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;

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.
Overload for basic_json specializations. The function is equivalent to executing
```
return *this;
```
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

copy of the JSON value, converted to ValueType
a copy of *this, converted into BasicJsonType
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

Complexity

Depends on the json_serializer<ValueType>::from_json() implementation for overloads (1) and (2); constant for overload (3).

Notes

Undefined behavior for pointers

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

Undefined behavior for numeric conversions

Conversions between numeric types are performed by the corresponding from_json() implementation using the target C++ type. When converting between numeric types, the library does not check whether the source value is representable by the target type.

If the source value is outside the range of the target type, the behavior is the same as the corresponding C++ conversion. In particular, converting a floating-point value to an integer type that cannot represent the value results in undefined behavior.

See Number conversion for more information.

std::optional conversions

Prior to version 3.13.0, get<std::optional<T>>() (and other conversions to std::optional<T>) failed to compile in every configuration, due to an internal implementation bug that made the from_json overload for std::optional unreachable regardless of the JSON_USE_IMPLICIT_CONVERSIONS setting. This has been fixed.

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]