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api/basic_json_document/memory_usage/index.md

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+# nlohmann::basic_json_document::memory_usage
+
+```
+std::size_t memory_usage() const noexcept;
+```
+
+Returns the number of bytes held by the document: the node index, the decoded-string buffer (for strings that contain escapes), and, for an owned document, its copy of the source text.
+
+## Return value
+
+The number of bytes the document holds, `0` for a [discarded](https://json.nlohmann.me/api/basic_json_document/is_discarded/index.md) document.
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+The exact value depends on the platform, the allocator, and the library's own layout, and may change between versions; do not rely on it being a specific number, and do not compare it across different builds or platforms. Compare it for the same document over time, or between documents built with the same binary, instead -- for instance to observe the effect of [`shrink_to_fit()`](https://json.nlohmann.me/api/basic_json_document/shrink_to_fit/index.md).
+
+## Examples
+
+Example
+
+```
+#include <iostream>
+#include <string>
+#include <nlohmann/json_view.hpp>
+
+using json_document = nlohmann::json_document;
+
+int main()
+{
+    std::cout << std::boolalpha;
+
+    // long enough that copying it needs a real (heap) allocation, so the
+    // comparison below does not depend on the standard library's small
+    // string optimization threshold
+    const std::string text = std::string(200, ' ') + "[1, 2, 3, 4, 5]";
+
+    // memory_usage() is not portable across platforms/allocators/compilers, so
+    // compare it relatively instead of printing the raw byte count
+    json_document borrowed = json_document::parse(text);
+    json_document owned = json_document::parse_copy(text);
+
+    // the owned document additionally stores its own copy of the source text
+    std::cout << (owned.memory_usage() > borrowed.memory_usage()) << '\n';
+
+    // a document with more values needs a larger index
+    json_document small = json_document::parse(std::string("[1]"));
+    json_document large = json_document::parse(std::string("[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]"));
+    std::cout << (large.memory_usage() > small.memory_usage()) << '\n';
+}
+```
+
+Output:
+
+```
+true
+true
+```
+
+## See also
+
+- [node_count](https://json.nlohmann.me/api/basic_json_document/node_count/index.md) - the number of index entries
+- [shrink_to_fit](https://json.nlohmann.me/api/basic_json_document/shrink_to_fit/index.md) - release unused index capacity
+
+## Version history
+
+- Added in version 3.13.0 unreleased.