allocator_type array array_t 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 basic_json basic_json basic_json basic_json basic_json basic_json basic_json basic_json basic_json basic_json basic_json basic_json basic_json begin begin boolean_t cbegin cend clear const_pointer const_reference const_reverse_iterator count crbegin crend difference_type dump empty end end erase erase erase erase find find front front get get get get_allocator get_ptr get_ptr get_ref get_ref insert insert insert insert insert 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_wrapper iterator_wrapper max_size 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[] parse parse parse parse_event_t parser_callback_t pointer push_back push_back push_back rbegin rbegin reference rend rend reverse_iterator size size_type string_t swap swap swap swap type value value value_t value_type ~basic_json

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 = int64_t, class NumberUnsignedType = uint64_t, class NumberFloatType = double, template< typename U > class AllocatorType = std::allocator> reference nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType >::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 std::domain_error if the JSON value is not an object; example: "cannot use at() with boolean" std::out_of_range if the key key is is not stored in the object; that is, find(key) == end(); example: "key "the fast" not found" Complexity Logarithmic in the size of the container. Example The example below shows how object elements can be read and written using at. #include <json.hpp> using json = nlohmann::json; int main() { // create JSON object json object = { {"the good", "il buono"}, {"the bad", "il cativo"}, {"the ugly", "il brutto"} }; // output element with key "the ugly" std::cout << object.at("the ugly") << '\n'; // change element with key "the bad" object.at("the bad") = "il cattivo"; // output changed array std::cout << object << '\n'; // try to write at a nonexisting key try { object.at("the fast") = "il rapido"; } catch (std::out_of_range& e) { std::cout << "out of range: " << e.what() << '\n'; } } Output (play with this example online): "il brutto" {"the bad":"il cattivo","the good":"il buono","the ugly":"il brutto"} out of range: key 'the fast' not found The example code above can be translated with g++ -std=c++11 -Isrc doc/examples/at__object_t_key_type.cpp -o at__object_t_key_type 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 Definition at line 3016 of file json.hpp.