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api/basic_json_view/size.md

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+# <small>nlohmann::basic_json_view::</small>size
+
+```cpp
+size_type size() const noexcept;
+```
+
+Returns the number of elements, as [`BasicJsonType::size()`](../basic_json/size.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       | `0`                       |
+| discarded  | `0`                       |
+| boolean    | `1`                       |
+| string     | `1`                       |
+| number     | `1`                       |
+| object     | number of key/value pairs |
+| array      | number of elements        |
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant: for an object or array, the element count is stored in the index, not counted on demand.
+
+## Notes
+
+As for [`BasicJsonType::size()`](../basic_json/size.md), this does not return the length of a string value -- it is
+`1` for a string, regardless of its length.
+
+If the source text has an object with a duplicate key, every occurrence counts towards its `size()` -- unlike
+[`materialize()`](materialize.md) (and [`BasicJsonType::parse()`](../basic_json/parse.md)), which keeps only the last
+value for a repeated key. This means `#!cpp v.size()` can be larger than `#!cpp v.materialize().size()`. See the
+[Notes on duplicate keys](operator[].md#notes) of `operator[]` for why lookups and iteration disagree on how many
+members there are.
+
+## Examples
+
+??? example
+
+    The example below uses `size()` and [`empty()`](empty.md) 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
+
+- [empty](empty.md) - return whether the value has no elements
+- [`BasicJsonType::size`](../basic_json/size.md) - the corresponding function of `basic_json`
+
+## Version history
+
+- Added in version 3.13.0.