api/basic_json/at/index.md - third_party/json - Git at Google

 # nlohmann::basic_json::at

 ```
 // (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;
 ```

 1. Returns a reference to the array element at specified location `idx`, with bounds checking.
 1. Returns a reference to the object element with specified key `key`, with bounds checking.
 1. See 2. This overload is only available if `KeyType` is comparable with `typename object_t::key_type` and `typename object_comparator_t::is_transparent` denotes a type.
 1. Returns a reference to the element at specified JSON pointer `ptr`, with bounds checking.

 ## Template parameters

 `KeyType` : A type for an object key other than [`json_pointer`](https://json.nlohmann.me/api/json_pointer/index.md) that is comparable with [`string_t`](https://json.nlohmann.me/api/basic_json/string_t/index.md) using [`object_comparator_t`](https://json.nlohmann.me/api/basic_json/object_comparator_t/index.md). This can also be a string view (C++17).

 ## Parameters

 `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

 ## Return value

 1. reference to the element at index `idx`
 1. reference to the element at key `key`
 1. reference to the element at key `key`
 1. reference to the element pointed to by `ptr`

 ## Exception safety

 Strong exception safety: if an exception occurs, the original value stays intact.

 ## Exceptions

 1. The function can throw the following exceptions:
    - Throws [`type_error.304`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error304) if the JSON value is not an array; in this case, calling `at` with an index makes no sense. See the example below.
    - Throws [`out_of_range.401`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range401) if the index `idx` is out of range of the array; that is, `idx >= size()`. See the example below.
 1. The function can throw the following exceptions:
    - Throws [`type_error.304`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error304) if the JSON value is not an object; in this case, calling `at` with a key makes no sense. See the example below.
    - Throws [`out_of_range.403`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range403) if the key `key` is not stored in the object; that is, `find(key) == end()`. See the example below.
 1. See 2.
 1. The function can throw the following exceptions:
    - Throws [`parse_error.106`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error106) if an array index in the passed JSON pointer `ptr` begins with '0'. See the example below.
    - Throws [`parse_error.109`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error109) if an array index in the passed JSON pointer `ptr` is not a number. See the example below.
    - Throws [`out_of_range.401`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range401) if an array index in the passed JSON pointer `ptr` is out of range. See the example below.
    - Throws [`out_of_range.402`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range402) 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.
    - Throws [`out_of_range.403`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range403) if the JSON pointer describes a key of an object which cannot be found. See the example below.
    - Throws [`out_of_range.404`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range404) if the JSON pointer `ptr` can not be resolved. See the example below.
    - Throws [`out_of_range.410`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range410) if an array index in the passed JSON pointer `ptr` exceeds the range of `size_type` (e.g., on 32-bit platforms).

 ## Complexity

 1. Constant.
 1. Logarithmic in the size of the container.
 1. Logarithmic in the size of the container.
 1. Logarithmic in the size of the container.

 ## Examples

 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'
 ```

 ## See also

 - documentation on [checked access](https://json.nlohmann.me/features/element_access/checked_access/index.md)
 - [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) for unchecked access by reference
 - [`value`](https://json.nlohmann.me/api/basic_json/value/index.md) for access with default value

 ## Version history

 1. Added in version 1.0.0.
 1. Added in version 1.0.0.
 1. Added in version 3.11.0.
 1. Added in version 2.0.0.
	# nlohmann::basic_json::at

	```
	// (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;
	```

	1. Returns a reference to the array element at specified location `idx`, with bounds checking.
	1. Returns a reference to the object element with specified key `key`, with bounds checking.
	1. See 2. This overload is only available if `KeyType` is comparable with `typename object_t::key_type` and `typename object_comparator_t::is_transparent` denotes a type.
	1. Returns a reference to the element at specified JSON pointer `ptr`, with bounds checking.

	## Template parameters

	`KeyType` : A type for an object key other than [`json_pointer`](https://json.nlohmann.me/api/json_pointer/index.md) that is comparable with [`string_t`](https://json.nlohmann.me/api/basic_json/string_t/index.md) using [`object_comparator_t`](https://json.nlohmann.me/api/basic_json/object_comparator_t/index.md). This can also be a string view (C++17).

	## Parameters

	`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

	## Return value

	1. reference to the element at index `idx`
	1. reference to the element at key `key`
	1. reference to the element at key `key`
	1. reference to the element pointed to by `ptr`

	## Exception safety

	Strong exception safety: if an exception occurs, the original value stays intact.

	## Exceptions

	1. The function can throw the following exceptions:
	- Throws [`type_error.304`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error304) if the JSON value is not an array; in this case, calling `at` with an index makes no sense. See the example below.
	- Throws [`out_of_range.401`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range401) if the index `idx` is out of range of the array; that is, `idx >= size()`. See the example below.
	1. The function can throw the following exceptions:
	- Throws [`type_error.304`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error304) if the JSON value is not an object; in this case, calling `at` with a key makes no sense. See the example below.
	- Throws [`out_of_range.403`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range403) if the key `key` is not stored in the object; that is, `find(key) == end()`. See the example below.
	1. See 2.
	1. The function can throw the following exceptions:
	- Throws [`parse_error.106`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error106) if an array index in the passed JSON pointer `ptr` begins with '0'. See the example below.
	- Throws [`parse_error.109`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error109) if an array index in the passed JSON pointer `ptr` is not a number. See the example below.
	- Throws [`out_of_range.401`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range401) if an array index in the passed JSON pointer `ptr` is out of range. See the example below.
	- Throws [`out_of_range.402`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range402) 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.
	- Throws [`out_of_range.403`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range403) if the JSON pointer describes a key of an object which cannot be found. See the example below.
	- Throws [`out_of_range.404`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range404) if the JSON pointer `ptr` can not be resolved. See the example below.
	- Throws [`out_of_range.410`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range410) if an array index in the passed JSON pointer `ptr` exceeds the range of `size_type` (e.g., on 32-bit platforms).

	## Complexity

	1. Constant.
	1. Logarithmic in the size of the container.
	1. Logarithmic in the size of the container.
	1. Logarithmic in the size of the container.

	## Examples

	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'
	```

	## See also

	- documentation on [checked access](https://json.nlohmann.me/features/element_access/checked_access/index.md)
	- [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) for unchecked access by reference
	- [`value`](https://json.nlohmann.me/api/basic_json/value/index.md) for access with default value

	## Version history

	1. Added in version 1.0.0.
	1. Added in version 1.0.0.
	1. Added in version 3.11.0.
	1. Added in version 2.0.0.