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api/basic_json_view/contains/index.md

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+# nlohmann::basic_json_view::contains
+
+```
+// (1)
+bool contains(string_view_t key) const;
+bool contains(const char* key) const;
+bool contains(const string_t& key) const;
+
+// (2)
+bool contains(const json_pointer& ptr) const;
+```
+
+1. Checks whether the value is an object with a member with key `key`.
+1. Checks whether a JSON pointer `ptr` can be resolved, starting at this value.
+
+## Parameters
+
+`key` (in) : key value to check its existence
+
+`ptr` (in) : JSON pointer to check its existence
+
+## Return value
+
+1. `true` if the value is an object and has a member with key `key`, `false` otherwise
+1. `true` if `ptr` can be resolved to a value starting at this view, `false` otherwise
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+1. Linear in the number of members: as for [`ordered_json`](https://json.nlohmann.me/api/ordered_json/index.md), members are compared one after another, in document order, stopping at the first match. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes constant time on average.
+1. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at that level or the index into the array -- as for [`operator[]`](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/#complexity) and [`at`](https://json.nlohmann.me/api/basic_json_view/at/#complexity) with a JSON pointer.
+
+## Notes
+
+Duplicate keys: the first member wins
+
+If the source text repeats a key, this resolves to the *first* member with it, not to the last one that [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) and `parse()` keep. See the [Notes on duplicate keys](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/#notes) of `operator[]` and [Duplicate keys](https://json.nlohmann.me/features/json_view/#duplicate-keys) for the reasons and for how to get the last value.
+
+Overload 1 always returns `false` when the value is not an object -- including a [discarded](https://json.nlohmann.me/api/basic_json_view/is_discarded/index.md) view.
+
+Postconditions
+
+If `v.contains(key)` returns `true`, then `v[key]` is not [discarded](https://json.nlohmann.me/api/basic_json_view/is_discarded/index.md) and `v.at(key)` does not throw. If `v.contains(ptr)` returns `true`, then `v[ptr]` is not discarded and `v.at(ptr)` does not throw.
+
+Overload 2 never throws
+
+Unlike [`BasicJsonType::contains(const json_pointer&)`](https://json.nlohmann.me/api/basic_json/contains/index.md), which can throw for certain malformed pointers (for instance an empty array-index reference token), overload 2 never throws: a missing key, an out-of-range or malformed array index, a `"-"` index, or a reference token used on a primitive all simply make it return `false`.
+
+## Examples
+
+Example: (1) check with key
+
+The example below counts how many of a batch of records carry an optional `retry_of` field, using `contains()` to check without ever materializing a single record of the batch.
+
+```
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json_document = nlohmann::json_document;
+
+int main()
+{
+    // count how many of many incoming records carry an optional "retry_of"
+    // field -- contains() only walks the flat index, so scanning a large
+    // batch like this never builds a single nlohmann::json value
+    json_document batch = json_document::parse(R"(
+      [
+        {"id": 1},
+        {"id": 2, "retry_of": 1},
+        {"id": 3},
+        {"id": 4, "retry_of": 3}
+      ]
+    )");
+
+    const auto records = batch.root();
+    std::size_t retries = 0;
+    for (std::size_t i = 0; i < records.size(); ++i)
+    {
+        if (records[i].contains("retry_of"))
+        {
+            ++retries;
+        }
+    }
+    std::cout << retries << " of " << records.size() << " records are retries\n";
+}
+```
+
+Output:
+
+```
+2 of 4 records are retries
+```
+
+Example: (2) check with JSON pointer
+
+The example below checks an optional, nested field with a JSON pointer, and shows two pointers that `contains()` resolves to `false` without throwing.
+
+```
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json_document = nlohmann::json_document;
+using json_pointer = nlohmann::json::json_pointer;
+
+int main()
+{
+    // "retry_of" is only present on some records, nested a level down
+    // inside "meta"
+    json_document batch = json_document::parse(R"(
+      [
+        {"id": 1, "meta": {}},
+        {"id": 2, "meta": {"retry_of": 1}}
+      ]
+    )");
+
+    const auto records = batch.root();
+    const json_pointer retry_of("/meta/retry_of");
+    for (std::size_t i = 0; i < records.size(); ++i)
+    {
+        const auto record = records[i];
+        if (record.contains(retry_of))
+        {
+            std::cout << "record " << i << " is a retry of " << record[retry_of].get<int>() << '\n';
+        }
+        else
+        {
+            std::cout << "record " << i << " is original\n";
+        }
+    }
+
+    // contains() with a JSON pointer never throws -- not even for a
+    // pointer that indexes into a primitive ("/0/id/x") or uses a
+    // malformed array index ("/01"), either of which would need a
+    // try/catch with json::contains(ptr)
+    std::cout << std::boolalpha << records.contains(json_pointer("/0/id/x")) << '\n';
+    std::cout << std::boolalpha << records.contains(json_pointer("/01")) << '\n';
+}
+```
+
+Output:
+
+```
+record 0 is original
+record 1 is a retry of 1
+false
+false
+```
+
+## See also
+
+- [find](https://json.nlohmann.me/api/basic_json_view/find/index.md) - find a value in an object
+- [count](https://json.nlohmann.me/api/basic_json_view/count/index.md) - returns the number of occurrences of a key
+- [at](https://json.nlohmann.me/api/basic_json_view/at/index.md), [operator[]](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/index.md) - resolve a JSON pointer and throw, or return a discarded view
+- [`BasicJsonType::contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) - the corresponding function of `basic_json`
+
+## Version history
+
+- Added in version 3.13.0 unreleased.