JSON for Modern C++ 3.9.1

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~basic_json
| ## ◆ value() [4/4] template<template< typename U, typename V, typename... Args > class ObjectType = std::map, template< typename U, typename... Args > class ArrayType = std::vector, class StringType = std::string, class BooleanType = bool, class NumberIntegerType = std::int64_t, class NumberUnsignedType = std::uint64_t, class NumberFloatType = double, template< typename U > class AllocatorType = std::allocator, template< typename T, typename SFINAE=void > class JSONSerializer = adl_serializer, class BinaryType = std::vector<std::uint8_t>> template<class ValueType , typename std::enable_if< detail::is_getable< basic_json_t, ValueType >::value &&!std::is_same< value_t, ValueType >::value, int >::type = 0>
ValueType nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType >::value | ( | const typename object_t::key_type & | key,
|  | const ValueType & | default_value
| ) |  | const
| inline
  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; } Note Unlike at(const typename object_t::key_type&), this function does not throw if the given key key was not found. Unlike operator[](const typename object_t::key_type& key), this function does not implicitly add an element to the position defined by key. This function is furthermore also applicable to const objects. Parameters
  [in] | key | key of the element to access
  [in] | default_value | the value to return if key is not found
  Template Parameters
  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 and the default value default_value must be compatible.
  Returns copy of the element at key key or default_value if key is not found Exceptions
  type_error.302 | if default_value does not match the type of the value at key
  type_error.306 | if the JSON value is not an object; in that case, using `value()` with a key makes no sense.
  Complexity Logarithmic in the size of the container. Example The example below shows how object elements can be queried with a default value. 1 #include <iostream> 2 #include <nlohmann/json.hpp> 3 4 using json = nlohmann::json; 5 6 int main() 7 { 8 // create a JSON object with different entry types 9 json j = 10 { 11 {"integer", 1}, 12 {"floating", 42.23}, 13 {"string", "hello world"}, 14 {"boolean", true}, 15 {"object", {{"key1", 1}, {"key2", 2}}}, 16 {"array", {1, 2, 3}} 17 }; 18 19 // access existing values 20 int v_integer = j.value("integer", 0); 21 double v_floating = j.value("floating", 47.11); 22 23 // access nonexisting values and rely on default value 24 std::string v_string = j.value("nonexisting", "oops"); 25 bool v_boolean = j.value("nonexisting", false); 26 27 // output values 28 std::cout << std::boolalpha << v_integer << " " << v_floating 29 << " " << v_string << " " << v_boolean << "\n"; 30 } Output (play with this example online):
      1 42.23 oops false
  The example code above can be translated with
      g++ -std=c++11 -Isingle_include doc/examples/basic_json__value.cpp -o basic_json__value
See also at(const typename object_t::key_type&) for access by reference with range checking operator[](const typename object_t::key_type&) for unchecked access by reference Since version 1.0.0 Definition at line 20312 of file json.hpp.

nlohmann::basic_json::out_of_range
detail::out_of_range out_of_range
exception indicating access out of the defined range
Definition: json.hpp:16768
nlohmann::json
basic_json<> json
default JSON class
Definition: json.hpp:2940
nlohmann::basic_json::at
reference at(size_type idx)
access specified array element with bounds checking
Definition: json.hpp:19816