JSON for Modern C++ 2.1.1
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◆ 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< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer >::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
std::domain_errorif the JSON value is not an object; example: "cannot use at() with boolean"
std::out_of_rangeif the key key is is not stored in the object; that is, find(key) == end(); example: "key "the fast" not found"
Complexity\n Logarithmic in the size of the container.
Example\n The example below shows how object elements can be read using
at().
1#include <json.hpp>
2
3using json = nlohmann::json;
4
5int main()
6{
7 // create JSON object
8 json object =
9 {
10 {"the good", "il buono"},
11 {"the bad", "il cattivo"},
12 {"the ugly", "il brutto"}
13 };
14
15 // output element with key "the ugly"
16 std::cout << object.at("the ugly") << '\n';
17
18 // try to read from a nonexisting key
19 try
20 {
21 std::cout << object.at("the fast") << '\n';
22 }
23 catch (std::out_of_range)
24 {
25 std::cout << "out of range" << '\n';
26 }
27}
static basic_json array(std::initializer_list< basic_json > init=std::initializer_list< basic_json >())
explicitly create an array from an initializer list
Definition json.hpp:2165
reference at(size_type idx)
access specified array element with bounds checking
Definition json.hpp:3531
a class to store JSON values
Definition json.hpp:1040
basic_json<> json
default JSON class
Definition json.hpp:12369

Output (play with this example online):
"il brutto"
out of range

The example code above can be translated with
g++ -std=c++11 -Isrc doc/examples/at__object_t_key_type_const.cpp -o at__object_t_key_type_const 
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

Definition at line 3668 of file json.hpp.