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nlohmann
669 lines
18 KiB
Markdown
669 lines
18 KiB
Markdown
# nlohmann::basic_json::at
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```
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// (1)
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reference at(size_type idx);
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const_reference at(size_type idx) const;
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// (2)
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reference at(const typename object_t::key_type& key);
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const_reference at(const typename object_t::key_type& key) const;
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// (3)
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template<typename KeyType>
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reference at(KeyType&& key);
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template<typename KeyType>
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const_reference at(KeyType&& key) const;
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// (4)
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reference at(const json_pointer& ptr);
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const_reference at(const json_pointer& ptr) const;
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```
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1. Returns a reference to the array element at specified location `idx`, with bounds checking.
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1. Returns a reference to the object element with specified key `key`, with bounds checking.
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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.
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1. Returns a reference to the element at specified JSON pointer `ptr`, with bounds checking.
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## Template parameters
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`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).
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## Parameters
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`idx` (in) : index of the element to access
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`key` (in) : object key of the elements to access
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`ptr` (in) : JSON pointer to the desired element
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## Return value
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1. reference to the element at index `idx`
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1. reference to the element at key `key`
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1. reference to the element at key `key`
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1. reference to the element pointed to by `ptr`
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## Exception safety
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Strong exception safety: if an exception occurs, the original value stays intact.
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## Exceptions
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1. The function can throw the following exceptions:
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- 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.
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- 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.
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1. The function can throw the following exceptions:
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- 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.
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- 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.
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1. See 2.
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1. The function can throw the following exceptions:
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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).
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## Complexity
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1. Constant.
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1. Logarithmic in the size of the container.
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1. Logarithmic in the size of the container.
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1. Logarithmic in the size of the container.
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## Examples
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Example: (1) access specified array element with bounds checking
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The example below shows how array elements can be read and written using `at()`. It also demonstrates the different exceptions that can be thrown.
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```
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create JSON array
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json array = {"first", "2nd", "third", "fourth"};
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// output element at index 2 (third element)
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std::cout << array.at(2) << '\n';
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// change element at index 1 (second element) to "second"
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array.at(1) = "second";
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// output changed array
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std::cout << array << '\n';
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// exception type_error.304
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try
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{
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// use at() on a non-array type
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json str = "I am a string";
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str.at(0) = "Another string";
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}
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catch (const json::type_error& e)
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{
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std::cout << e.what() << '\n';
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}
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// exception out_of_range.401
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try
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{
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// try to write beyond the array limit
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array.at(5) = "sixth";
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}
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catch (const json::out_of_range& e)
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{
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std::cout << e.what() << '\n';
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}
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}
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```
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Output:
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```
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"third"
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["first","second","third","fourth"]
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[json.exception.type_error.304] cannot use at() with string
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[json.exception.out_of_range.401] array index 5 is out of range
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```
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Example: (1) access specified array element with bounds checking
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The example below shows how array elements can be read using `at()`. It also demonstrates the different exceptions that can be thrown.
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```
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create JSON array
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const json array = {"first", "2nd", "third", "fourth"};
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// output element at index 2 (third element)
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std::cout << array.at(2) << '\n';
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// exception type_error.304
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try
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{
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// use at() on a non-array type
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const json str = "I am a string";
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std::cout << str.at(0) << '\n';
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}
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catch (const json::type_error& e)
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{
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std::cout << e.what() << '\n';
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}
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// exception out_of_range.401
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try
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{
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// try to read beyond the array limit
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std::cout << array.at(5) << '\n';
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}
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catch (const json::out_of_range& e)
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{
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std::cout << e.what() << '\n';
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}
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}
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```
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Output:
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```
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"third"
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[json.exception.type_error.304] cannot use at() with string
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[json.exception.out_of_range.401] array index 5 is out of range
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```
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Example: (2) access specified object element with bounds checking
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The example below shows how object elements can be read and written using `at()`. It also demonstrates the different exceptions that can be thrown.
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```
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create JSON object
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json object =
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{
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{"the good", "il buono"},
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{"the bad", "il cattivo"},
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{"the ugly", "il brutto"}
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};
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// output element with key "the ugly"
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std::cout << object.at("the ugly") << '\n';
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// change element with key "the bad"
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object.at("the bad") = "il cattivo";
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// output changed array
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std::cout << object << '\n';
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// exception type_error.304
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try
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{
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// use at() on a non-object type
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json str = "I am a string";
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str.at("the good") = "Another string";
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}
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catch (const json::type_error& e)
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{
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std::cout << e.what() << '\n';
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}
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// exception out_of_range.401
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try
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{
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// try to write at a nonexisting key
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object.at("the fast") = "il rapido";
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}
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catch (const json::out_of_range& e)
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{
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std::cout << e.what() << '\n';
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}
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}
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```
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Output:
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```
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"il brutto"
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{"the bad":"il cattivo","the good":"il buono","the ugly":"il brutto"}
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[json.exception.type_error.304] cannot use at() with string
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[json.exception.out_of_range.403] key 'the fast' not found
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```
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Example: (2) access specified object element with bounds checking
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The example below shows how object elements can be read using `at()`. It also demonstrates the different exceptions that can be thrown.
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```
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create JSON object
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const json object =
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{
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{"the good", "il buono"},
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{"the bad", "il cattivo"},
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{"the ugly", "il brutto"}
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};
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// output element with key "the ugly"
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std::cout << object.at("the ugly") << '\n';
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// exception type_error.304
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try
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{
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// use at() on a non-object type
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const json str = "I am a string";
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std::cout << str.at("the good") << '\n';
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}
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catch (const json::type_error& e)
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{
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std::cout << e.what() << '\n';
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}
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// exception out_of_range.401
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try
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{
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// try to read from a nonexisting key
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std::cout << object.at("the fast") << '\n';
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}
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catch (const json::out_of_range)
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{
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std::cout << "out of range" << '\n';
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}
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}
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```
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Output:
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```
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"il brutto"
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[json.exception.type_error.304] cannot use at() with string
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out of range
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```
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Example: (3) access specified object element using string_view with bounds checking
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The example below shows how object elements can be read and written using `at()`. It also demonstrates the different exceptions that can be thrown.
