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111 lines
2.5 KiB
Markdown
111 lines
2.5 KiB
Markdown
# <small>nlohmann::basic_json::</small>back
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```cpp
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reference back();
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const_reference back() const;
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```
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Returns a reference to the last element in the container. For a JSON container `c`, the expression `c.back()` is
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equivalent to
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```cpp
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auto tmp = c.end();
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--tmp;
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return *tmp;
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```
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## Return value
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In the case of a structured type (array or object), a reference to the last element is returned. In the case of number,
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string, boolean, or binary values, a reference to the value is returned.
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## Exception safety
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Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
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## Exceptions
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If the JSON value is `#!json null`, exception
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[`invalid_iterator.214`](../../home/exceptions.md#jsonexceptioninvalid_iterator214) is thrown.
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## Complexity
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Constant.
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## Notes
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!!! info "Precondition"
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The array or object must not be empty. Calling `back` on an empty array or object yields undefined behavior.
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## Examples
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??? example
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The following code shows an example for `back()`.
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```cpp
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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 values
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json j_boolean = true;
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json j_number_integer = 17;
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json j_number_float = 23.42;
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json j_object = {{"one", 1}, {"two", 2}};
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json j_object_empty(json::value_t::object);
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json j_array = {1, 2, 4, 8, 16};
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json j_array_empty(json::value_t::array);
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json j_string = "Hello, world";
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// call back()
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std::cout << j_boolean.back() << '\n';
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std::cout << j_number_integer.back() << '\n';
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std::cout << j_number_float.back() << '\n';
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std::cout << j_object.back() << '\n';
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//std::cout << j_object_empty.back() << '\n'; // undefined behavior
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std::cout << j_array.back() << '\n';
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//std::cout << j_array_empty.back() << '\n'; // undefined behavior
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std::cout << j_string.back() << '\n';
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// back() called on a null value
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try
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{
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json j_null;
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j_null.back();
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}
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catch (const json::invalid_iterator& 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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```json
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true
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17
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23.42
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2
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16
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"Hello, world"
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[json.exception.invalid_iterator.214] cannot get value
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```
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## See also
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- [front](front.md) to access the first element
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## Version history
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- Added in version 1.0.0.
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- Adjusted code to return reference to binary values in version 3.8.0.
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