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api/basic_json/exception/index.md

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+# nlohmann::basic_json::exception
+
+```
+class exception : public std::exception;
+```
+
+This class is an extension of [`std::exception`](https://en.cppreference.com/w/cpp/error/exception) objects with a member `id` for exception ids. It is used as the base class for all exceptions thrown by the `basic_json` class. This class can hence be used as "wildcard" to catch exceptions, see example below.
+
+```
+classDiagram
+  direction LR
+
+    class std_exception ["std::exception"] {
+        <<interface>>
+    }
+
+    class json_exception ["basic_json::exception"] {
+        +const int id
+        +const char* what() const
+    }
+
+    class json_parse_error ["basic_json::parse_error"] {
+        +const std::size_t byte
+    }
+
+    class json_invalid_iterator ["basic_json::invalid_iterator"]
+    class json_type_error ["basic_json::type_error"]
+    class json_out_of_range ["basic_json::out_of_range"]
+    class json_other_error ["basic_json::other_error"]
+
+    std_exception <|-- json_exception
+    json_exception <|-- json_parse_error
+    json_exception <|-- json_invalid_iterator
+    json_exception <|-- json_type_error
+    json_exception <|-- json_out_of_range
+    json_exception <|-- json_other_error
+
+    style json_exception fill:#CCCCFF
+```
+
+Subclasses:
+
+- [`parse_error`](https://json.nlohmann.me/api/basic_json/parse_error/index.md) for exceptions indicating a parse error
+- [`invalid_iterator`](https://json.nlohmann.me/api/basic_json/invalid_iterator/index.md) for exceptions indicating errors with iterators
+- [`type_error`](https://json.nlohmann.me/api/basic_json/type_error/index.md) for exceptions indicating executing a member function with a wrong type
+- [`out_of_range`](https://json.nlohmann.me/api/basic_json/out_of_range/index.md) for exceptions indicating access out of the defined range
+- [`other_error`](https://json.nlohmann.me/api/basic_json/other_error/index.md) for exceptions indicating other library errors
+
+## Member functions
+
+- **what** - returns explanatory string
+
+## Member variables
+
+- **id** - the id of the exception
+
+## Notes
+
+To have nothrow-copy-constructible exceptions, we internally use `std::runtime_error` which can cope with arbitrary-length error messages. Intermediate strings are built with static functions and then passed to the actual constructor.
+
+## Examples
+
+Example
+
+The following code shows how arbitrary library exceptions can be caught.
+
+```
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    try
+    {
+        // calling at() for a non-existing key
+        json j = {{"foo", "bar"}};
+        json k = j.at("non-existing");
+    }
+    catch (const json::exception& e)
+    {
+        // output exception information
+        std::cout << "message: " << e.what() << '\n'
+                  << "exception id: " << e.id << std::endl;
+    }
+}
+```
+
+Output:
+
+```
+message: [json.exception.out_of_range.403] key 'non-existing' not found
+exception id: 403
+```
+
+## See also
+
+[List of exceptions](https://json.nlohmann.me/home/exceptions/index.md)
+
+## Version history
+
+- Since version 3.0.0.