JSON for Modern C++ 3.10.4

◆ 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]ptrJSON pointer to the desired element
Returns
reference to the element pointed to by ptr
Exceptions
parse_error.106if an array index in the passed JSON pointer ptr begins with '0'. See example below.
parse_error.109if an array index in the passed JSON pointer ptr is not a number. See example below.
out_of_range.401if an array index in the passed JSON pointer ptr is out of range. See example below.
out_of_range.402if 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.403if the JSON pointer describes a key of an object which cannot be found. See example below.
out_of_range.404if 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
4using json = nlohmann::json;
5
6int 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}
value_type & reference
the type of an element reference
Definition: json.hpp:17791
detail::out_of_range out_of_range
exception indicating access out of the defined range
Definition: json.hpp:17771
reference at(size_type idx)
access specified array element with bounds checking
Definition: json.hpp:20987
detail::parse_error parse_error
exception indicating a parse error
Definition: json.hpp:17765
const value_type & const_reference
the type of an element const reference
Definition: json.hpp:17793
basic_json<> json
default JSON class
Definition: json.hpp:3472

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 25784 of file json.hpp.