JSON for Modern C++  3.6.1

◆ operator[]() [1/8]

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[] ( size_type  idx)
inline

Returns a reference to the element at specified location idx.

Note
If idx is beyond the range of the array (i.e., idx >= size()), then the array is silently filled up with null values to make idx a valid reference to the last stored element.
Parameters
[in]idxindex of the element to access
Returns
reference to the element at index idx
Exceptions
type_error.305if the JSON value is not an array or null; in that cases, using the [] operator with an index makes no sense.
Complexity
Constant if idx is in the range of the array. Otherwise linear in idx - size().
Example
The example below shows how array elements can be read and written using [] operator. Note the addition of null values.
1 #include <iostream>
2 #include <nlohmann/json.hpp>
3 
4 using json = nlohmann::json;
5 
6 int main()
7 {
8  // create a JSON array
9  json array = {1, 2, 3, 4, 5};
10 
11  // output element at index 3 (fourth element)
12  std::cout << array[3] << '\n';
13 
14  // change last element to 6
15  array[array.size() - 1] = 6;
16 
17  // output changed array
18  std::cout << array << '\n';
19 
20  // write beyond array limit
21  array[10] = 11;
22 
23  // output changed array
24  std::cout << array << '\n';
25 }
basic_json<> json
default JSON class
Definition: json.hpp:871
size_type size() const noexcept
returns the number of elements
Definition: json.hpp:17326
static JSON_NODISCARD basic_json array(initializer_list_t init={})
explicitly create an array from an initializer list
Definition: json.hpp:14263

Output (play with this example online):
4
[1,2,3,4,6]
[1,2,3,4,6,null,null,null,null,null,11]
The example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/operatorarray__size_type.cpp -o operatorarray__size_type 
Since
version 1.0.0

Definition at line 15885 of file json.hpp.