// (1) reference at(size_type idx); const_reference at(size_type idx) const; // (2) reference at(const typename object_t::key_type& key); const_reference at(const typename object_t::key_type& key) const; // (3) template<typename KeyType> reference at(KeyType&& key); template<typename KeyType> const_reference at(KeyType&& key) const; // (4) reference at(const json_pointer& ptr); const_reference at(const json_pointer& ptr) const;
idx, with bounds checking.key, with bounds checking.KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.ptr, with bounds checking.KeyType : A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17).
idx (in) : index of the element to access
key (in) : object key of the elements to access
ptr (in) : JSON pointer to the desired element
idxkeykeyptrStrong exception safety: if an exception occurs, the original value stays intact.
type_error.304 if the JSON value is not an array; in this case, calling at with an index makes no sense. See the example below.out_of_range.401 if the index idx is out of range of the array; that is, idx >= size(). See the example below.type_error.304 if the JSON value is not an object; in this case, calling at with a key makes no sense. See the example below.out_of_range.403 if the key key is not stored in the object; that is, find(key) == end(). See the example below.parse_error.106 if an array index in the passed JSON pointer ptr begins with ‘0’. See the example below.parse_error.109 if an array index in the passed JSON pointer ptr is not a number. See the example below.out_of_range.401 if an array index in the passed JSON pointer ptr is out of range. See the example below.out_of_range.402 if the array index ‘-’ is used in the passed JSON pointer ptr. As at provides checked access (and no elements are implicitly inserted), the index ‘-’ is always invalid. See the example below.out_of_range.403 if the JSON pointer describes a key of an object which cannot be found. See the example below.out_of_range.404 if the JSON pointer ptr can not be resolved. See the example below.out_of_range.410 if an array index in the passed JSON pointer ptr exceeds the range of size_type (e.g., on 32-bit platforms).Example: (1) access specified array element with bounds checking
The example below shows how array elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// create JSON array
json array = {"first", "2nd", "third", "fourth"};
// output element at index 2 (third element)
std::cout << array.at(2) << '\n';
// change element at index 1 (second element) to "second"
array.at(1) = "second";
// output changed array
std::cout << array << '\n';
// exception type_error.304
try
{
// use at() on a non-array type
json str = "I am a string";
str.at(0) = "Another string";
}
catch (const json::type_error& e)
{
std::cout << e.what() << '\n';
}
// exception out_of_range.401
try
{
// try to write beyond the array limit
array.at(5) = "sixth";
}
catch (const json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
}
Output:
"third" ["first","second","third","fourth"] [json.exception.type_error.304] cannot use at() with string [json.exception.out_of_range.401] array index 5 is out of range
Example: (1) access specified array element with bounds checking
The example below shows how array elements can be read using at(). It also demonstrates the different exceptions that can be thrown.
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// create JSON array
const json array = {"first", "2nd", "third", "fourth"};
// output element at index 2 (third element)
std::cout << array.at(2) << '\n';
// exception type_error.304
try
{
// use at() on a non-array type
const json str = "I am a string";
std::cout << str.at(0) << '\n';
}
catch (const json::type_error& e)
{
std::cout << e.what() << '\n';
}
// exception out_of_range.401
try
{
// try to read beyond the array limit
std::cout << array.at(5) << '\n';
}
catch (const json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
}
Output:
"third" [json.exception.type_error.304] cannot use at() with string [json.exception.out_of_range.401] array index 5 is out of range
Example: (2) access specified object element with bounds checking
The example below shows how object elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// create JSON object
json object =
{
{"the good", "il buono"},
{"the bad", "il cattivo"},
{"the ugly", "il brutto"}
};
// output element with key "the ugly"
std::cout << object.at("the ugly") << '\n';
// change element with key "the bad"
object.at("the bad") = "il cattivo";
// output changed array
std::cout << object << '\n';
// exception type_error.304
try
{
// use at() on a non-object type
json str = "I am a string";
str.at("the good") = "Another string";
}
catch (const json::type_error& e)
{
std::cout << e.what() << '\n';
}
// exception out_of_range.401
try
{
// try to write at a nonexisting key
object.at("the fast") = "il rapido";
}
catch (const json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
}
Output:
"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
Example: (2) access specified object element with bounds checking
The example below shows how object elements can be read using at(). It also demonstrates the different exceptions that can be thrown.
