JSON for Modern C++ 3.10.4

◆ 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, class BinaryType = std::vector<std::uint8_t>>
const_reference nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType >::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
see operator[](const typename object_t::key_type&) for unchecked access by reference
see 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
4using json = nlohmann::json;
5
6int main()
7{
8 // create JSON object
9 const 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}
detail::out_of_range out_of_range
exception indicating access out of the defined range
Definition: json.hpp:17771
detail::type_error type_error
exception indicating executing a member function with a wrong type
Definition: json.hpp:17769
basic_json<> json
default JSON class
Definition: json.hpp:3472

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 21146 of file json.hpp.