110 lines
3.7 KiB
Markdown
110 lines
3.7 KiB
Markdown
# nlohmann::basic_json::exception
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```
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class exception : public std::exception;
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```
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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.
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```
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classDiagram
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direction LR
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class std_exception ["std::exception"] {
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<<interface>>
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}
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class json_exception ["basic_json::exception"] {
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+const int id
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+const char* what() const
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}
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class json_parse_error ["basic_json::parse_error"] {
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+const std::size_t byte
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}
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class json_invalid_iterator ["basic_json::invalid_iterator"]
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class json_type_error ["basic_json::type_error"]
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class json_out_of_range ["basic_json::out_of_range"]
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class json_other_error ["basic_json::other_error"]
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std_exception <|-- json_exception
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json_exception <|-- json_parse_error
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json_exception <|-- json_invalid_iterator
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json_exception <|-- json_type_error
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json_exception <|-- json_out_of_range
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json_exception <|-- json_other_error
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style json_exception fill:#CCCCFF
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```
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Class hierarchy of the library's exceptions; arrows point from a derived class to its base class, and the highlighted box is `basic_json::exception`.
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Subclasses:
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- [`parse_error`](https://json.nlohmann.me/api/basic_json/parse_error/index.md) for exceptions indicating a parse error
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- [`invalid_iterator`](https://json.nlohmann.me/api/basic_json/invalid_iterator/index.md) for exceptions indicating errors with iterators
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- [`type_error`](https://json.nlohmann.me/api/basic_json/type_error/index.md) for exceptions indicating executing a member function with a wrong type
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- [`out_of_range`](https://json.nlohmann.me/api/basic_json/out_of_range/index.md) for exceptions indicating access out of the defined range
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- [`other_error`](https://json.nlohmann.me/api/basic_json/other_error/index.md) for exceptions indicating other library errors
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## Member functions
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- **what** - returns explanatory string
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## Member variables
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- **id** - the id of the exception
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## Notes
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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.
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## Examples
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Example
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The following code shows how arbitrary library exceptions can be caught.
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```
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#include <iostream>
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#include <nlohmann/json.hpp>
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using nlohmann::json;
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int main()
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{
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try
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{
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// calling at() for a non-existing key
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json j = {{"foo", "bar"}};
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json k = j.at("non-existing");
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}
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catch (const json::exception& e)
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{
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// output exception information
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std::cout << "message: " << e.what() << '\n'
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<< "exception id: " << e.id << '\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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message: [json.exception.out_of_range.403] key 'non-existing' not found
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exception id: 403
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```
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## See also
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- [List of exceptions](https://json.nlohmann.me/home/exceptions/index.md) - overview of all exceptions thrown by the library
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- [Working Without Exceptions](https://json.nlohmann.me/features/no_exceptions/index.md) - using the library without exceptions
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- [`JSON_DIAGNOSTICS`](https://json.nlohmann.me/api/macros/json_diagnostics/index.md) - extended exception messages with the path of the failing value
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- [Parsing and Exceptions](https://json.nlohmann.me/features/parsing/parse_exceptions/index.md) - parsing alternatives that do not throw
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- [Debugging](https://json.nlohmann.me/home/debugging/index.md) - debugging aids
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## Version history
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- Added in version 3.0.0.
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