JSON for Modern C++ 2.1.1
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◆ operator[]() [5/10]

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, template< typename T, typename SFINAE=void > class JSONSerializer = adl_serializer>
reference nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer >::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\n Constant if idx is in the range of the array. Otherwise
linear in idx - size().
Example\n 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
3using json = nlohmann::json;
4
5int 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}
size_type size() const noexcept
returns the number of elements
Definition json.hpp:5063
static basic_json array(std::initializer_list< basic_json > init=std::initializer_list< basic_json >())
explicitly create an array from an initializer list
Definition json.hpp:2165
a class to store JSON values
Definition json.hpp:1040
basic_json<> json
default JSON class
Definition json.hpp:12369

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 3714 of file json.hpp.