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; 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';
}