JSON for Modern C++  2.0.6

§ operator[]() [1/10]

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[] ( 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
std::domain_errorif JSON is not an array or null; example: "cannot use operator[] with string"
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 <json.hpp>
2 
3 using json = nlohmann::json;
4 
5 int main()
6 {
7  // create a JSON array
8  json array = {1, 2, 3, 4, 5};
9 
10  // output element at index 3 (fourth element)
11  std::cout << array[3] << '\n';
12 
13  // change last element to 6
14  array[array.size() - 1] = 6;
15 
16  // output changed array
17  std::cout << array << '\n';
18 
19  // write beyond array limit
20  array[10] = 11;
21 
22  // output changed array
23  std::cout << array << '\n';
24 }
basic_json<> json
default JSON class
Definition: json.hpp:10155
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 -Isrc doc/examples/operatorarray__size_type.cpp -o operatorarray__size_type 
Since
version 1.0.0

Definition at line 3334 of file json.hpp.