JSON for Modern C++  2.0.3
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>
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
std::out_of_rangeif the JSON pointer can not be resolved
std::domain_errorif an array index begins with '0'
std::invalid_argumentif an array index was not a number
Example
The behavior is shown in the example.
1 #include <json.hpp>
2 
3 using json = nlohmann::json;
4 
5 int main()
6 {
7  // create a JSON value
8  json j =
9  {
10  {"number", 1}, {"string", "foo"}, {"array", {1, 2}}
11  };
12 
13  // read-only access
14 
15  // output element with JSON pointer "/number"
16  std::cout << j["/number"_json_pointer] << '\n';
17  // output element with JSON pointer "/string"
18  std::cout << j["/string"_json_pointer] << '\n';
19  // output element with JSON pointer "/array"
20  std::cout << j["/array"_json_pointer] << '\n';
21  // output element with JSON pointer "/array/1"
22  std::cout << j["/array/1"_json_pointer] << '\n';
23 
24  // writing access
25 
26  // change the string
27  j["/string"_json_pointer] = "bar";
28  // output the changed string
29  std::cout << j["string"] << '\n';
30 
31  // "change" a nonexisting object entry
32  j["/boolean"_json_pointer] = true;
33  // output the changed object
34  std::cout << j << '\n';
35 
36  // change an array element
37  j["/array/1"_json_pointer] = 21;
38  // "change" an array element with nonexisting index
39  j["/array/4"_json_pointer] = 44;
40  // output the changed array
41  std::cout << j["array"] << '\n';
42 
43  // "change" the arry element past the end
44  j["/array/-"_json_pointer] = 55;
45  // output the changed array
46  std::cout << j["array"] << '\n';
47 }
basic_json<> json
default JSON class
Definition: json.hpp:10122
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 -Isrc doc/examples/operatorjson_pointer.cpp -o operatorjson_pointer 
Since
version 2.0.0

Definition at line 9493 of file json.hpp.