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
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clear
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detail::external_constructor
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emplace
emplace_back
empty
end
end
erase
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erase
error_handler_t
exception
find
find
flatten
from_bson
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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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input_format_t
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items
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meta
number_float_t
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object_t
operator value_t
operator ValueType
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parser_callback_t
patch
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rbegin
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to_bson
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type
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~basic_json
| ## ◆ at() [3/6]
reference nlohmann::basic_json::at | ( | const typename object_t::key_type & | key | ) |
| inline
  Returns a reference to the element at with specified key key, with bounds checking. Parameters
  [in] | key | key of the element to access
  Returns reference to the element at key key Exceptions
  type_error.304 | if the JSON value is not an object; in this case, calling `at` with a key makes no sense. See example below.
  out_of_range.403 | if the key key is is not stored in the object; that is, `find(key) == end()`. See example below.
  Exception safety Strong guarantee: if an exception is thrown, there are no changes in the JSON value. Complexity Logarithmic in the size of the container. See also operator[](const typename object_t::key_type&) for unchecked access by reference value() for access by value with a default value Since version 1.0.0 Example The example below shows how object elements can be read and written using `at()`. It also demonstrates the different exceptions that can be thrown. 1 #include <iostream> 2 #include <nlohmann/json.hpp> 3 4 using json = nlohmann::json; 5 6 int main() 7 { 8 // create JSON object 9 json object = 10 { 11 {"the good", "il buono"}, 12 {"the bad", "il cattivo"}, 13 {"the ugly", "il brutto"} 14 }; 15 16 // output element with key "the ugly" 17 std::cout << object.at("the ugly") << '\n'; 18 19 // change element with key "the bad" 20 object.at("the bad") = "il cattivo"; 21 22 // output changed array 23 std::cout << object << '\n'; 24 25 26 // exception type_error.304 27 try 28 { 29 // use at() on a non-object type 30 json str = "I am a string"; 31 str.at("the good") = "Another string"; 32 } 33 catch (json::type_error& e) 34 { 35 std::cout << e.what() << '\n'; 36 } 37 38 // exception out_of_range.401 39 try 40 { 41 // try to write at a nonexisting key 42 object.at("the fast") = "il rapido"; 43 } 44 catch (json::out_of_range& e) 45 { 46 std::cout << e.what() << '\n'; 47 } 48 } Output (play with this example online):
      "il brutto"
      {"the bad":"il cattivo","the good":"il buono","the ugly":"il brutto"}
      [json.exception.type_error.304] cannot use at() with string
      [json.exception.out_of_range.403] key 'the fast' not found
  The example code above can be translated with
      g++ -std=c++11 -Isingle_include doc/examples/at__object_t_key_type.cpp -o at__object_t_key_type
Definition at line 19183 of file json.hpp.

nlohmann::basic_json::type_error
detail::type_error type_error
exception indicating executing a member function with a wrong type
Definition: json.hpp:16045
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