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api/basic_json/get_ptr/index.md

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+# nlohmann::basic_json::get_ptr
+
+```
+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`](https://json.nlohmann.me/api/basic_json/array_t/index.md), [`object_t`](https://json.nlohmann.me/api/basic_json/object_t/index.md), [`string_t`](https://json.nlohmann.me/api/basic_json/string_t/index.md), [`boolean_t`](https://json.nlohmann.me/api/basic_json/boolean_t/index.md), [`number_integer_t`](https://json.nlohmann.me/api/basic_json/number_integer_t/index.md), or [`number_unsigned_t`](https://json.nlohmann.me/api/basic_json/number_unsigned_t/index.md), [`number_float_t`](https://json.nlohmann.me/api/basic_json/number_float_t/index.md), or [`binary_t`](https://json.nlohmann.me/api/basic_json/binary_t/index.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; `nullptr` otherwise
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+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.
+
+```
+#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 `nullptr` is returned if the value and the requested pointer type does not match.
+
+```
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create a JSON number
+    json value = 17;
+
+    // explicitly getting pointers
+    auto p1 = value.get_ptr<const json::number_integer_t*>();
+    auto p2 = value.get_ptr<json::number_integer_t*>();
+    auto p3 = value.get_ptr<json::number_integer_t* const>();
+    auto p4 = value.get_ptr<const json::number_integer_t* const>();
+    auto p5 = value.get_ptr<json::number_float_t*>();
+
+    // print the pointees
+    std::cout << *p1 << ' ' << *p2 << ' ' << *p3 << ' ' << *p4 << '\n';
+    std::cout << std::boolalpha << (p5 == nullptr) << '\n';
+}
+```
+
+Output:
+
+```
+17 17 17 17
+true
+```
+
+## See also
+
+- [get_ref()](https://json.nlohmann.me/api/basic_json/get_ref/index.md) get a reference value
+
+## Version history
+
+- Added in version 1.0.0.
+- Extended to binary types in version 3.8.0.