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api/basic_json/is_discarded/index.md

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+# nlohmann::basic_json::is_discarded
+
+```
+constexpr bool is_discarded() const noexcept;
+```
+
+This function returns `true` for a JSON value if either:
+
+- the value was discarded during parsing with a callback function (see [`parser_callback_t`](https://json.nlohmann.me/api/basic_json/parser_callback_t/index.md)), or
+- the value is the result of parsing invalid JSON with parameter `allow_exceptions` set to `false`; see [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md) for more information.
+
+## Return value
+
+`true` if type is discarded, `false` otherwise.
+
+## Exception safety
+
+No-throw guarantee: this member function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+Comparisons
+
+Discarded values are never compared equal with [`operator==`](https://json.nlohmann.me/api/basic_json/operator_eq/index.md). That is, checking whether a JSON value `j` is discarded will only work via:
+
+```
+j.is_discarded()
+```
+
+because
+
+```
+j == json::value_t::discarded
+```
+
+will always be `false`.
+
+Removal during parsing with callback functions
+
+When a value is discarded by a callback function (see [`parser_callback_t`](https://json.nlohmann.me/api/basic_json/parser_callback_t/index.md)) during parsing, then it is removed when it is part of a structured value. For instance, if the second value of an array is discarded, instead of `[null, discarded, false]`, the array `[null, false]` is returned. If the top-level value itself is discarded by the callback, the `parse` call returns a `null` value.
+
+After a successful parse, this function always returns `false`: discarded values can only occur during parsing and are either removed when inside a structured value or replaced by `null` at the top level. The exception is parsing with `allow_exceptions` set to `false`: a parse error then yields a discarded value for which this function returns `true` (see [`parse`](https://json.nlohmann.me/api/basic_json/parse/index.md)).
+
+## Examples
+
+Example
+
+The following code exemplifies `is_discarded()` for all JSON types.
+
+```
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON values
+    json j_null;
+    json j_boolean = true;
+    json j_number_integer = 17;
+    json j_number_unsigned_integer = 12345678987654321u;
+    json j_number_float = 23.42;
+    json j_object = {{"one", 1}, {"two", 2}};
+    json j_array = {1, 2, 4, 8, 16};
+    json j_string = "Hello, world";
+    json j_binary = json::binary({1, 2, 3});
+
+    // call is_discarded()
+    std::cout << std::boolalpha;
+    std::cout << j_null.is_discarded() << '\n';
+    std::cout << j_boolean.is_discarded() << '\n';
+    std::cout << j_number_integer.is_discarded() << '\n';
+    std::cout << j_number_unsigned_integer.is_discarded() << '\n';
+    std::cout << j_number_float.is_discarded() << '\n';
+    std::cout << j_object.is_discarded() << '\n';
+    std::cout << j_array.is_discarded() << '\n';
+    std::cout << j_string.is_discarded() << '\n';
+    std::cout << j_binary.is_discarded() << '\n';
+}
+```
+
+Output:
+
+```
+false
+false
+false
+false
+false
+false
+false
+false
+false
+```
+
+## Version history
+
+- Added in version 1.0.0.