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// create JSON object
const json object =
{
{"the good", "il buono"},
{"the bad", "il cattivo"},
{"the ugly", "il brutto"}
};
// output element with key "the ugly"
std::cout << object.at("the ugly") << '\n';
// exception type_error.304
try
{
// use at() on a non-object type
const json str = "I am a string";
std::cout << str.at("the good") << '\n';
}
catch (const json::type_error& e)
{
std::cout << e.what() << '\n';
}
// exception out_of_range.401
try
{
// try to read from a nonexisting key
std::cout << object.at("the fast") << '\n';
}
catch (const json::out_of_range)
{
std::cout << "out of range" << '\n';
}
}
Output:
"il brutto" [json.exception.type_error.304] cannot use at() with string out of range
Example: (3) access specified object element using string_view with bounds checking
The example below shows how object elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.
#include <iostream>
#include <string_view>
#include <nlohmann/json.hpp>
using namespace std::string_view_literals;
using json = nlohmann::json;
int main()
{
// create JSON object
json object =
{
{"the good", "il buono"},
{"the bad", "il cattivo"},
{"the ugly", "il brutto"}
};
// output element with key "the ugly" using string_view
std::cout << object.at("the ugly"sv) << '\n';
// change element with key "the bad" using string_view
object.at("the bad"sv) = "il cattivo";
// output changed array
std::cout << object << '\n';
// exception type_error.304
try
{
// use at() with string_view on a non-object type
json str = "I am a string";
str.at("the good"sv) = "Another string";
}
catch (const json::type_error& e)
{
std::cout << e.what() << '\n';
}
// exception out_of_range.401
try
{
// try to write at a nonexisting key using string_view
object.at("the fast"sv) = "il rapido";
}
catch (const json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
}
Output:
"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
Example: (3) access specified object element using string_view with bounds checking
The example below shows how object elements can be read using at(). It also demonstrates the different exceptions that can be thrown.
#include <iostream>
#include <string_view>
#include <nlohmann/json.hpp>
using namespace std::string_view_literals;
using json = nlohmann::json;
int main()
{
// create JSON object
const json object =
{
{"the good", "il buono"},
{"the bad", "il cattivo"},
{"the ugly", "il brutto"}
};
// output element with key "the ugly" using string_view
std::cout << object.at("the ugly"sv) << '\n';
// exception type_error.304
try
{
// use at() with string_view on a non-object type
const json str = "I am a string";
std::cout << str.at("the good"sv) << '\n';
}
catch (const json::type_error& e)
{
std::cout << e.what() << '\n';
}
// exception out_of_range.401
try
{
// try to read from a nonexisting key using string_view
std::cout << object.at("the fast"sv) << '\n';
}
catch (const json::out_of_range& e)
{
std::cout << "out of range" << '\n';
}
}
Output:
"il brutto" [json.exception.type_error.304] cannot use at() with string out of range
Example: (4) access specified element via JSON Pointer
The example below shows how object elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
using namespace nlohmann::literals;
int main()
{
// create a JSON value
json j =
{
{"number", 1}, {"string", "foo"}, {"array", {1, 2}}
};
// read-only access
// output element with JSON pointer "/number"
std::cout << j.at("/number"_json_pointer) << '\n';
// output element with JSON pointer "/string"
std::cout << j.at("/string"_json_pointer) << '\n';
// output element with JSON pointer "/array"
std::cout << j.at("/array"_json_pointer) << '\n';
// output element with JSON pointer "/array/1"
std::cout << j.at("/array/1"_json_pointer) << '\n';
// writing access
// change the string
j.at("/string"_json_pointer) = "bar";
// output the changed string
std::cout << j["string"] << '\n';
// change an array element
j.at("/array/1"_json_pointer) = 21;
