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() [6/6] 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 json_pointer &  ptr ) const inline Returns a const reference to the element at with specified JSON pointer ptr, with bounds checking. Parameters [in] ptr JSON pointer to the desired element Returns reference to the element pointed to by ptr Exceptions parse_error.106 if an array index in the passed JSON pointer ptr begins with '0'. See example below. parse_error.109 if an array index in the passed JSON pointer ptr is not a number. See example below. out_of_range.401 if an array index in the passed JSON pointer ptr is out of range. See example below. out_of_range.402 if the array index '-' is used in the passed JSON pointer ptr. As at provides checked access (and no elements are implicitly inserted), the index '-' is always invalid. See example below. out_of_range.404 if the JSON pointer ptr can not be resolved. See example below. Exception safety Strong guarantee: if an exception is thrown, there are no changes in the JSON value. Complexity Constant. Since version 2.0.0 Example The behavior is shown in the example. #include "json.hpp" using json = nlohmann::json; int main() { // create a JSON value const json j = { {"number", 1}, {"string", "foo"}, {"array", {1, 2}} }; // read-only access // output element with JSON pointer "/number" std::cout << j.at("/number"_json_pointer) << '\n'; // output element with JSON pointer "/string" std::cout << j.at("/string"_json_pointer) << '\n'; // output element with JSON pointer "/array" std::cout << j.at("/array"_json_pointer) << '\n'; // output element with JSON pointer "/array/1" std::cout << j.at("/array/1"_json_pointer) << '\n'; // out_of_range.109 try { // try to use an array index that is not a number json::const_reference ref = j.at("/array/one"_json_pointer); } catch (json::parse_error& e) { std::cout << e.what() << '\n'; } // out_of_range.401 try { // try to use a an invalid array index json::const_reference ref = j.at("/array/4"_json_pointer); } catch (json::out_of_range& e) { std::cout << e.what() << '\n'; } // out_of_range.402 try { // try to use the array index '-' json::const_reference ref = j.at("/array/-"_json_pointer); } catch (json::out_of_range& e) { std::cout << e.what() << '\n'; } // out_of_range.404 try { // try to use a JSON pointer that cannot be resolved json::const_reference ref = j.at("/number/foo"_json_pointer); } catch (json::out_of_range& e) { std::cout << e.what() << '\n'; } } Output (play with this example online): 1 "foo" [1,2] 2 [json.exception.parse_error.109] parse error: array index 'one' is not a number [json.exception.out_of_range.401] array index 4 is out of range [json.exception.out_of_range.402] array index '-' (2) is out of range [json.exception.out_of_range.404] unresolved reference token 'foo' The example code above can be translated with g++ -std=c++11 -Isrc doc/examples/at_json_pointer_const.cpp -o at_json_pointer_const Definition at line 13369 of file json.hpp.