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```
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#include <iostream>
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#include <string_view>
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#include <nlohmann/json.hpp>
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using namespace std::string_view_literals;
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using json = nlohmann::json;
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int main()
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{
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// create JSON object
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json object =
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{
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{"the good", "il buono"},
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{"the bad", "il cattivo"},
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{"the ugly", "il brutto"}
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};
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// output element with key "the ugly" using string_view
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std::cout << object.at("the ugly"sv) << '\n';
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// change element with key "the bad" using string_view
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object.at("the bad"sv) = "il cattivo";
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// output changed array
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std::cout << object << '\n';
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// exception type_error.304
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try
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{
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// use at() with string_view on a non-object type
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json str = "I am a string";
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str.at("the good"sv) = "Another string";
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}
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catch (const json::type_error& e)
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{
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std::cout << e.what() << '\n';
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}
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// exception out_of_range.401
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try
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{
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// try to write at a nonexisting key using string_view
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object.at("the fast"sv) = "il rapido";
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}
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catch (const json::out_of_range& e)
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{
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std::cout << e.what() << '\n';
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}
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}
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```
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Output:
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```
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"il brutto"
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{"the bad":"il cattivo","the good":"il buono","the ugly":"il brutto"}
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[json.exception.type_error.304] cannot use at() with string
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[json.exception.out_of_range.403] key 'the fast' not found
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```
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Example: (3) access specified object element using string_view with bounds checking
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The example below shows how object elements can be read using `at()`. It also demonstrates the different exceptions that can be thrown.
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```
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#include <iostream>
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#include <string_view>
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#include <nlohmann/json.hpp>
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using namespace std::string_view_literals;
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using json = nlohmann::json;
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int main()
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{
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// create JSON object
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const json object =
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{
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{"the good", "il buono"},
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{"the bad", "il cattivo"},
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{"the ugly", "il brutto"}
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};
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// output element with key "the ugly" using string_view
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std::cout << object.at("the ugly"sv) << '\n';
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// exception type_error.304
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try
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{
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// use at() with string_view on a non-object type
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const json str = "I am a string";
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std::cout << str.at("the good"sv) << '\n';
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}
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catch (const json::type_error& e)
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{
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std::cout << e.what() << '\n';
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}
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// exception out_of_range.401
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try
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{
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// try to read from a nonexisting key using string_view
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std::cout << object.at("the fast"sv) << '\n';
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}
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catch (const json::out_of_range& e)
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{
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std::cout << "out of range" << '\n';
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}
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}
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```
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Output:
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```
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"il brutto"
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[json.exception.type_error.304] cannot use at() with string
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out of range
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```
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Example: (4) access specified element via JSON Pointer
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The example below shows how object elements can be read and written using `at()`. It also demonstrates the different exceptions that can be thrown.
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```
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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using namespace nlohmann::literals;
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int main()
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{
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// create a JSON value
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json j =
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{
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{"number", 1}, {"string", "foo"}, {"array", {1, 2}}
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};
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// read-only access
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// output element with JSON pointer "/number"
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std::cout << j.at("/number"_json_pointer) << '\n';
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// output element with JSON pointer "/string"
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std::cout << j.at("/string"_json_pointer) << '\n';
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// output element with JSON pointer "/array"
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std::cout << j.at("/array"_json_pointer) << '\n';
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// output element with JSON pointer "/array/1"
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std::cout << j.at("/array/1"_json_pointer) << '\n';
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// writing access
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// change the string
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j.at("/string"_json_pointer) = "bar";
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// output the changed string
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std::cout << j["string"] << '\n';
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// change an array element
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j.at("/array/1"_json_pointer) = 21;
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// output the changed array
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std::cout << j["array"] << '\n';
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// out_of_range.106
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try
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{
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// try to use an array index with leading '0'
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json::reference ref = j.at("/array/01"_json_pointer);
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}
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catch (const json::parse_error& e)
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{
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std::cout << e.what() << '\n';
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}
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// out_of_range.109
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try
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{
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// try to use an array index that is not a number
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json::reference ref = j.at("/array/one"_json_pointer);
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}
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catch (const json::parse_error& e)
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{
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std::cout << e.what() << '\n';
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}
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// out_of_range.401
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try
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{
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// try to use an invalid array index
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json::reference ref = j.at("/array/4"_json_pointer);
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}
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catch (const json::out_of_range& e)
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{
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std::cout << e.what() << '\n';
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}
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// out_of_range.402
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try
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{
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// try to use the array index '-'
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json::reference ref = j.at("/array/-"_json_pointer);
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}
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catch (const json::out_of_range& e)
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{
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std::cout << e.what() << '\n';
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}
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// out_of_range.403
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try
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{
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// try to use a JSON pointer to a nonexistent object key
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json::const_reference ref = j.at("/foo"_json_pointer);
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}
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catch (const json::out_of_range& e)
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{
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std::cout << e.what() << '\n';
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}
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// out_of_range.404
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try
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{
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// try to use a JSON pointer that cannot be resolved
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json::reference ref = j.at("/number/foo"_json_pointer);
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}
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catch (const json::out_of_range& e)
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{
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std::cout << e.what() << '\n';
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}
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}
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```
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Output:
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```
|
|
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.
|