JSON for Modern C++  3.7.0

◆ at() [4/6]

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 ( const typename object_t::key_type &  key) const
inline

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

Parameters
[in]keykey of the element to access
Returns
const 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 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 
20  // exception type_error.304
21  try
22  {
23  // use at() on a non-object type
24  const json str = "I am a string";
25  std::cout << str.at("the good") << '\n';
26  }
27  catch (json::type_error& e)
28  {
29  std::cout << e.what() << '\n';
30  }
31 
32  // exception out_of_range.401
33  try
34  {
35  // try to read from a nonexisting key
36  std::cout << object.at("the fast") << '\n';
37  }
38  catch (json::out_of_range)
39  {
40  std::cout << "out of range" << '\n';
41  }
42 }
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"
[json.exception.type_error.304] cannot use at() with string
out of range
The <a href= https://github.com/nlohmann/json/blob/develop/doc/examples/ at__object_t_key_type_const.cpp>example code above can be translated with
g++ -std=c++11 -Isingle_include doc/examples/
at__object_t_key_type_const.cpp -o 
at__object_t_key_type_const 

Definition at line 17494 of file json.hpp.