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api/basic_json_view/empty.md

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+# <small>nlohmann::basic_json_view::</small>empty
+
+```cpp
+bool empty() const noexcept;
+```
+
+Checks whether [`size()`](size.md) is `0`, as [`BasicJsonType::empty()`](../basic_json/empty.md) would for the same
+value.
+
+## Return value
+
+The return value depends on the type and is defined as follows:
+
+| Value type          | return value    |
+|----------------------|-----------------|
+| null                  | `#!cpp true`     |
+| discarded             | `#!cpp true`     |
+| boolean               | `#!cpp false`    |
+| string                | `#!cpp false`    |
+| number                | `#!cpp false`    |
+| object                | `#!cpp object_t::empty()` |
+| array                 | `#!cpp array_t::empty()`  |
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+As for [`BasicJsonType::empty()`](../basic_json/empty.md), this does not return whether a string value is empty -- it
+is `#!cpp false` for any string, regardless of its length.
+
+## Examples
+
+??? example
+
+    The example below uses [`size()`](size.md) and `empty()` to decide whether a parsed message is worth acting on,
+    without materializing it into a `BasicJsonType` value.
+
+    ```cpp
+    #include <iostream>
+    #include <nlohmann/json_view.hpp>
+    
+    using json_document = nlohmann::json_document;
+    
+    int main()
+    {
+        std::cout << std::boolalpha;
+    
+        // decide whether a batch is worth processing before building any
+        // nlohmann::json value for it
+        json_document batch = json_document::parse(R"([1, 2, 3, 4, 5])");
+        json_document empty_batch = json_document::parse("[]");
+    
+        std::cout << batch.root().empty() << ' ' << batch.root().size() << '\n';
+        std::cout << empty_batch.root().empty() << ' ' << empty_batch.root().size() << '\n';
+    
+        // as for basic_json: null has size 0, every other scalar has size 1
+        json_document n = json_document::parse("null");
+        json_document s = json_document::parse(R"("hi")");
+        std::cout << n.root().size() << ' ' << s.root().size() << '\n';
+    }
+    
+    ```
+
+    Output:
+
+    ```json
+    false 5
+    true 0
+    0 1
+    
+    ```
+
+## See also
+
+- [size](size.md) - return the number of elements
+- [`BasicJsonType::empty`](../basic_json/empty.md) - the corresponding function of `basic_json`
+
+## Version history
+
+- Added in version 3.13.0.