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api/basic_json_view/size/index.md

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+# nlohmann::basic_json_view::size
+
+```
+size_type size() const noexcept;
+```
+
+Returns the number of elements, as [`BasicJsonType::size()`](https://json.nlohmann.me/api/basic_json/size/index.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()`](https://json.nlohmann.me/api/basic_json/size/index.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()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) (and [`BasicJsonType::parse()`](https://json.nlohmann.me/api/basic_json/parse/index.md)), which keeps only the last value for a repeated key. This means `v.size()` can be larger than `v.materialize().size()`. See the [Notes on duplicate keys](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/#notes) of `operator[]` for why lookups and iteration disagree on how many members there are.
+
+## Examples
+
+Example
+
+The example below uses `size()` and [`empty()`](https://json.nlohmann.me/api/basic_json_view/empty/index.md) to decide whether a parsed message is worth acting on, without materializing it into a `BasicJsonType` value.
+
+```
+#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:
+
+```
+false 5
+true 0
+0 1
+```
+
+## See also
+
+- [empty](https://json.nlohmann.me/api/basic_json_view/empty/index.md) - return whether the value has no elements
+- [`BasicJsonType::size`](https://json.nlohmann.me/api/basic_json/size/index.md) - the corresponding function of `basic_json`
+
+## Version history
+
+- Added in version 3.13.0 unreleased.