json/api/basic_json/value/index.md

nlohmann::basic_json::value

// (1)
template<class ValueType>
ValueType value(const typename object_t::key_type& key,
                ValueType&& default_value) const;

// (2)
template<class ValueType, class KeyType>
ValueType value(KeyType&& key,
                ValueType&& default_value) const;

// (3)
template<class ValueType>
ValueType value(const json_pointer& ptr,
                const ValueType& default_value) const;

This is equivalent to Python's dict.get(key, default).

1. Returns either a copy of an object's element at the specified key key or a given default value if no element with key key exists.

   The function is basically equivalent to executing

   try {
      return at(key);
   } catch(out_of_range) {
      return default_value;
   }
   

2. See 1. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.

3. Returns either a copy of an object's element at the specified JSON pointer ptr or a given default value if no value at ptr exists.

   The function is basically equivalent to executing

   try {
      return at(ptr);
   } catch(out_of_range) {
      return default_value;
   }
   

Differences to at and operator[]

• Unlike at, this function does not throw if the given key/ptr was not found.
• Unlike operator[], this function does not implicitly add an element to the position defined by key/ptr key. This function is furthermore also applicable to const objects.

Integer keys

Calling this function with an integer key argument (for example, value(0, 1)) does not compile in C++11, where object_comparator_t is not transparent: such an argument would otherwise implicitly convert to a null const char* and, from there, cause undefined behavior when constructing a std::string for the object key. To access an array element with a default value, use at together with a try/catch block, or compare against size instead.

Template parameters

KeyType : A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17).

ValueType : type compatible to JSON values, for instance int for JSON integer numbers, bool for JSON booleans, or std::vector types for JSON arrays. Note the type of the expected value at key/ptr and the default value default_value must be compatible.

Parameters

key (in) : key of the element to access

default_value (in) : the value to return if key/ptr found no value

ptr (in) : a JSON pointer to the element to access

Return value

1. copy of the element at key key or default_value if key is not found
2. copy of the element at key key or default_value if key is not found
3. copy of the element at JSON Pointer ptr or default_value if no value for ptr is found

Exception safety

Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

Exceptions

1. The function can throw the following exceptions:
   • Throws type_error.302 if default_value does not match the type of the value at key
   • Throws type_error.306 if the JSON value is not an object; in that case, using value() with a key makes no sense.
2. See 1.
3. The function can throw the following exceptions:
   • Throws type_error.302 if default_value does not match the type of the value at ptr
   • Throws type_error.306 if the JSON value is not an array or object; in that case, using value() with a JSON pointer makes no sense.
   • Throws parse_error.106 if an array index in the passed JSON pointer ptr begins with '0'.
   • Throws parse_error.109 if an array index in the passed JSON pointer ptr is not a number.

Complexity

1. Logarithmic in the size of the container.
2. Logarithmic in the size of the container.
3. Logarithmic in the size of the container.

Notes

Return type

The value function is a template, and the return type of the function is determined by the type of the provided default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit unsigned integer. We get exactly that value when using operator[]. However, when we call value and provide 0 as default value, then -1 is returned. This occurs, because 0 has type int which overflows when handling the value 18446744073709551615.

To address this issue, either provide a correctly typed default value or use the template parameter to specify the desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default value is not used as the return value.

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

using json = nlohmann::json;

int main()
{
    json j = json::parse(R"({"uint64": 18446744073709551615})");

    std::cout << "operator[]:                " << j["uint64"] << '\n'
              << "default value (int):       " << j.value("uint64", 0) << '\n'
              << "default value (uint64_t):  " << j.value("uint64", std::uint64_t(0)) << '\n'
              << "explicit return value type: " << j.value<std::uint64_t>("uint64", 0) << '\n';
}

Output:

operator[]:                18446744073709551615
default value (int):       -1
default value (uint64_t):  18446744073709551615
explicit return value type: 18446744073709551615

Deprecation

Overload (3) also accepts a json_pointer whose template argument is a basic_json specialization (e.g., nlohmann::json_pointer<nlohmann::json>) instead of a string type. This is deprecated since version 3.11.0 and will be removed in a future major version; use basic_json::json_pointer (for json, nlohmann::json_pointer<std::string>) instead.

You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

See the migration guide for how to update existing code.