// output the changed array
std::cout << j["array"] << '\n';
// out_of_range.106
try
{
// try to use an array index with leading '0'
json::reference ref = j.at("/array/01"_json_pointer);
}
catch (const json::parse_error& e)
{
std::cout << e.what() << '\n';
}
// out_of_range.109
try
{
// try to use an array index that is not a number
json::reference ref = j.at("/array/one"_json_pointer);
}
catch (const json::parse_error& e)
{
std::cout << e.what() << '\n';
}
// out_of_range.401
try
{
// try to use an invalid array index
json::reference ref = j.at("/array/4"_json_pointer);
}
catch (const json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
// out_of_range.402
try
{
// try to use the array index '-'
json::reference ref = j.at("/array/-"_json_pointer);
}
catch (const json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
// out_of_range.403
try
{
// try to use a JSON pointer to a nonexistent object key
json::const_reference ref = j.at("/foo"_json_pointer);
}
catch (const json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
// out_of_range.404
try
{
// try to use a JSON pointer that cannot be resolved
json::reference ref = j.at("/number/foo"_json_pointer);
}
catch (const json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
}
Output:
1 "foo" [1,2] 2 "bar" [1,21] [json.exception.parse_error.106] parse error: array index '01' must not begin with '0' [json.exception.parse_error.109] parse error: array index 'one' is not a number [json.exception.out_of_range.401] array index 4 is out of range [json.exception.out_of_range.402] array index '-' (2) is out of range [json.exception.out_of_range.403] key 'foo' not found [json.exception.out_of_range.404] unresolved reference token 'foo'
Example: (4) access specified element via JSON Pointer
The example below shows how object elements can be read using at(). It also demonstrates the different exceptions that can be thrown.
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
using namespace nlohmann::literals;
int main()
{
// create a JSON value
const json j =
{
{"number", 1}, {"string", "foo"}, {"array", {1, 2}}
};
// read-only access
// output element with JSON pointer "/number"
std::cout << j.at("/number"_json_pointer) << '\n';
// output element with JSON pointer "/string"
std::cout << j.at("/string"_json_pointer) << '\n';
// output element with JSON pointer "/array"
std::cout << j.at("/array"_json_pointer) << '\n';
// output element with JSON pointer "/array/1"
std::cout << j.at("/array/1"_json_pointer) << '\n';
// out_of_range.109
try
{
// try to use an array index that is not a number
json::const_reference ref = j.at("/array/one"_json_pointer);
}
catch (const json::parse_error& e)
{
std::cout << e.what() << '\n';
}
// out_of_range.401
try
{
// try to use an invalid array index
json::const_reference ref = j.at("/array/4"_json_pointer);
}
catch (const json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
// out_of_range.402
try
{
// try to use the array index '-'
json::const_reference ref = j.at("/array/-"_json_pointer);
}
catch (const json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
// out_of_range.403
try
{
// try to use a JSON pointer to a nonexistent object key
json::const_reference ref = j.at("/foo"_json_pointer);
}
catch (const json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
// out_of_range.404
try
{
// try to use a JSON pointer that cannot be resolved
json::const_reference ref = j.at("/number/foo"_json_pointer);
}
catch (const json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
}
Output:
1 "foo" [1,2] 2 [json.exception.parse_error.109] parse error: array index 'one' is not a number [json.exception.out_of_range.401] array index 4 is out of range [json.exception.out_of_range.402] array index '-' (2) is out of range [json.exception.out_of_range.403] key 'foo' not found [json.exception.out_of_range.404] unresolved reference token 'foo'
operator[] for unchecked access by referencevalue for access with default value