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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"].template get<bool>();
    auto v2 = json_types["number"]["integer"].template get<int>();
    auto v3 = json_types["number"]["integer"].template get<short>();
    auto v4 = json_types["number"]["floating-point"].template get<float>();
    auto v5 = json_types["number"]["floating-point"].template get<int>();
    auto v6 = json_types["string"].template get<std::string>();
    auto v7 = json_types["array"].template get<std::vector<short>>();
    auto v8 = json_types.template 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.template get<const json::number_integer_t*>();
    auto p2 = value.template get<json::number_integer_t*>();
    auto p3 = value.template get<json::number_integer_t* const>();
    auto p4 = value.template get<const json::number_integer_t* const>();
    auto p5 = value.template 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.