Examples

Example: (1) access specified object element with default value

The example below shows how object elements can be queried with a default value.

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

using json = nlohmann::json;

int main()
{
    // create a JSON object with different entry types
    json j =
    {
        {"integer", 1},
        {"floating", 42.23},
        {"string", "hello world"},
        {"boolean", true},
        {"object", {{"key1", 1}, {"key2", 2}}},
        {"array", {1, 2, 3}}
    };

    // access existing values
    int v_integer = j.value("integer", 0);
    double v_floating = j.value("floating", 47.11);

    // access nonexisting values and rely on default value
    std::string v_string = j.value("nonexisting", "oops");
    bool v_boolean = j.value("nonexisting", false);

    // output values
    std::cout << std::boolalpha << v_integer << " " << v_floating
              << " " << v_string << " " << v_boolean << "\n";
}

Output:

1 42.23 oops false

Example: (2) access specified object element using string_view with default value

The example below shows how object elements can be queried with a default value.

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

using namespace std::string_view_literals;
using json = nlohmann::json;

int main()
{
    // create a JSON object with different entry types
    json j =
    {
        {"integer", 1},
        {"floating", 42.23},
        {"string", "hello world"},
        {"boolean", true},
        {"object", {{"key1", 1}, {"key2", 2}}},
        {"array", {1, 2, 3}}
    };

    // access existing values
    int v_integer = j.value("integer"sv, 0);
    double v_floating = j.value("floating"sv, 47.11);

    // access nonexisting values and rely on default value
    std::string v_string = j.value("nonexisting"sv, "oops");
    bool v_boolean = j.value("nonexisting"sv, false);

    // output values
    std::cout << std::boolalpha << v_integer << " " << v_floating
              << " " << v_string << " " << v_boolean << "\n";
}

Output:

1 42.23 oops false

Example: (3) access specified object element via JSON Pointer with default value

The example below shows how object elements can be queried with a default value.

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

using json = nlohmann::json;
using namespace nlohmann::literals;

int main()
{
    // create a JSON object with different entry types
    json j =
    {
        {"integer", 1},
        {"floating", 42.23},
        {"string", "hello world"},
        {"boolean", true},
        {"object", {{"key1", 1}, {"key2", 2}}},
        {"array", {1, 2, 3}}
    };

    // access existing values
    int v_integer = j.value("/integer"_json_pointer, 0);
    double v_floating = j.value("/floating"_json_pointer, 47.11);

    // access nonexisting values and rely on default value
    std::string v_string = j.value("/nonexisting"_json_pointer, "oops");
    bool v_boolean = j.value("/nonexisting"_json_pointer, false);

    // output values
    std::cout << std::boolalpha << v_integer << " " << v_floating
              << " " << v_string << " " << v_boolean << "\n";
}

Output:

1 42.23 oops false

Example: (1) type_error.302 and type_error.306 exceptions

The example below shows how value() throws type_error.302 when the default value's type does not match the type of the stored value, and type_error.306 when value() is called on a JSON value that is not an object.

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

using json = nlohmann::json;

int main()
{
    // create a JSON object with a string value
    json j = {{"name", "the good"}};

    // exception type_error.302
    try
    {
        int v = j.value("name", 0);
        std::cout << v << '\n';
    }
    catch (const json::type_error& e)
    {
        std::cout << e.what() << '\n';
    }

    // exception type_error.306
    try
    {
        json str = "I am a string";
        auto v = str.value("name", 0);
        std::cout << v << '\n';
    }
    catch (const json::type_error& e)
    {
        std::cout << e.what() << '\n';
    }
}

Output:

[json.exception.type_error.302] type must be number, but is string
[json.exception.type_error.306] cannot use value() with string

See also

• see at for access by reference with range checking
• see operator[] for unchecked access by reference

Version history

1. Added in version 1.0.0. Changed parameter default_value type from const ValueType& to ValueType&& in version 3.11.0. Deleted overload for integral key types added in version 3.13.0 unreleased to reject such calls at compile time instead of causing undefined behavior at runtime.
2. Added in version 3.11.0. Made ValueType the first template parameter in version 3.11.2.
3. Added in version 2.0.2. Extended to work with arrays in version 3.13.0 unreleased, including fixing an issue where resolving ptr through an array unexpectedly threw out_of_range instead of returning the resolved element (or default_value, as documented).