JSON for Modern C++  3.7.0

◆ operator[]() [7/8]

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>
reference nlohmann::basic_json::operator[] ( const json_pointer ptr)
inline

Uses a JSON pointer to retrieve a reference to the respective JSON value. No bound checking is performed. Similar to operator[](const typename object_t::key_type&), null values are created in arrays and objects if necessary.

In particular:

  • If the JSON pointer points to an object key that does not exist, it is created an filled with a null value before a reference to it is returned.
  • If the JSON pointer points to an array index that does not exist, it is created an filled with a null value before a reference to it is returned. All indices between the current maximum and the given index are also filled with null.
  • The special value - is treated as a synonym for the index past the end.
Parameters
[in]ptra JSON pointer
Returns
reference to the element pointed to by ptr
Complexity
Constant.
Exceptions
parse_error.106if an array index begins with '0'
parse_error.109if an array index was not a number
out_of_range.404if the JSON pointer can not be resolved
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["/number"_json_pointer] << '\n';
18  // output element with JSON pointer "/string"
19  std::cout << j["/string"_json_pointer] << '\n';
20  // output element with JSON pointer "/array"
21  std::cout << j["/array"_json_pointer] << '\n';
22  // output element with JSON pointer "/array/1"
23  std::cout << j["/array/1"_json_pointer] << '\n';
24 
25  // writing access
26 
27  // change the string
28  j["/string"_json_pointer] = "bar";
29  // output the changed string
30  std::cout << j["string"] << '\n';
31 
32  // "change" a nonexisting object entry
33  j["/boolean"_json_pointer] = true;
34  // output the changed object
35  std::cout << j << '\n';
36 
37  // change an array element
38  j["/array/1"_json_pointer] = 21;
39  // "change" an array element with nonexisting index
40  j["/array/4"_json_pointer] = 44;
41  // output the changed array
42  std::cout << j["array"] << '\n';
43 
44  // "change" the array element past the end
45  j["/array/-"_json_pointer] = 55;
46  // output the changed array
47  std::cout << j["array"] << '\n';
48 }
basic_json<> json
default JSON class
Definition: json.hpp:2355

Output (play with this example online):
1
"foo"
[1,2]
2
"bar"
{"array":[1,2],"boolean":true,"number":1,"string":"bar"}
[1,21,null,null,44]
[1,21,null,null,44,55]
The example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/operatorjson_pointer.cpp -o operatorjson_pointer 
Since
version 2.0.0

Definition at line 21697 of file json.hpp.