JSON for Modern C++ 3.7.3

accept
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basic_json
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~basic_json
| ## ◆ basic_json() [5/10]
nlohmann::basic_json::basic_json | ( | initializer_list_t | init,
|  | bool | type_deduction = `true`,
|  | value_t | manual_type = `value_t::array`
| ) |  |
| inline
  Creates a JSON value of type array or object from the passed initializer list init. In case type_deduction is `true` (default), the type of the JSON value to be created is deducted from the initializer list init according to the following rules: If the list is empty, an empty JSON object value `{}` is created. If the list consists of pairs whose first element is a string, a JSON object value is created where the first elements of the pairs are treated as keys and the second elements are as values. In all other cases, an array is created. The rules aim to create the best fit between a C++ initializer list and JSON values. The rationale is as follows: The empty initializer list is written as `{}` which is exactly an empty JSON object. C++ has no way of describing mapped types other than to list a list of pairs. As JSON requires that keys must be of type string, rule 2 is the weakest constraint one can pose on initializer lists to interpret them as an object. In all other cases, the initializer list could not be interpreted as JSON object type, so interpreting it as JSON array type is safe. With the rules described above, the following JSON values cannot be expressed by an initializer list: the empty array (`[]`): use array(initializer_list_t) with an empty initializer list in this case arrays whose elements satisfy rule 2: use array(initializer_list_t) with the same initializer list in this case Note When used without parentheses around an empty initializer list, basic_json() is called instead of this function, yielding the JSON null value. Parameters
  [in] | init | initializer list with JSON values
  [in] | type_deduction | internal parameter; when set to `true`, the type of the JSON value is deducted from the initializer list init; when set to `false`, the type provided via manual_type is forced. This mode is used by the functions array(initializer_list_t) and object(initializer_list_t).
  [in] | manual_type | internal parameter; when type_deduction is set to `false`, the created JSON value will use the provided type (only value_t::array and value_t::object are valid); when type_deduction is set to `true`, this parameter has no effect
  Exceptions
  type_error.301 | if type_deduction is `false`, manual_type is `value_t::object`, but init contains an element which is not a pair whose first element is a string. In this case, the constructor could not create an object. If type_deduction would have be `true`, an array would have been created. See object(initializer_list_t) for an example.
  Complexity Linear in the size of the initializer list init. Exception safety Strong guarantee: if an exception is thrown, there are no changes to any JSON value. Example The example below shows how JSON values are created from initializer lists. 1 #include <iostream> 2 #include <nlohmann/json.hpp> 3 4 using json = nlohmann::json; 5 6 int main() 7 { 8 // create JSON values 9 json j_empty_init_list = json({}); 10 json j_object = { {"one", 1}, {"two", 2} }; 11 json j_array = {1, 2, 3, 4}; 12 json j_nested_object = { {"one", {1}}, {"two", {1, 2}} }; 13 json j_nested_array = { {{1}, "one"}, {{1, 2}, "two"} }; 14 15 // serialize the JSON value 16 std::cout << j_empty_init_list << '\n'; 17 std::cout << j_object << '\n'; 18 std::cout << j_array << '\n'; 19 std::cout << j_nested_object << '\n'; 20 std::cout << j_nested_array << '\n'; 21 } Output (play with this example online):
      {}
      {"one":1,"two":2}
      [1,2,3,4]
      {"one":[1],"two":[1,2]}
      [[[1],"one"],[[1,2],"two"]]
  The example code above can be translated with
      g++ -std=c++11 -Isingle_include doc/examples/basic_json__list_init_t.cpp -o basic_json__list_init_t
See also array(initializer_list_t) – create a JSON array value from an initializer list object(initializer_list_t) – create a JSON object value from an initializer list Since version 1.0.0 Definition at line 17387 of file json.hpp.

nlohmann::json
basic_json<> json
default JSON class
Definition: json.hpp:2774