JSON for Modern C++  3.5.0

◆ at() [2/6]

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>
const_reference nlohmann::basic_json::at ( size_type  idx) const
inline

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

Parameters
[in]idxindex of the element to access
Returns
const 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 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  const json array = {"first", "2nd", "third", "fourth"};
10 
11  // output element at index 2 (third element)
12  std::cout << array.at(2) << '\n';
13 
14 
15  // exception type_error.304
16  try
17  {
18  // use at() on a non-array type
19  const json str = "I am a string";
20  std::cout << str.at(0) << '\n';
21  }
22  catch (json::type_error& e)
23  {
24  std::cout << e.what() << '\n';
25  }
26 
27  // exception out_of_range.401
28  try
29  {
30  // try to read beyond the array limit
31  std::cout << array.at(5) << '\n';
32  }
33  catch (json::out_of_range& e)
34  {
35  std::cout << e.what() << '\n';
36  }
37 }
basic_json<> json
default JSON class
Definition: json.hpp:110
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"
[json.exception.type_error.304] cannot use at() with string
[json.exception.out_of_range.401] array index 5 is out of range
^^ The <a href= https://github.com/nlohmann/json/blob/develop/doc/examples/ at__size_type_const.cpp>example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/
at__size_type_const.cpp -o 
at__size_type_const 

Definition at line 15345 of file json.hpp.