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nlohmann
481 lines
13 KiB
Markdown
481 lines
13 KiB
Markdown
# nlohmann::basic_json::operator[]
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```
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// (1)
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reference operator[](size_type idx);
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const_reference operator[](size_type idx) const;
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// (2)
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reference operator[](typename object_t::key_type key);
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const_reference operator[](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 operator[](KeyType&& key);
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template<typename KeyType>
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const_reference operator[](KeyType&& key) const;
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// (4)
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reference operator[](const json_pointer& ptr);
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const_reference operator[](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`.
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1. Returns a reference to the object element with specified key `key`. The non-const qualified overload takes the key by value.
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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 with specified JSON pointer `ptr`.
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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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## Iterator invalidation
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For the non-const versions 1. and 4., when passing an **array** index that does not exist, it is created and filled with a `null` value before a reference to it is returned. For this, a reallocation can happen, in which case all iterators (including the [`end()`](https://json.nlohmann.me/api/basic_json/end/index.md) iterator) and all references to the elements are invalidated.
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For [`ordered_json`](https://json.nlohmann.me/api/ordered_json/index.md), also passing an **object key** to the non-const versions 2., 3., and 4., a reallocation can happen which again invalidates all iterators and all references.
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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 element 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. (const) reference to the element at index `idx`
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1. (const) reference to the element at key `key`
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1. (const) reference to the element at key `key`
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1. (const) 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.305`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error305) if the JSON value is not an array or null; in that case, using the `[]` operator with an index makes no sense.
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1. The function can throw the following exceptions:
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- Throws [`type_error.305`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error305) if the JSON value is not an object or null; in that case, using the `[]` operator with a key makes no sense.
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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'.
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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.
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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` for the const version.
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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.
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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 if `idx` is in the range of the array. Otherwise, linear in `idx - size()`.
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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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## Notes
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Undefined behavior and runtime assertions
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The following cases apply to the **const** overloads; the non-const overloads instead insert the missing element (see the notes below).
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1. If the element at index `idx` does not exist, the behavior is undefined.
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1. If the element with key `key` does not exist, the behavior is undefined and is **guarded by a [runtime assertion](https://json.nlohmann.me/features/assertions/index.md)**!
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1. The non-const version may add values: If `idx` is beyond the range of the array (i.e., `idx >= size()`), then the array is silently filled up with `null` values to make `idx` a valid reference to the last stored element. In case the value was `null` before, it is converted to an array.
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1. If `key` is not found in the object, then it is silently added to the object and filled with a `null` value to make `key` a valid reference. In case the value was `null` before, it is converted to an object.
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1. See 2.
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1. `null` values are created in arrays and objects if necessary.
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In particular:
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- If the JSON pointer points to an object key that does not exist, it is created and filled with a `null` value before a reference to it is returned.
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- If the JSON pointer points to an array index that does not exist, it is created and filled with a `null` value before a reference to it is returned. All indices between the current maximum and the given index are also filled with `null`.
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- The special value `-` is treated as a synonym for the index past the end.
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## Examples
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Example: (1) access specified array element
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The example below shows how array elements can be read and written using `[]` operator. Note the addition of `null` values.
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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 a JSON array
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json array = {1, 2, 3, 4, 5};
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// output element at index 3 (fourth element)
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std::cout << array[3] << '\n';
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// change last element to 6
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array[array.size() - 1] = 6;
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// output changed array
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std::cout << array << '\n';
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// write beyond array limit
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array[10] = 11;
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// output changed array
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std::cout << array << '\n';
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}
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```
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Output:
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```
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4
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[1,2,3,4,6]
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[1,2,3,4,6,null,null,null,null,null,11]
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```
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Example: (1) access specified array element (const)
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The example below shows how array elements can be read using the `[]` operator.
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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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}
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```
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Output:
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```
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"third"
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```
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Example: (2) access specified object element
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The example below shows how object elements can be read and written using the `[]` operator.
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```
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#include <iostream>
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#include <iomanip>
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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 a JSON object
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json object =
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{
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{"one", 1}, {"two", 2}, {"three", 2.9}
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};
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// output element with key "two"
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std::cout << object["two"] << "\n\n";
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// change element with key "three"
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object["three"] = 3;
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// output changed array
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std::cout << std::setw(4) << object << "\n\n";
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// mention nonexisting key
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object["four"];
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// write to nonexisting key
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object["five"]["really"]["nested"] = true;
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// output changed object
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std::cout << std::setw(4) << object << '\n';
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}
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```
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Output:
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```
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2
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{
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"one": 1,
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"three": 3,
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"two": 2
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}
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{
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"five": {
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"really": {
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"nested": true
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}
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},
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"four": null,
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"one": 1,
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"three": 3,
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"two": 2
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}
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```
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Example: (2) access specified object element (const)
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The example below shows how object elements can be read using the `[]` operator.
