JSON for Modern C++  3.5.0

◆ at() [1/6]

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::at ( size_type  idx)
inline

Returns a reference to the element at specified location idx, with bounds checking.

Parameters
[in]idxindex of the element to access
Returns
reference to the element at index idx
Exceptions
type_error.304if the JSON value is not an array; in this case, calling at with an index makes no sense. See example below.
out_of_range.401if the index idx is out of range of the array; that is, idx >= size(). See example below.
Exception safety^^ Strong guarantee: if an exception is thrown, there are no
changes in the JSON value.
Complexity^^ Constant.
Since
version 1.0.0
Example^^ The example below shows how array elements can be read and
written using at(). It also demonstrates the different exceptions that can be thrown. ^^
1 #include <iostream>
2 #include <nlohmann/json.hpp>
3 
4 using json = nlohmann::json;
5 
6 int main()
7 {
8  // create JSON array
9  json array = {"first", "2nd", "third", "fourth"};
10 
11  // output element at index 2 (third element)
12  std::cout << array.at(2) << '\n';
13 
14  // change element at index 1 (second element) to "second"
15  array.at(1) = "second";
16 
17  // output changed array
18  std::cout << array << '\n';
19 
20 
21  // exception type_error.304
22  try
23  {
24  // use at() on a non-array type
25  json str = "I am a string";
26  str.at(0) = "Another string";
27  }
28  catch (json::type_error& e)
29  {
30  std::cout << e.what() << '\n';
31  }
32 
33  // exception out_of_range.401
34  try
35  {
36  // try to write beyond the array limit
37  array.at(5) = "sixth";
38  }
39  catch (json::out_of_range& e)
40  {
41  std::cout << e.what() << '\n';
42  }
43 }
static basic_json array(initializer_list_t init={})
explicitly create an array from an initializer list
Definition: json.hpp:13877
basic_json<> json
default JSON class
Definition: json.hpp:110
reference at(size_type idx)
access specified array element with bounds checking
Definition: json.hpp:15293
detail::out_of_range out_of_range
exception indicating access out of the defined range
Definition: json.hpp:12622
detail::type_error type_error
exception indicating executing a member function with a wrong type
Definition: json.hpp:12620
Output (play with this example online):^^
"third"
["first","second","third","fourth"]
[json.exception.type_error.304] cannot use at() with string
[json.exception.out_of_range.401] array index 5 is out of range
^^ The example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/at__size_type.cpp -o at__size_type 

Definition at line 15293 of file json.hpp.