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nlohmann
127 lines
3.3 KiB
Markdown
127 lines
3.3 KiB
Markdown
# nlohmann::basic_json::items
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```
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iteration_proxy<iterator> items() noexcept;
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iteration_proxy<const_iterator> items() const noexcept;
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```
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This function allows accessing `iterator::key()` and `iterator::value()` during range-based for loops. In these loops, a reference to the JSON values is returned, so there is no access to the underlying iterator.
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For loop without `items()` function:
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```
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for (auto it = j_object.begin(); it != j_object.end(); ++it)
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{
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std::cout << "key: " << it.key() << ", value:" << it.value() << '\n';
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}
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```
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Range-based for loop without `items()` function:
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```
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for (auto it : j_object)
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{
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// "it" is of type json::reference and has no key() member
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std::cout << "value: " << it << '\n';
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}
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```
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Range-based for loop with `items()` function:
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```
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for (auto& el : j_object.items())
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{
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std::cout << "key: " << el.key() << ", value:" << el.value() << '\n';
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}
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```
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The `items()` function also allows using [structured bindings](https://en.cppreference.com/w/cpp/language/structured_binding) (C++17):
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```
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for (auto& [key, val] : j_object.items())
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{
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std::cout << "key: " << key << ", value:" << val << '\n';
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}
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```
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## Return value
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iteration proxy object wrapping the current value with an interface to use in range-based for loops
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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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## Complexity
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Constant.
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## Notes
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When iterating over an array, `key()` will return the index of the element as string (see example). For primitive types (e.g., numbers), `key()` returns an empty string.
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Lifetime issues
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Using `items()` on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See [#2040](https://github.com/nlohmann/json/issues/2040) for more information.
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## Examples
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Example
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The following code shows an example for `items()`.
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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 values
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json j_object = {{"one", 1}, {"two", 2}};
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json j_array = {1, 2, 4, 8, 16};
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// example for an object
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for (auto& x : j_object.items())
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{
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std::cout << "key: " << x.key() << ", value: " << x.value() << '\n';
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}
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// example for an array
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for (auto& x : j_array.items())
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{
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std::cout << "key: " << x.key() << ", value: " << x.value() << '\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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key: one, value: 1
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key: two, value: 2
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key: 0, value: 1
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key: 1, value: 2
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key: 2, value: 4
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key: 3, value: 8
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key: 4, value: 16
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```
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## See also
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- [begin](https://json.nlohmann.me/api/basic_json/begin/index.md) returns an iterator to the first element
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- [end](https://json.nlohmann.me/api/basic_json/end/index.md) returns an iterator to one past the last element
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## Version history
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- Added `iterator_wrapper` in version 3.0.0.
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- Added `items` and deprecated `iterator_wrapper` in version 3.1.0.
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- Added structured binding support in version 3.5.0.
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Deprecation
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This function replaces the static function `iterator_wrapper` which was introduced in version 1.0.0, but has been deprecated in version 3.1.0. Function `iterator_wrapper` will be removed in version 4.0.0. Please replace all occurrences of `iterator_wrapper(j)` with `j.items()`.
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You should be warned by your compiler with a `-Wdeprecated-declarations` warning if you are using a deprecated function.
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