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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 a JSON object
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const json object =
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{
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{"one", 1}, {"two", 2}, {"three", 2.9}
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};
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// output element with key "two"
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std::cout << object["two"] << '\n';
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}
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```
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Output:
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```
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2
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```
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Example: (3) access specified object element using string_view
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The example below shows how object elements can be read using the `[]` operator.
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```
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#include <iostream>
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#include <iomanip>
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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 a JSON object
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json object =
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{
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{"one", 1}, {"two", 2}, {"three", 2.9}
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};
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// output element with key "two"
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std::cout << object["two"sv] << "\n\n";
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// change element with key "three"
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object["three"sv] = 3;
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// output changed array
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std::cout << std::setw(4) << object << "\n\n";
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// mention nonexisting key
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object["four"sv];
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// write to nonexisting key
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object["five"sv]["really"sv]["nested"sv] = true;
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// output changed object
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std::cout << std::setw(4) << object << '\n';
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}
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```
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Output:
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```
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2
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{
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"one": 1,
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"three": 3,
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"two": 2
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}
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{
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"five": {
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"really": {
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"nested": true
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}
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},
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"four": null,
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"one": 1,
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"three": 3,
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"two": 2
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}
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```
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Example: (3) access specified object element using string_view (const)
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The example below shows how object elements can be read using the `[]` operator.
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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 a JSON object
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const json object =
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{
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{"one", 1}, {"two", 2}, {"three", 2.9}
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};
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// output element with key "two"
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std::cout << object["two"sv] << '\n';
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}
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```
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Output:
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```
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2
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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 values can be read and written using JSON Pointers.
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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["/number"_json_pointer] << '\n';
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// output element with JSON pointer "/string"
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std::cout << j["/string"_json_pointer] << '\n';
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// output element with JSON pointer "/array"
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std::cout << j["/array"_json_pointer] << '\n';
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// output element with JSON pointer "/array/1"
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std::cout << j["/array/1"_json_pointer] << '\n';
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// writing access
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// change the string
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j["/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" a nonexisting object entry
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j["/boolean"_json_pointer] = true;
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// output the changed object
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std::cout << j << '\n';
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// change an array element
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j["/array/1"_json_pointer] = 21;
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// "change" an array element with nonexisting index
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j["/array/4"_json_pointer] = 44;
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// output the changed array
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std::cout << j["array"] << '\n';
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// "change" the array element past the end
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j["/array/-"_json_pointer] = 55;
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// output the changed array
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std::cout << j["array"] << '\n';
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}
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```
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Output:
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```
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1
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"foo"
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[1,2]
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2
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"bar"
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{"array":[1,2],"boolean":true,"number":1,"string":"bar"}
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[1,21,null,null,44]
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[1,21,null,null,44,55]
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```
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Example: (4) access specified element via JSON Pointer (const)
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The example below shows how values can be read using JSON Pointers.
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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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const 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["/number"_json_pointer] << '\n';
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// output element with JSON pointer "/string"
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std::cout << j["/string"_json_pointer] << '\n';
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// output element with JSON pointer "/array"
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std::cout << j["/array"_json_pointer] << '\n';
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// output element with JSON pointer "/array/1"
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std::cout << j["/array/1"_json_pointer] << '\n';
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}
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```
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Output:
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```
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1
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"foo"
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[1,2]
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2
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```
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## See also
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- documentation on [unchecked access](https://json.nlohmann.me/features/element_access/unchecked_access/index.md)
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- documentation on [runtime assertions](https://json.nlohmann.me/features/assertions/index.md)
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- see [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) for access by reference with range checking
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- see [`value`](https://json.nlohmann.me/api/basic_json/value/index.md) for access with default value
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## Version history
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1. Added in version 1.0.0.
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1. Added in version 1.0.0. Added overloads for `T* key` in version 1.1.0. Removed overloads for `T* key` (replaced by 3) in version 3.11.0.
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1. Added in version 3.11.0.
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1. Added in version 2.0.0.
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