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api/basic_json_document/shrink_to_fit.md

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+# <small>nlohmann::basic_json_document::</small>shrink_to_fit
+
+```cpp
+void shrink_to_fit();
+```
+
+Releases capacity that is no longer needed, both of the node index and of the buffer for decoded strings (strings that
+contained escape sequences), e.g. after [`read()`](read.md) replaced a large document with a much smaller one. Like
+`#!cpp std::vector::shrink_to_fit()`, this is a non-binding request: the library may keep more capacity than strictly
+necessary.
+
+## Exception safety
+
+Strong guarantee: if an exception is thrown, there are no changes to the document.
+
+## Exceptions
+
+May throw `#!cpp std::bad_alloc` if the reallocation fails; on exception, the document is unchanged.
+
+## Complexity
+
+Linear in [`node_count()`](node_count.md) plus the length of the decoded strings.
+
+## Notes
+
+!!! warning "Invalidates views"
+
+    Unlike moving the document, `shrink_to_fit()` **invalidates every view taken from this document before the
+    call**, including a previously obtained [`root()`](root.md): the node index is moved into a new, smaller
+    allocation, and the old one is freed. Take a fresh view from [`root()`](root.md) after calling this function.
+
+This is unlike `#!cpp std::vector::shrink_to_fit()`, which promises nothing about validity but in practice often
+leaves iterators alone when it did not need to reallocate; here, an implementation that avoids a reallocation when
+possible would be an internal optimization only, not a guarantee to rely on.
+
+## Examples
+
+??? example
+
+    ```cpp
+    #include <iostream>
+    #include <sstream>
+    #include <string>
+    #include <nlohmann/json_view.hpp>
+    
+    using json_document = nlohmann::json_document;
+    
+    int main()
+    {
+        std::cout << std::boolalpha;
+    
+        // build a large array (many nodes), then read a small one into the same
+        // document: the index grown for the large input is still allocated
+        std::ostringstream big;
+        big << '[';
+        for (int i = 0; i < 500; ++i)
+        {
+            if (i != 0)
+            {
+                big << ',';
+            }
+            big << i;
+        }
+        big << ']';
+    
+        json_document doc;
+        doc.read(big.str());
+        const std::size_t big_nodes = doc.node_count();
+    
+        doc.read(std::string("[1]"));
+        std::cout << (doc.node_count() < big_nodes) << '\n'; // far fewer live nodes now
+        const std::size_t before = doc.memory_usage();
+    
+        // shrink_to_fit() moves the index into a block sized for what is actually
+        // used. This INVALIDATES every view taken from this document before the
+        // call (they point into the old, now-freed block) -- take fresh ones from
+        // root() afterward.
+        doc.shrink_to_fit();
+        const std::size_t after = doc.memory_usage();
+        std::cout << (after <= before) << '\n';
+    
+        // a freshly taken view is valid and correct
+        std::cout << doc.root().is_array() << ' ' << doc.root().size() << '\n';
+    }
+    
+    ```
+
+    Output:
+
+    ```json
+    true
+    true
+    true 1
+    
+    ```
+
+## See also
+
+- [node_count](node_count.md) - the number of index entries
+- [memory_usage](memory_usage.md) - the number of bytes held by the document
+- [root](root.md) - the view of the root value
+
+## Version history
+
+- Added in version 3.13.0.