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# nlohmann::basic_json::get

    // (1)
    template<typename ValueType>
    ValueType get() const noexcept(
        noexcept(JSONSerializer<ValueType>::from_json(
            std::declval<const basic_json_t&>(), std::declval<ValueType&>())));

    // (2)
    template<typename BasicJsonType>
    BasicJsonType get() const;

    // (3)
    template<typename PointerType>
    PointerType get_ptr();

    template<typename PointerType>
    constexpr const PointerType get_ptr() const noexcept;

1. Explicit type conversion between the JSON value and a compatible value which is CopyConstructible and DefaultConstructible. The value is converted by calling the `json_serializer<ValueType>` `from_json()` method.

   The function is equivalent to executing

       ValueType ret;
       JSONSerializer<ValueType>::from_json(*this, ret);
       return ret;

   This overload is chosen if:

   - `ValueType` is not `basic_json`,
   - `json_serializer<ValueType>` has a `from_json()` method of the form `void from_json(const basic_json&, ValueType&)`, and
   - `json_serializer<ValueType>` does not have a `from_json()` method of the form `ValueType from_json(const basic_json&)`

   If the type is not CopyConstructible and not DefaultConstructible, the value is converted by calling the `json_serializer<ValueType>` `from_json()` method.

   The function is then equivalent to executing

       return JSONSerializer<ValueTypeCV>::from_json(*this);

   This overload is chosen if:

   - `ValueType` is not `basic_json` and
   - `json_serializer<ValueType>` has a `from_json()` method of the form `ValueType from_json(const basic_json&)`

   If `json_serializer<ValueType>` has both overloads of `from_json()`, the latter one is chosen.

2. Overload for `basic_json` specializations. The function is equivalent to executing

       return *this;

3. Explicit pointer access to the internally stored JSON value. No copies are made.

## Template parameters

`ValueType`
  the value type to return
`BasicJsonType`
  a specialization of `basic_json`
`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

1. copy of the JSON value, converted to `ValueType`
2. a copy of `*this`, converted into `BasicJsonType`
3. pointer to the internally stored JSON value if the requested pointer type fits to the JSON value; `nullptr` otherwise

## Exceptions

Depends on what `json_serializer<ValueType>` `from_json()` method throws

## Notes

Undefined behavior

Writing data to the pointee (overload 3) of the result yields an undefined state.

## Examples

Example

The example below shows several conversions from JSON values to other types. There a few things to note: (1) Floating-point numbers can be converted to integers, (2) A JSON array can be converted to a standard `std::vector<short>`, (3) A JSON object can be converted to C++ associative containers such as `std::unordered_map<std::string, json>`.

    #include <iostream>
    #include <unordered_map>
    #include <nlohmann/json.hpp>

    using json = nlohmann::json;

    int main()
    {
        // create a JSON value with different types
        json json_types =
        {
            {"boolean", true},
            {
                "number", {
                    {"integer", 42},
                    {"floating-point", 17.23}
                }
            },
            {"string", "Hello, world!"},
            {"array", {1, 2, 3, 4, 5}},
            {"null", nullptr}
        };

        // use explicit conversions
        auto v1 = json_types["boolean"].get<bool>();
        auto v2 = json_types["number"]["integer"].get<int>();
        auto v3 = json_types["number"]["integer"].get<short>();
        auto v4 = json_types["number"]["floating-point"].get<float>();
        auto v5 = json_types["number"]["floating-point"].get<int>();
        auto v6 = json_types["string"].get<std::string>();
        auto v7 = json_types["array"].get<std::vector<short>>();
        auto v8 = json_types.get<std::unordered_map<std::string, json>>();

        // print the conversion results
        std::cout << v1 << '\n';
        std::cout << v2 << ' ' << v3 << '\n';
        std::cout << v4 << ' ' << v5 << '\n';
        std::cout << v6 << '\n';

        for (auto i : v7)
        {
            std::cout << i << ' ';
        }
        std::cout << "\n\n";

        for (auto i : v8)
        {
            std::cout << i.first << ": " << i.second << '\n';
        }
    }

Output:

    1
    42 42
    17.23 17
    Hello, world!
    1 2 3 4 5

    string: "Hello, world!"
    number: {"floating-point":17.23,"integer":42}
    null: null
    boolean: true
    array: [1,2,3,4,5]

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.

    #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<const json::number_integer_t*>();
        auto p2 = value.get<json::number_integer_t*>();
        auto p3 = value.get<json::number_integer_t* const>();
        auto p4 = value.get<const json::number_integer_t* const>();
        auto p5 = value.get<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

## Version history

1. Since version 2.1.0.
2. Since version 2.1.0. Extended to work with other specializations of `basic_json` in version 3.2.0.
3. Since version 1.0.0.

Last update: May 1, 2022
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