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() [2/6] 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 ( size_type  idx ) const inline Returns a const reference to the element at specified location idx, with bounds checking. Parameters [in] idx index of the element to access Returns const reference to the element at index idx Exceptions type_error.304 if the JSON value is not an array; in this case, calling at with an index makes no sense. See example below. out_of_range.401 if the index idx is out of range of the array; that is, idx >= size(). See example below. Exception safety Strong guarantee: if an exception is thrown, there are no changes in the JSON value. Complexity Constant. Since version 1.0.0 Example The example below shows how array 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 array const json array = {"first", "2nd", "third", "fourth"}; // output element at index 2 (third element) std::cout << array.at(2) << '\n'; // exception type_error.304 try { // use at() on a non-array type const json str = "I am a string"; std::cout << str.at(0) << '\n'; } catch (json::type_error& e) { std::cout << e.what() << '\n'; } // exception out_of_range.401 try { // try to read beyond the array limit std::cout << array.at(5) << '\n'; } catch (json::out_of_range& e) { std::cout << e.what() << '\n'; } } Output (play with this example online): "third" [json.exception.type_error.304] cannot use at() with string [json.exception.out_of_range.401] array index 5 is out of range The example code above can be translated with g++ -std=c++11 -Isrc doc/examples/ at__size_type_const.cpp -o at__size_type_const Definition at line 3831 of file json.hpp.