JSON for Modern C++ 3.7.3

accept
accept
accept
allocator_type
array
array_t
at
at
at
at
at
at
back
back
basic_json
basic_json
basic_json
basic_json
basic_json
basic_json
basic_json
basic_json
basic_json
basic_json
begin
begin
binary
binary
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clear
const_iterator
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detail::external_constructor
diff
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dump
emplace
emplace_back
empty
end
end
erase
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erase
erase
error_handler_t
exception
find
find
flatten
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from_ubjson
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from_ubjson
front
front
get
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get_allocator
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get_to
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is_array
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items
items
iterator
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json_pointer
json_sax_t
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meta
number_float_t
number_integer_t
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object
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object_t
operator value_t
operator ValueType
operator!=
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parse
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rbegin
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string_t
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type
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value
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~basic_json
| ## ◆ value() [3/4]
string_t nlohmann::basic_json::value | ( | const typename object_t::key_type & | key,
|  | const char * | default_value
| ) |  | const
| inline
  access specified object element with default value 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 19596 of file json.hpp.

nlohmann::basic_json::at
reference at(size_type idx)
access specified array element with bounds checking
Definition: json.hpp:19080
nlohmann::basic_json::out_of_range
detail::out_of_range out_of_range
exception indicating access out of the defined range
Definition: json.hpp:16047
nlohmann::json
basic_json<> json
default JSON class
Definition: json.hpp:2774