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 begin begin boolean_t cbegin cend clear const_iterator const_pointer const_reference const_reverse_iterator count crbegin crend detail::external_constructor diff difference_type dump emplace emplace_back empty end end erase erase erase erase exception find find flatten from_cbor from_msgpack front front get get get get get get_allocator get_ptr get_ptr get_ref get_ref insert insert insert insert insert insert invalid_iterator is_array is_boolean is_discarded is_null is_number is_number_float is_number_integer is_number_unsigned is_object is_primitive is_string is_structured iterator iterator_wrapper iterator_wrapper json_serializer max_size meta number_float_t number_integer_t number_unsigned_t object object_t operator value_t operator ValueType operator!= operator!= operator!= operator+= operator+= operator+= operator+= operator< operator< operator< operator<< operator<< operator<= operator<= operator<= operator= operator== operator== operator== operator> operator> operator> operator>= operator>= operator>= operator>> operator>> operator[] operator[] operator[] operator[] operator[] operator[] operator[] operator[] operator[] operator[] other_error out_of_range parse parse parse parse parse parse parse_error parse_event_t parser_callback_t patch pointer push_back push_back push_back push_back rbegin rbegin reference rend rend reverse_iterator size size_type string_t swap swap swap swap to_cbor to_msgpack type type_error type_name unflatten value value value value value_t value_type ~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> const_reference nlohmann::basic_json::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. #include "json.hpp" using json = nlohmann::json; int main() { // create JSON object json object = { {"the good", "il buono"}, {"the bad", "il cattivo"}, {"the ugly", "il brutto"} }; // output element with key "the ugly" std::cout << object.at("the ugly") << '\n'; // exception type_error.304 try { // use at() on a non-object type const json str = "I am a string"; std::cout << str.at("the good") << '\n'; } catch (json::type_error& e) { std::cout << e.what() << '\n'; } // exception out_of_range.401 try { // try to read from a nonexisting key std::cout << object.at("the fast") << '\n'; } catch (json::out_of_range) { std::cout << "out of range" << '\n'; } } Output (play with this example online): "il brutto" [json.exception.type_error.304] cannot use at() with string out of range The example code above can be translated with g++ -std=c++11 -Isrc doc/examples/ at__object_t_key_type_const.cpp -o at__object_t_key_type_const Definition at line 3937 of file json.hpp.