JSON for Modern C++ 3.9.1

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accept
accept
accept
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array_t
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back
back
basic_json
basic_json
basic_json
basic_json
basic_json
basic_json
basic_json
basic_json
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basic_json
begin
begin
binary
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boolean_t
cbegin
cbor_tag_handler_t
cend
clear
const_iterator
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const_reference
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detail::external_constructor
diff
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dump
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end
end
erase
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error_handler_t
exception
find
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from_bson
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front
front
get
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get_ref
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get_to
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items
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json_sax_t
json_serializer
m_value
max_size
merge_patch
meta
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number_integer_t
number_unsigned_t
object
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operator ValueType
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parser_callback_t
patch
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~basic_json
| ## ◆ at() [1/6] 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>>
reference nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType >::at | ( | const json_pointer & | ptr | ) |
| inline
  Returns a 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.403 | if the JSON pointer describes a key of an object which cannot be found. 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. 1 #include <iostream> 2 #include <nlohmann/json.hpp> 3 4 using json = nlohmann::json; 5 6 int main() 7 { 8 // create a JSON value 9 json j = 10 { 11 {"number", 1}, {"string", "foo"}, {"array", {1, 2}} 12 }; 13 14 // read-only access 15 16 // output element with JSON pointer "/number" 17 std::cout << j.at("/number"_json_pointer) << '\n'; 18 // output element with JSON pointer "/string" 19 std::cout << j.at("/string"_json_pointer) << '\n'; 20 // output element with JSON pointer "/array" 21 std::cout << j.at("/array"_json_pointer) << '\n'; 22 // output element with JSON pointer "/array/1" 23 std::cout << j.at("/array/1"_json_pointer) << '\n'; 24 25 // writing access 26 27 // change the string 28 j.at("/string"_json_pointer) = "bar"; 29 // output the changed string 30 std::cout << j["string"] << '\n'; 31 32 // change an array element 33 j.at("/array/1"_json_pointer) = 21; 34 // output the changed array 35 std::cout << j["array"] << '\n'; 36 37 38 // out_of_range.106 39 try 40 { 41 // try to use an array index with leading '0' 42 json::reference ref = j.at("/array/01"_json_pointer); 43 } 44 catch (json::parse_error& e) 45 { 46 std::cout << e.what() << '\n'; 47 } 48 49 // out_of_range.109 50 try 51 { 52 // try to use an array index that is not a number 53 json::reference ref = j.at("/array/one"_json_pointer); 54 } 55 catch (json::parse_error& e) 56 { 57 std::cout << e.what() << '\n'; 58 } 59 60 // out_of_range.401 61 try 62 { 63 // try to use a an invalid array index 64 json::reference ref = j.at("/array/4"_json_pointer); 65 } 66 catch (json::out_of_range& e) 67 { 68 std::cout << e.what() << '\n'; 69 } 70 71 // out_of_range.402 72 try 73 { 74 // try to use the array index '-' 75 json::reference ref = j.at("/array/-"_json_pointer); 76 } 77 catch (json::out_of_range& e) 78 { 79 std::cout << e.what() << '\n'; 80 } 81 82 // out_of_range.403 83 try 84 { 85 // try to use a JSON pointer to an nonexistent object key 86 json::const_reference ref = j.at("/foo"_json_pointer); 87 } 88 catch (json::out_of_range& e) 89 { 90 std::cout << e.what() << '\n'; 91 } 92 93 // out_of_range.404 94 try 95 { 96 // try to use a JSON pointer that cannot be resolved 97 json::reference ref = j.at("/number/foo"_json_pointer); 98 } 99 catch (json::out_of_range& e) 100 { 101 std::cout << e.what() << '\n'; 102 } 103 } nlohmann::basic_json::reference value_type & reference the type of an element reference Definition: json.hpp:17303 nlohmann::basic_json::out_of_range detail::out_of_range out_of_range exception indicating access out of the defined range Definition: json.hpp:17283 nlohmann::basic_json::at reference at(size_type idx) access specified array element with bounds checking Definition: json.hpp:20443 nlohmann::basic_json::parse_error detail::parse_error parse_error exception indicating a parse error Definition: json.hpp:17277 nlohmann::basic_json::const_reference const value_type & const_reference the type of an element const reference Definition: json.hpp:17305 nlohmann::json basic_json<> json default JSON class Definition: json.hpp:3385 Output (play with this example online):
      1
      "foo"
      [1,2]
      2
      "bar"
      [1,21]
      [json.exception.parse_error.106] parse error: array index '01' must not begin with '0'
      [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.403] key 'foo' not found
      [json.exception.out_of_range.404] unresolved reference token 'foo'
  The example code above can be translated with
      g++ -std=c++11 -Isingle_include doc/examples/at_json_pointer.cpp -o at_json_pointer
Definition at line 25180 of file json.hpp.