JSON for Modern C++  3.7.0

◆ at() [3/6]

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 ( const typename object_t::key_type &  key)
inline

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

Parameters
[in]keykey of the element to access
Returns
reference to the element at key key
Exceptions
type_error.304if the JSON value is not an object; in this case, calling at with a key makes no sense. See example below.
out_of_range.403if the key key is is not stored in the object; that is, find(key) == end(). See example below.
Exception safety
Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
Complexity
Logarithmic in the size of the container.
See also
operator[](const typename object_t::key_type&) for unchecked access by reference
value() for access by value with a default value
Since
version 1.0.0
Example
The example below shows how object 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 object
9  json object =
10  {
11  {"the good", "il buono"},
12  {"the bad", "il cattivo"},
13  {"the ugly", "il brutto"}
14  };
15 
16  // output element with key "the ugly"
17  std::cout << object.at("the ugly") << '\n';
18 
19  // change element with key "the bad"
20  object.at("the bad") = "il cattivo";
21 
22  // output changed array
23  std::cout << object << '\n';
24 
25 
26  // exception type_error.304
27  try
28  {
29  // use at() on a non-object type
30  json str = "I am a string";
31  str.at("the good") = "Another string";
32  }
33  catch (json::type_error& e)
34  {
35  std::cout << e.what() << '\n';
36  }
37 
38  // exception out_of_range.401
39  try
40  {
41  // try to write at a nonexisting key
42  object.at("the fast") = "il rapido";
43  }
44  catch (json::out_of_range& e)
45  {
46  std::cout << e.what() << '\n';
47  }
48 }
basic_json<> json
default JSON class
Definition: json.hpp:2355
detail::out_of_range out_of_range
exception indicating access out of the defined range
Definition: json.hpp:14645
detail::type_error type_error
exception indicating executing a member function with a wrong type
Definition: json.hpp:14643

Output (play with this example online):
"il brutto"
{"the bad":"il cattivo","the good":"il buono","the ugly":"il brutto"}
[json.exception.type_error.304] cannot use at() with string
[json.exception.out_of_range.403] key 'the fast' not found
The example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/at__object_t_key_type.cpp -o at__object_t_key_type 

Definition at line 17438 of file json.hpp.