JSON for Modern C++ 3.9.1

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accept
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~basic_json
| ## ◆ at() [4/6] 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>>
const_reference nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType >::at | ( | const typename object_t::key_type & | key | ) | const
| inline
  Returns a const reference to the element at with specified key key, with bounds checking. Parameters
  [in] | key | key of the element to access
  Returns const 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 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 const 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 20 // exception type_error.304 21 try 22 { 23 // use at() on a non-object type 24 const json str = "I am a string"; 25 std::cout << str.at("the good") << '\n'; 26 } 27 catch (json::type_error& e) 28 { 29 std::cout << e.what() << '\n'; 30 } 31 32 // exception out_of_range.401 33 try 34 { 35 // try to read from a nonexisting key 36 std::cout << object.at("the fast") << '\n'; 37 } 38 catch (json::out_of_range) 39 { 40 std::cout << "out of range" << '\n'; 41 } 42 } Output (play with this example online):
      "il brutto"
      [json.exception.type_error.304] cannot use at() with string
      out of range
  The <a href= https://github.com/nlohmann/json/blob/develop/doc/examples/ at__object_t_key_type_const.cpp>example code above can be translated with
      g++ -std=c++11 -Isingle_include doc/examples/
      at__object_t_key_type_const.cpp -o
      at__object_t_key_type_const
Definition at line 19975 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::type_error
detail::type_error type_error
exception indicating executing a member function with a wrong type
Definition: json.hpp:16766