json/docs/mkdocs/docs/api/basic_json/get_ptr.md

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, object_t, string_t, boolean_t, number_integer_t, or number_unsigned_t, number_float_t, or binary_t. 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 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 a reference value
• Converting values - getting values out, with or without copying

Version history

• Added in version 1.0.0.
• Extended to binary types in version 3.8.0.