deploy: 7c9208bfb3

2026-07-09 12:05:10 +00:00 · 2026-07-08 18:19:46 +00:00
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+# nlohmann::basic_json::contains
+
+```
+// (1)
+bool contains(const typename object_t::key_type& key) const;
+
+// (2)
+template<typename KeyType>
+bool contains(KeyType&& key) const;
+
+// (3)
+bool contains(const json_pointer& ptr) const;
+```
+
+1. Check whether an element exists in a JSON object with a key equivalent to `key`. If the element is not found or the JSON value is not an object, `false` is returned.
+1. See 1. 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.
+1. Check whether the given JSON pointer `ptr` can be resolved in the current JSON value.
+
+## Template parameters
+
+`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).
+
+## Parameters
+
+`key` (in) : key value to check its existence.
+
+`ptr` (in) : JSON pointer to check its existence.
+
+## Return value
+
+1. `true` if an element with specified `key` exists. If no such element with such a key is found or the JSON value is not an object, `false` is returned.
+1. See 1.
+1. `true` if the JSON pointer can be resolved to a stored value, `false` otherwise.
+
+## Exception safety
+
+Strong exception safety: if an exception occurs, the original value stays intact.
+
+## Exceptions
+
+1. The function does not throw exceptions.
+1. The function does not throw exceptions.
+1. The function does not throw exceptions.
+
+## Complexity
+
+Logarithmic in the size of the JSON object.
+
+## Notes
+
+- This method always returns `false` when executed on a JSON type that is not an object.
+- This method can be executed on any JSON value type.
+
+Postconditions
+
+If `j.contains(x)` returns `true` for a key or JSON pointer `x`, then it is safe to call `j[x]`.
+
+## Examples
+
+Example: (1) check with key
+
+The example shows how `contains()` is used.
+
+```
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+using namespace nlohmann::literals;
+
+int main()
+{
+    // create some JSON values
+    json j_object = R"( {"key": "value"} )"_json;
+    json j_array = R"( [1, 2, 3] )"_json;
+
+    // call contains
+    std::cout << std::boolalpha <<
+              "j_object contains 'key': " << j_object.contains("key") << '\n' <<
+              "j_object contains 'another': " << j_object.contains("another") << '\n' <<
+              "j_array contains 'key': " << j_array.contains("key") << std::endl;
+}
+```
+
+Output:
+
+```
+j_object contains 'key': true
+j_object contains 'another': false
+j_array contains 'key': false
+```
+
+Example: (2) check with key using string_view
+
+The example shows how `contains()` is used.
+
+```
+#include <iostream>
+#include <string_view>
+#include <nlohmann/json.hpp>
+
+using namespace std::string_view_literals;
+using json = nlohmann::json;
+using namespace nlohmann::literals;
+
+int main()
+{
+    // create some JSON values
+    json j_object = R"( {"key": "value"} )"_json;
+    json j_array = R"( [1, 2, 3] )"_json;
+
+    // call contains
+    std::cout << std::boolalpha <<
+              "j_object contains 'key': " << j_object.contains("key"sv) << '\n' <<
+              "j_object contains 'another': " << j_object.contains("another"sv) << '\n' <<
+              "j_array contains 'key': " << j_array.contains("key"sv) << std::endl;
+}
+```
+
+Output:
+
+```
+j_object contains 'key': true
+j_object contains 'another': false
+j_array contains 'key': false
+```
+
+Example: (3) check with JSON pointer
+
+The example shows how `contains()` is used.
+
+```
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+using namespace nlohmann::literals;
+
+int main()
+{
+    // create a JSON value
+    json j =
+    {
+        {"number", 1}, {"string", "foo"}, {"array", {1, 2}}
+    };
+
+    std::cout << std::boolalpha
+              << j.contains("/number"_json_pointer) << '\n'
+              << j.contains("/string"_json_pointer) << '\n'
+              << j.contains("/array"_json_pointer) << '\n'
+              << j.contains("/array/1"_json_pointer) << '\n'
+              << j.contains("/array/-"_json_pointer) << '\n'
+              << j.contains("/array/4"_json_pointer) << '\n'
+              << j.contains("/baz"_json_pointer) << std::endl;
+
+    try
+    {
+        // try to use an array index with leading '0'
+        j.contains("/array/01"_json_pointer);
+    }
+    catch (const json::parse_error& e)
+    {
+        std::cout << e.what() << '\n';
+    }
+
+    try
+    {
+        // try to use an array index that is not a number
+        j.contains("/array/one"_json_pointer);
+    }
+    catch (const json::parse_error& e)
+    {
+        std::cout << e.what() << '\n';
+    }
+}
+```
+
+Output:
+
+```
+true
+true
+true
+true
+false
+false
+false
+```
+
+## See also
+
+- [find](https://json.nlohmann.me/api/basic_json/find/index.md) find a value in an object
+- [count](https://json.nlohmann.me/api/basic_json/count/index.md) returns the number of occurrences of a key
+
+## Version history
+
+1. Added in version 3.11.0.
+1. Added in version 3.6.0. Extended template `KeyType` to support comparable types in version 3.11.0.
+1. Added in version 3.7.0.