std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\n';
+        std::cout << d_unsigned.root().is_number_unsigned() << '\n';
+        std::cout << d_float.root().is_number_float() << '\n';
+        std::cout << d_string.root().is_string() << '\n';
+        std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\n';
+        std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\n';
+    
+        // JSON text can never produce a binary value: is_binary() is always false
+        std::cout << d_array.root().is_binary() << '\n';
+    
+        // a default-constructed view, and the root of a document that failed to
+        // parse without exceptions, are both discarded
+        nlohmann::json_view invalid;
+        json_document failed = json_document::parse("not json", /* allow_exceptions */ false);
+        std::cout << invalid.is_discarded() << '\n';
+        std::cout << failed.root().is_discarded() << '\n';
+    
+        // type() returns the same value_t enumeration as basic_json::type()
+        std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\n';
+    }
+    
+    ```
+
+    Output:
+
+    ```json
+    true
+    true
+    true true
+    true
+    true
+    true
+    true true
+    true false
+    false
+    true
+    true
+    true
+    
+    ```
+
+## See also
+
+- [operator[]](operator[].md) - access specified element; yields a discarded view where an element is missing
+- [(constructor)](basic_json_view.md) - the default constructor creates a discarded view
+- [is_discarded (basic_json_document)](../basic_json_document/is_discarded.md) - return whether the last parse failed
+- [`BasicJsonType::is_discarded`](../basic_json/is_discarded.md) - the corresponding function of `basic_json`
+
+## Version history
+
+- Added in version 3.13.0.