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api/basic_json/get_ptr.md

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+# <small>nlohmann::basic_json::</small>get_ptr
+
+```cpp
+template<typename PointerType>
+PointerType get_ptr() noexcept;
+
+template<typename PointerType>
+constexpr const PointerType get_ptr() const noexcept;
+```
+
+Implicit pointer access to the internally stored JSON value. No copies are made.
+
+## Template parameters
+
+`PointerType`
+:   pointer type; must be a pointer to [`array_t`](array_t.md), [`object_t`](object_t.md), [`string_t`](string_t.md),
+    [`boolean_t`](boolean_t.md), [`number_integer_t`](number_integer_t.md), or
+    [`number_unsigned_t`](number_unsigned_t.md), [`number_float_t`](number_float_t.md), or [`binary_t`](binary_t.md).
+    Other types will not compile.
+
+## Return value
+
+pointer to the internally stored JSON value if the requested pointer type fits to the JSON value; `#!cpp nullptr`
+otherwise
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+!!! danger "Undefined behavior"
+
+    The pointer becomes invalid if the underlying JSON object changes.
+
+    Consider the following example code where the pointer `ptr` changes after the array is resized. As a result,
+    reading or writing to `ptr` after the array change would be undefined behavior. The address of the first array
+    element changes, because the underlying `std::vector` is resized after adding a fifth element.
+
+    ```cpp
+    #include <iostream>
+    #include <nlohmann/json.hpp>
+    
+    using json = nlohmann::json;
+    
+    int main()
+    {
+        json j = {1, 2, 3, 4};
+        auto* ptr = j[0].get_ptr<std::int64_t*>();
+        std::cout << "value at " << ptr << " is " << *ptr << std::endl;
+    
+        j.push_back(5);
+    
+        ptr = j[0].get_ptr<std::int64_t*>();
+        std::cout << "value at " << ptr << " is " << *ptr << std::endl;
+    }
+    ```
+
+    Output:
+
+    ```
+    value at 0x6000012fc1c8 is 1
+    value at 0x6000029fc088 is 1
+    ```
+
+## Examples
+
+??? example
+
+    The example below shows how pointers to internal values of a JSON value can be requested. Note that no type
+    conversions are made and a `#!cpp nullptr` is returned if the value and the requested pointer type does not match.
+    
+    ```cpp
+    --8<-- "examples/get_ptr.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/get_ptr.output"
+    ```
+
+## See also
+
+- [get_ref()](get_ref.md) get a reference value
+
+## Version history
+
+- Added in version 1.0.0.
+- Extended to binary types in version 3.8.0.